Carbon, Nitrogen and Hydrogen in Iron-Based Solid Solutions: Similarities and Differences in their Effect on Structure and Properties
Interstitial N, C and H atoms in iron-based solid solutions are compared in terms of their effect on the structure and properties. Electronic structure and stacking fault energy, atomic distribution, interaction of interstitial atoms with dislocations and vacancies, mobility of dislocations, mechani...
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Інститут металофізики ім. Г.В. Курдюмова НАН України
2016
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| Cite this: | Carbon, Nitrogen and Hydrogen in Iron-Based Solid Solutions: Similarities and Differences in their Effect on Structure and Properties / V. G. Gavriljuk // Металлофизика и новейшие технологии. — 2016. — Т. 38, № 1. — С. 67-98. — Бібліогр.: 89 назв. — англ. |
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| author | Gavriljuk, V.G. |
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| citation_txt | Carbon, Nitrogen and Hydrogen in Iron-Based Solid Solutions: Similarities and Differences in their Effect on Structure and Properties / V. G. Gavriljuk // Металлофизика и новейшие технологии. — 2016. — Т. 38, № 1. — С. 67-98. — Бібліогр.: 89 назв. — англ. |
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| description | Interstitial N, C and H atoms in iron-based solid solutions are compared in terms of their effect on the structure and properties. Electronic structure and stacking fault energy, atomic distribution, interaction of interstitial atoms with dislocations and vacancies, mobility of dislocations, mechanisms of deformation and fracture are compared based on theoretical calculations and experimental observations. As shown, nitrogen and hydrogen increase the electron density of states at the Fermi level of f.c.c. iron, whereas carbon decreases it. Correspondingly, the concentration of free electrons increases within the nitrogen and hydrogen iron-based solid solutions and decreases in the carbon ones. A correlation is revealed between the character of interatomic bonds and the short-range atomic order in the studied solid solutions: nitrogen assists short-range atomic ordering in the spatial distribution of alloying elements, whereas carbon promotes their clustering. As consequence, nitrogen increases thermodynamical stability of austenitic steels, whereas carbon makes steel sensitive to precipitation of carbides from the solid solution that deteriorates corrosive characteristics. The most impressive is a correlation between the change in the electronic structure and properties of dislocations. In contrast to prevailing covalent bonds in carbon steels, the enhanced metallic character of interatomic bonds, as caused by nitrogen, increases mobility of dislocations that results in excellent plasticity and fracture toughness. However, the same effect caused by hydrogen is a cause of the hydrogen embrittlement through the hydrogen-enhanced localized plasticity. A unique similarity with hydrogen embrittlement becomes apparent in the course of impact loading of austenitic nitrogen steels, where, due to the absence of sufficient time for relaxation of stresses, the nitrogen-enhanced localized plasticity occurs resulting in a pseudo-brittle fracture. The different is only the mechanism for localization of plastic deformation: the shortrange atomic ordering caused by nitrogen and the increased concentration of superabundant vacancies due to hydrogen dissolution.
Проаналізовано вплив елементів втілення N, C і H в твердих розчинах на основі заліза на їхню структуру і властивості. На основі теоретичних розрахунків і експериментальних результатів порівнюються електронна структура і енергія дефектів пакування, розподіл атомів у твердих розчинах, взаємодія атомів втілення з дислокаціями та вакансіями, рухомість дислокацій, механізми пластичної деформації та руйнування. Встановлено, що Нітроґен і Гідроґен збільшують густину електронних станів на рівні Фермі ГЦК-заліза, в той час як Карбон зменшує її. Відповідно, концентрація вільних електронів підвищується в твердих розчинах Нітроґену і Гідроґену на основі γ-заліза і зменшується при розчиненні Карбону. Виявлено кореляцію між характером міжатомового зв’язку і близьким атомовим порядком в аустенітних сталях: Нітроґен сприяє близькому атомовому упорядкуванню в розподілі леґувальних елементів, в той час як розчинення Карбону супроводжується їх кластеризацією. Як наслідок, Нітроґен підвищує термодинамічну стабільність аустенітних сталей, а Карбон робить сталь чутливою до виділення карбідів із твердого розчину, що погіршує корозійні властивості. Найбільш вражаючою є кореляція між електронною структурою і властивостями дислокацій. На відміну від переважаючих ковалентних зв’язків у вуглецевих сталях, посилений Нітроґеном їхній металічний характер підвищує рухливість дислокацій, наслідком чого є висока пластичність і в’язкість руйнування. Але аналогічний вплив Гідроґену є причиною водневого окрихчування через посилену Гідроґеном локалізовану пластичність. Унікальна схожість з водневим окрихчуванням має місце, якщо аустенітна азотиста сталь піддається ударному навантаженню. Внаслідок недостатнього часу для релаксації напружень, посилена Нітроґеном локалізована пластичність призводить до псевдокрихкого руйнування. Відмінним є лише механізм локалізації пластичної деформації: близьке атомове упорядкування, спричинене Гідроґеном, і збільшення концентрації надлишкових вакансій у випадку розчинення Гідроґену.
Выполнен анализ влияния элементов внедрения N, C и H в твёрдых растворах на основе железа на их структуру и свойства. На основе теоретических расчётов и экспериментальных данных сравниваются электронная структура и энергия дефектов упаковки, распределение атомов в твёрдых растворах, взаимодействие атомов внедрения с дислокациями и вакансиями, подвижность дислокаций, механизмы пластической деформации и разрушения. Установлено, что азот и водород повышают плотность электронных состояний на уровне Ферми ГЦК-железа, в то время как углерод уменьшает её. Соответственно, концентрация свободных электронов увеличивается в твёрдых растворах азота и водорода на основе γ-железа и уменьшается при растворении углерода. Найдена корреляция между характером межатомных связей и ближним атомным порядком в аустенитных сталях: азот способствует ближнему атомному упорядочению в распределении легирующих элементов, в то время как растворение углерода сопровождается их кластеризацией. Как следствие, азот увеличивает термодинамическую стабильность аустенитных сталей, а углерод делает сталь чувствительной к выделению карбидов из твёрдого раствора, что ухудшает коррозионные свойства. Наиболее впечатляющей является корреляция между электронной структурой и свойствами дислокаций. В отличие от превалирующих ковалентных связей в углеродистых сталях, усиленный азотом их металлический характер увеличивает подвижность дислокаций, следствием чего является высокая пластичность и вязкость разрушения. Однако аналогичное влияние водорода является причиной водородного охрупчивания стали из-за усиленной водородом локализованной пластичности. Уникальное сходство с водородной хрупкостью имеет место, если аустенитная азотистая сталь подвергается ударному нагружению. Вследствие недостаточного времени для релаксации напряжений, усиленная азотом локализованная пластичность приводит к псевдохрупкому разрушению. Различным является лишь механизм локализации пластической деформации: ближнее атомное упорядочение, обусловленное азотом, и повышение концентрации избыточных вакансий в случае растворения водорода.
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67
ДЕФЕКТЫ КРИСТАЛЛИЧЕСКОЙ РЕШЁТКИ
PACS numbers:61.66.Dk, 61.72.J-,61.72.Lk,61.72.Nn,62.20.mj,62.20.mm,64.75.Nx, 71.55.Ak
Carbon, Nitrogen, and Hydrogen in Iron-Based Solid Solutions:
Similarities and Differences in Their Effect on Structure and
Properties
V. G. Gavriljuk
G. V. Kurdyumov Institute for Metal Physics, N.A.S. of Ukraine,
36 Academician Vernadsky Blvd.,
UA-03680 Kyiv, Ukraine
Interstitial N, C and H atoms in iron-based solid solutions are compared in
terms of their effect on the structure and properties. Electronic structure
and stacking fault energy, atomic distribution, interaction of interstitial at-
oms with dislocations and vacancies, mobility of dislocations, mechanisms of
deformation and fracture are compared based on theoretical calculations and
experimental observations. As shown, nitrogen and hydrogen increase the
electron density of states at the Fermi level of f.c.c. iron, whereas carbon de-
creases it. Correspondingly, the concentration of free electrons increases
within the nitrogen and hydrogen iron-based solid solutions and decreases in
the carbon ones. A correlation is revealed between the character of interatom-
ic bonds and the short-range atomic order in the studied solid solutions: ni-
trogen assists short-range atomic ordering in the spatial distribution of al-
loying elements, whereas carbon promotes their clustering. As consequence,
nitrogen increases thermodynamical stability of austenitic steels, whereas
carbon makes steel sensitive to precipitation of carbides from the solid solu-
tion that deteriorates corrosive characteristics. The most impressive is a cor-
relation between the change in the electronic structure and properties of dis-
locations. In contrast to prevailing covalent bonds in carbon steels, the en-
hanced metallic character of interatomic bonds, as caused by nitrogen, in-
creases mobility of dislocations that results in excellent plasticity and frac-
ture toughness. However, the same effect caused by hydrogen is a cause of
the hydrogen embrittlement through the hydrogen-enhanced localized plas-
Corresponding author: Valentin Gennadievich Gavriljuk
E-mail: gavriljuk@rambler.ru
Please cite this article as: V. G. Gavriljuk, Carbon, Nitrogen, and Hydrogen in
Iron-Based Solid Solutions: Similarities and Differences in Their Effect on Structure
and Properties, Metallofiz. Noveishie Tekhnol., 38, No. 1: 67—98 (2016),
DOI: 10.15407/MFiNT.38.0067.
Металлофиз. новейшие технол. / Metallofiz. Noveishie Tekhnol.
2016, т. 38, № 1, сс. 67—98 / DOI: 10.15407/MFiNT.38.0067
Оттиски доступны непосредственно от издателя
Фотокопирование разрешено только
в соответствии с лицензией
2016 ИМФ (Институт металлофизики
им. Г. В. Курдюмова НАН Украины)
Напечатано в Украине.
68 V. G. GAVRILJUK
ticity. A unique similarity with hydrogen embrittlement becomes apparent in
the course of impact loading of austenitic nitrogen steels, where, due to the
absence of sufficient time for relaxation of stresses, the nitrogen-enhanced
localized plasticity occurs resulting in a pseudo-brittle fracture. The differ-
ent is only the mechanism for localization of plastic deformation: the short-
range atomic ordering caused by nitrogen and the increased concentration of
superabundant vacancies due to hydrogen dissolution.
Key words: carbon, nitrogen, hydrogen, electronic structure, short-range
atomic order, thermodynamical stability, deformation, fracture.
Проаналізовано вплив елементів втілення N, C і H в твердих розчинах на
основі заліза на їхню структуру і властивості. На основі теоретичних роз-
рахунків і експериментальних результатів порівнюються електронна
структура і енергія дефектів пакування, розподіл атомів у твердих розчи-
нах, взаємодія атомів втілення з дислокаціями та вакансіями, рухомість
дислокацій, механізми пластичної деформації та руйнування. Встановле-
но, що Нітроґен і Гідроґен збільшують густину електронних станів на рі-
вні Фермі ГЦК-заліза, в той час як Карбон зменшує її. Відповідно, конце-
нтрація вільних електронів підвищується в твердих розчинах Нітроґену і
Гідроґену на основі -заліза і зменшується при розчиненні Карбону. Ви-
явлено кореляцію між характером міжатомового зв’язку і близьким ато-
мовим порядком в аустенітних сталях: Нітроґен сприяє близькому атомо-
вому упорядкуванню в розподілі леґувальних елементів, в той час як роз-
чинення Карбону супроводжується їх кластеризацією. Як наслідок, Ніт-
роґен підвищує термодинамічну стабільність аустенітних сталей, а Кар-
бон робить сталь чутливою до виділення карбідів із твердого розчину, що
погіршує корозійні властивості. Найбільш вражаючою є кореляція між
електронною структурою і властивостями дислокацій. На відміну від пе-
реважаючих ковалентних зв’язків у вуглецевих сталях, посилений Ніт-
роґеном їхній металічний характер підвищує рухливість дислокацій, на-
слідком чого є висока пластичність і в’язкість руйнування. Але аналогіч-
ний вплив Гідроґену є причиною водневого окрихчування через посилену
Гідроґеном локалізовану пластичність. Унікальна схожість з водневим
окрихчуванням має місце, якщо аустенітна азотиста сталь піддається
ударному навантаженню. Внаслідок недостатнього часу для релаксації
напружень, посилена Нітроґеном локалізована пластичність призводить
до псевдокрихкого руйнування. Відмінним є лише механізм локалізації
пластичної деформації: близьке атомове упорядкування, спричинене Гід-
роґеном, і збільшення концентрації надлишкових вакансій у випадку ро-
зчинення Гідроґену.
Ключові слова: сталь, Карбон, Нітроґен, Гідроґен, електронна структура,
близький атомовий порядок, термодинамічна стабільність, деформація,
руйнування.
Выполнен анализ влияния элементов внедрения N, C и H в твёрдых рас-
творах на основе железа на их структуру и свойства. На основе теоретиче-
ских расчётов и экспериментальных данных сравниваются электронная
структура и энергия дефектов упаковки, распределение атомов в твёрдых
CARBON, NITROGEN, AND HYDROGEN IN IRON-BASED SOLID SOLUTIONS 69
растворах, взаимодействие атомов внедрения с дислокациями и ваканси-
ями, подвижность дислокаций, механизмы пластической деформации и
разрушения. Установлено, что азот и водород повышают плотность элек-
тронных состояний на уровне Ферми ГЦК-железа, в то время как углерод
уменьшает её. Соответственно, концентрация свободных электронов уве-
личивается в твёрдых растворах азота и водорода на основе -железа и
уменьшается при растворении углерода. Найдена корреляция между ха-
рактером межатомных связей и ближним атомным порядком в аустенит-
ных сталях: азот способствует ближнему атомному упорядочению в рас-
пределении легирующих элементов, в то время как растворение углерода
сопровождается их кластеризацией. Как следствие, азот увеличивает
термодинамическую стабильность аустенитных сталей, а углерод делает
сталь чувствительной к выделению карбидов из твёрдого раствора, что
ухудшает коррозионные свойства. Наиболее впечатляющей является
корреляция между электронной структурой и свойствами дислокаций. В
отличие от превалирующих ковалентных связей в углеродистых сталях,
усиленный азотом их металлический характер увеличивает подвижность
дислокаций, следствием чего является высокая пластичность и вязкость
разрушения. Однако аналогичное влияние водорода является причиной
водородного охрупчивания стали из-за усиленной водородом локализо-
ванной пластичности. Уникальное сходство с водородной хрупкостью
имеет место, если аустенитная азотистая сталь подвергается ударному
нагружению. Вследствие недостаточного времени для релаксации напря-
жений, усиленная азотом локализованная пластичность приводит к псев-
дохрупкому разрушению. Различным является лишь механизм локали-
зации пластической деформации: ближнее атомное упорядочение, обу-
словленное азотом, и повышение концентрации избыточных вакансий в
случае растворения водорода.
Ключевые слова: сталь, углерод, азот, водород, электронная структура,
ближний атомный порядок, термодинамическая стабильность, деформа-
ция, разрушение.
(Received December 18, 2016)
1. INTRODUCTION
The carbon, nitrogen, and hydrogen solid solutions in the iron-based
alloys are characterized by different limits of solubility depending on
the crystal modification of the iron (, , ) and content of substitu-
tional alloying elements. As compared to carbon, nitrogen positively
affects mechanical and chemical properties of austenitic steels and, be
bound in carbonitrides, is effective for design of high strength low-
alloyed steels (HSLA). Carbon causes a number of harmful effects in
austenitic steels, particularly those concerned with brittle fracture
and bad corrosion resistance (see, e.g., [1, 2]). Studies of hydrogen in
steel are usually concerned with hydrogen brittleness, of which mech-
anism is still the subject of discussion (see reviews in [3—6]).
70 V. G. GAVRILJUK
So far, hydrogen effects in the iron-based solid solutions were not
compared with those of carbon or nitrogen. The aim of this article is to
fill this gap and discuss similarities and differences between C, N and
H in steel starting up from the atomic interactions, and consider possi-
ble consequences for thermodynamic stability of solid solutions, inter-
action with the crystal lattice defects and mechanical properties.
It is generally accepted that ‘structure’ stands for crystal lattice,
lattice defects and their distribution as well as for grain size. In the
solid solutions, the type of solute atoms and their distribution, as well
as precipitates are taken into account.
In fact, the structure of metals and alloys starts from localized or
free electrons. Under external force and resultant straining, the atoms
are being shifted from their positions, and mechanical response, plastic
deformation or brittle fracture, depends on the character of interatom-
ic bonds. In comparison with the nuclei, the response of electrons is
quicker by many orders of magnitude. The closed electron shell, so-
called ‘ion core’, can be excluded from the consideration because it does
not take part in chemical reactions and, under straining, can be only
slightly deformed, i.e., polarized. Only the external, i.e., valence elec-
trons are responsible for chemical bonds and deformation behaviour.
The prevailing localized valence electrons form covalent bonds be-
tween the atoms in the crystal lattice, which causes brittleness because
even a slight shift of the atoms under shear stress in the slip plane
leads to breaking the interatomic bonds. This is, e.g., the case of the
transition metals and alloys of group V and VI in the periodic table (Cr,
Mo, V, Nb, W). Free electrons are responsible for the metallic charac-
ter of interatomic bonds, and the higher their fraction is the more duc-
tile are metals and alloys.
In relation to phase transformations, valence electrons are responsi-
ble for the height of the energy barrier, which have to be overcome by
the atoms constituting a new crystal lattice, either during its nuclea-
tion or during their jumps through the interface between matrix and
new phase. Again, the stronger the covalent bonds between the atoms
are, the higher this energy barrier is. For this reason, the transition
metals of group V and VI do not reveal any polymorphic transfor-
mations.
Moreover, as will be shown, the control of interatomic bonds and the
free/localized electron ratio affects short-range atomic order in multi-
component solid solutions and, for this reason, their thermodynamic
stability.
2. ATOMIC INTERACTIONS
Ab initio calculations of the electronic structure in the f.c.c. iron con-
taining carbon, nitrogen or hydrogen, as studied in [7—9], have shown
CARBON, NITROGEN, AND HYDROGEN IN IRON-BASED SOLID SOLUTIONS 71
that carbon decreases density of electron states at the Fermi level of
f.c.c. iron, whereas nitrogen and hydrogen increase it (Fig. 1). In other
words, carbon is expected to promote the covalent character of intera-
tomic bonds, whereas nitrogen and hydrogen assist prevailing metallic
bonds between atoms.
This result is consistent with the experimental data obtained using
conduction electron spin resonance [10—13]). According to data in Fig.
2, a, the alloying of austenitic steels with nitrogen increases the con-
centration of free electrons at the Fermi level. In its influence on the
electronic structure of the -iron, hydrogen is similar to nitrogen.
Shown in Fig. 2, b is the spectrum of electron spin resonance in
Cr18Mn20N0.88 (% mass) hydrogen-charged and hydrogen-free austenit-
ic steel. The Ni-free steel was chosen in this experiment because, in
presence of nickel, the hydrogen charging causes ferromagnetism,
which makes impossible the observation of the ESR signal. Nitrogen in
this steel was introduced to provide stability of the austenitic phase at
temperatures down to 4.2 K. The obtained data confirm that, like ni-
trogen, hydrogen increases the concentration of free electrons at the
Fermi level.
Free electrons in nitrogen austenitic steels markedly contribute to
the magnetic susceptibility (Pauli paramagnetism), by more than one
order of the value larger in the comparison with carbon steels. In con-
trast, the localization of electrons at the atomic sites contributing to
the Curie—Weiss magnetic susceptibility is more pronounced in the
carbon steels (see Table 1).
In many relations, it is particularly important to know the spatial
distribution of the electron density over the crystal lattice. As an ex-
ample, Fig. 3 shows some results of ab initio calculations concerning
the distribution of external electrons (i.e. except for the electrons of
Fig. 1. Density of electron states per elementary cell in the f.c.c. iron and iron-
based solid solutions: Fe32C and Fe32N (a), Fe4H and FeH (b). The situation at
the Fermi level is shown in the insert at the left upper corners.
72 V. G. GAVRILJUK
the ion cores) in the octahedral sites of the -iron f.c.c. lattice occupied
by carbon, nitrogen or hydrogen atoms. It is seen that the electrons are
substantially removed from the interstitial site in the case of carbon,
whereas their density increases in the vicinity of nitrogen or hydrogen
atom located in the interstitial sites. These results suggest that carbon
ions in the austenite are positively charged, whereas nitrogen and hy-
drogen ions carry an effective negative electric charge.
Let us compare the results of these calculations with available ex-
perimental data. The measurements of the electron transfer in the bi-
nary Fe—C and Fe—N austenites [14, 15] have given the evidence that
carbon atoms in the -iron solid solution carry a positive electric
charge, whereas nitrogen atoms are likely to be negatively charged.
The data in Fig. 3 allow explaining such a different behaviour. The in-
creased concentration of free electrons at the interstitial sites occupied
Fig. 2. Concentration of free electrons at the Fermi level in the austenitic
steels alloyed with carbon and nitrogen (a) and signal of electron spin reso-
nance in the hydrogen-free and hydrogen-charged steel Cr18Mn10N0.88 (b). A
piece of the borate glass having 1015
spins was used as a reference sample for
determination of spin concentration. The intensity of ESR signal is normal-
ized to that of the reference sample A0.
CARBON, NITROGEN, AND HYDROGEN IN IRON-BASED SOLID SOLUTIONS 73
by the nitrogen atoms in the f.c.c. iron is consistent with the negative
effective charge. The positive effective charge of the carbon atoms fol-
lows from the decrease in the electron concentration at the site occu-
pied by carbon atoms as compared to that in the interstitial-free site.
Measurements of ESR evidence an increase in the concentration of the
localised d-electrons due to carbon in comparison with nitrogen in the
f.c.c. iron (see Table 1). Experimental data about the charge of hydro-
gen atoms in metals are controversial. The ab initio calculations sup-
port the idea of their negative effective charge.
3. ATOMIC DISTRIBUTION
A correlation between the character of interatomic bonds (the states
density and, correspondingly, the concentration of free electrons at
the Fermi level) and the atomic distribution in the iron-based solid so-
lutions has been found based on the experimental data obtained using
Mössbauer spectroscopy, electron spin resonance and small angle scat-
tering of neutrons.
3.1. Mössbauer spectroscopy is a useful tool for studying the short-
range atomic order in iron-based solid solutions due to its high resolu-
tion in the identification of different atomic configurations in the
crystal lattice [16—18].
Mössbauer spectra of binary Fe—C and Fe—N solid solutions and cor-
responding atomic configurations are presented in Fig. 4. The Fe—C
spectrum consists of a single line belonging to iron atoms Fe0 having no
carbon atoms as nearest neighbours. The doublet comes from the iron
atoms Fe1 with one carbon atom as nearest neighbour and Fe2—90 with
two carbon atoms in the nearest interstitial sites. These two different
configurations cause the same electric field gradient in the crystal lat-
tice (only its sign is different), which results in the same quadrupole
splitting of the spectrum.
TABLE 1. Some parameters of atomic interactions in Cr18Ni16Mn10 steels con-
taining nitrogen or carbon: c0, d1 are magnetic susceptibilities of free elec-
tron (Pauli paramagnetism) and isolated d-electron (Curie—Weiss) subsys-
tems; is the energy of superparamagnetic (Langevin) clusters in the tem-
perature units characterising the size of clusters.
Steel N(C) content, % mass 107c0 105d1 , K
N1 0.17 4.0 2.2 140
N2 0.4 10.8 0.9 140
C1 0.15 0.4 13.7 315
C2 0.43 0.8 80.0 715
74 V. G. GAVRILJUK
Such 90-pairs are never met in the Fe—N austenite. Instead, in addi-
tion to the Fe1 component caused by single nitrogen atoms, the spec-
trum contains a doublet from the Fe2—180 configurations caused by ni-
trogen atoms occupying interstitial sites within the second coordina-
tion sphere in the interstitial sublattice. Such a dumbbell-like configu-
ration is an element of the ordered Fe4N -phase.
In order to obtain the values of C—C and N—N interaction energies,
which could be consistent with the fractions of atomic configurations
derived from Mössbauer spectra, a modelling of these solid solutions
was carried out using the Monte Carlo method (Fig. 5). W1 and W2 are
the energies of interaction between any two interstitial atoms in the
first and second coordination spheres, respectively, under condition
that one of them is located in the co-ordinate origin. The areas marked
as C—C and N—N correspond to the values of C—C and N—N interactions
which are consistent with the fractions Fe1 (d), Fe2—90 (e) and Fe2—180 (f)
atoms in Fig. 4.
It is seen from Fig. 5 that the carbon distribution in austenitic steels
is characterized by a soft repulsion between C atoms in nearest inter-
stitial sites (a small W1 for the C—C area), so that, along with single
carbon atoms, some fraction of carbon pairs Fe2—90 can exist. However,
a hard C—C repulsion (large W2) is revealed for carbon atoms as the
neighbours in the second coordination sphere of the interstitial sublat-
Fig. 3. Spatial distribution of the valence electron density along (110) plane in
the lattice of solid solutions: Fe20Mn8Cr4C2 (a), Fe20Mn8Cr4N2 (b), FeH (c).
CARBON, NITROGEN, AND HYDROGEN IN IRON-BASED SOLID SOLUTIONS 75
tice. This means that dumbbell-like C—Fe—C configurations Fe2—180 do
not exist in the austenitic carbon steels, which makes impossible the
existence of an ordered Fe4C type structure.
In contrast, the N—N repulsion in the first coordination sphere is so
hard (large W1) that nitrogen atoms cannot be nearest neighbours in
the austenitic lattice, whereas the soft repulsion in the second coordi-
nation sphere (small W2) allows the N—Fe—N pairs which are clearly
identified in the Mössbauer spectra of austenitic nitrogen steels. This
is why the distribution of nitrogen atoms is characterized by short-
range atomic ordering. The carbon atoms in austenitic steels are prone
to form clusters.
The comparison of these experimental data with the aforementioned
effect of carbon and nitrogen on the electronic structure of the iron
austenite provides one with a suggestion that the increase in the
states’ density, i.e. the increased concentration of free electrons at the
Fermi level, assists the short-range atomic ordering, whereas the local-
ization of electrons at the atomic sites promotes clustering of solute
atoms [19].
This idea was confirmed in subsequent studies of combined alloying
of austenitic steels with carbon and nitrogen and development of a new
class of high-strength corrosion-resistant austenitic steels [2, 20, 21].
Mössbauer studies of the ternary Fe—0.9C—0.9N alloy have shown that
Fig. 4. Mössbauer spectra of binary austenitic solid solutions, at.%: Fe—9.1C
(a), Fe—9.3N (b) and corresponding atomic configurations(c—f).
76 V. G. GAVRILJUK
this change in the electronic structure results in an unordinary distri-
bution of the interstitial atoms in the solid solution, namely in the ten-
dency to not occupy the sites within the first two coordination spheres
of the interstitial sublattice. The experimental evidence for that is pre-
sented in Fig. 6. The spectrum consists of the austenitic and martensit-
ic parts. In contrast to the Fe—N austenite, the austenitic component
does not contain the doublet with the double quadrupole splitting from
dumbbell-like N—Fe—N nitrogen configurations (compare with Fig. 4,
b, f), and there is no component from 90 Fe2 carbon configurations in
the martensitic part, which is typical for Fe—C martensite (see the Fe—
C martensite spectrum in the upper right corner).
Mössbauer studies have also shown that carbon and nitrogen affect
the distribution of substitutional solutes in the multicomponent iron-
based alloys. The outer lines of the ferritic spectra of steels with basic
Cr15Mo1 composition containing carbon, nitrogen or carbon nitrogen
are presented in Fig. 7. After quenching, these steels were subjected to
tempering at 500C, so that carbon is not present in the solid solution.
The occurrence of carbides cannot affect the outer lines of the ferritic
spectra because the hyperfine field on the iron nuclei in the carbides is
much smaller in comparison with that in the ferrite.
Four components from the iron atoms having no, one, two and three
substitutional atoms as nearest neighbours can be distinguished in the
spectra presented in Fig. 7. The component Fe0 is clearly resolved and
can be properly used for the analysis. It is seen that the replacement of
carbon by nitrogen decreases the fraction of Fe0 atoms, which suggests
a more homogeneous distribution of the substitutional atoms. The
smallest fraction of the iron atoms not having neighbouring substitu-
tional atoms is found in the C N alloy and, therefore, the combined
Fig. 5. Areas of C—C and N—N atomic interactions (marked with grey colour)
within the first and second coordination spheres on the sublattice of intersti-
tial sites corresponding to the fractions of Fe1 (d), Fe2—90 (e) and Fe2—180 (f)
iron atoms obtained from Mössbauer spectra in Fig. 4.
CARBON, NITROGEN, AND HYDROGEN IN IRON-BASED SOLID SOLUTIONS 77
C N alloying retards clustering of chromium (molybdenum) atoms
and provides the most homogeneous atomic distribution.
3.2. Electron spin resonance (ESR). Three electron subsystems con-
tribute to the magnetic susceptibility in the iron-based solid solutions:
(i) the conduction (free) electrons denoted as c-subsystem (Pauli para-
magnetism), (ii) localized d-electrons named in the following as d1-
subsystem (Curie—Weiss paramagnetism) and (iii) clusters of d-atoms
acquiring their macroscopic moments due to spin polarisation (Lange-
vin superparamagnetism) denoted as d2-subsystem. The temperature
dependence of the ESR signal parameters was obtained from measure-
ments in [10] using a theory [22], which takes into account the ex-
change interaction between the free electrons (c-subsystem) and the
d1-, d2-subsystems.
As a result of this exchange (see for details [10]), magnetic suscepti-
bilities of c- and d-electron subsystems are
c c0(1 d), (1)
d d0(1 c0),
where is an exchange parameter, and d is the magnetic susceptibility
of d1-, d2-subsystems.
The magnetic susceptibility c0 of free electron subsystem not inter-
acting with d-subsystems is described as
,)2/1(
22
0 FBc Dg (2)
where g is the spectroscopic factor characterising a splitting of the
Fig. 6. Mössbauer spectrum of Fe—0.93C—0.91N (% mass) solid solution. For
the comparison, the Fe—C spectrum is shown in the upper right corner.
78 V. G. GAVRILJUK
electron levels under applied magnetic field, and it is determined from
the CESR spectra, B is the Bohr magneton, DF is the state density at
the Fermi level with energy E EF.
The value of c0 does not depend on the temperature in the low-
temperature range kBT EF. As was mentioned above, there are two
contributions to the magnetic susceptibility d:
d 1d1 2d2. (3)
The magnetic susceptibility of the isolated localized d-electrons d1
changes with the temperature according to the Curie—Weiss law:
d1(T) C1/T, (4)
Fig. 7. Outer lines of Mössbauer spectra of steels with (% mass) 15 Cr, 1 Mo
and 0.6 C or 0.62 N or 0.35 N 0.29 C after quenching from 1100C and tem-
pering at 650C for 2 hours.
CARBON, NITROGEN, AND HYDROGEN IN IRON-BASED SOLID SOLUTIONS 79
and the magnetic susceptibility of superparamagnetic clusters obeys
the Langevin law:
d2(T) C2L(/T), C2 d2 at T 1, (5)
L(/T) coth(/T) T/,
MH/kB,
where M is the cluster magnetic moment, H is the external magnetic
field, kB is the Boltzmann constant, is the energy of cluster in the
magnetic field in the temperature units, and it is proportional to the
number of d-atoms in the cluster.
The relation between g(T) and magnetic susceptibilities is written as
)),(//())(1()(
1
r1
1
r
TggTgTg dcc
(6)
where the relative magnetic susceptibility )(
1
r
T consists of three
above-mentioned contributions in a paramagnetic alloy:
./)(
1
1022
1
10011
1
r
dcddccdcT (6)
Figure 8, see also Table 1, shows the temperature dependence of
1
r
for austenitic steels Cr18Ni16Mn10C containing 0.15% or 0.43% of car-
bon and Cr18Ni16Mn10N0.4 (% mass). It is seen that the Curie—Weiss law
profoundly determines the )(1
r
T behaviour in the nitrogen-containing
steels, which suggests a preference of single d-atoms in the solid solu-
tion. In contrast, the Langevin superparamagnetism controls )(1
r
T , if
nitrogen is replaced by carbon, i.e. the clusters of d-atoms prevail in the
carbon austenitic steel.
The size of clusters can be estimated on the value of if some aver-
age value of the magnetic moment for each d-atom, e.g. 2.5B, is sug-
gested. Then, using the relation MH/kB, where M gBNa, and the
value of the external magnetic field H 0.3 T used in the experiment,
one obtain the number of d-atoms in the cluster, Na , which corre-
sponds to an average size of clusters in the carbon austenitic steels of
about 1 to 2 nm.
3.3. Small-angle scattering of neutrons. Experimental observations of
the nitrogen and carbon effects on the distribution of alloying ele-
ments in austenitic steels were performed using the small angle neu-
tron scattering, SANS [23]. This method allows one to obtain the in-
formation concerning the mass (chemical) inhomogeneities in solid so-
lutions on the scale of several tens nm. An incident neutron beam is
scattered on the inhomogeneities of mass distribution in the sample
and the weakening of its intensity is described as the total cross section
of scattering Stot ln(I0/I), where I0 and I are the intensities of the in-
80 V. G. GAVRILJUK
cident and passed neutron beams, respectively (see Fig. 9). As the nu-
clei of different elements differ in their neutron scatterings (the scat-
tering lengths for chromium bCr 0.35210
12
cm, manganese bMn
0.3610
12
cm, iron bFe 0.9610
12
cm, nickel bNi 1.0310
12
cm, bC
0.6610
12
cm, bN 0.9410
12
cm, see in detail [24]), the atomic clus-
ters have to enhance the neutron scattering.
As follows from Fig. 9, the alloying of austenitic CrNi and CrNiMn
steels with nitrogen decreases the total cross section of neutron scat-
tering, whereas carbon causes a striking increase of Stot.
It has to be emphasised that this effect is not due to scatterings of
neutrons on nitrogen or carbon atoms themselves because, if it would
be the case, the opposite effect is expected as the scattering cross sec-
tion of neutrons on the nitrogen atoms is about twice and the absorp-
tion cross section is more than 500 times as large as those on the carbon
ones.
Moreover, the fractions of nitrogen and carbon atoms in solid solu-
tions are too small in order to cause such a large scattering. Therefore,
it is an effect of nitrogen and carbon on the distribution of substitu-
Fig. 8. Temperature dependence of magnetic susceptibility of austenitic steels
Cr18Ni16Mn10N0.4 and Cr18Ni16Mn10C0.43 (% mass). The fitting of experimental
data to formula (7) is shown by solid lines. The insert shows contributions to
the magnetic susceptibility from free electrons (1), localized d-electrons (2)
and superparamagnetic clusters (3) as obtained from the fitting. Deviation
from the linear relation (Curie—Weiss law) is caused by the superparamagnet-
ic clusters of substitutional solute atoms.
CARBON, NITROGEN, AND HYDROGEN IN IRON-BASED SOLID SOLUTIONS 81
tional solutes, i.e. on the short-range atomic order in austenitic steels.
Based on the presented results, one can conclude that a clear tenden-
cy to clustering is observed in the carbon-containing steels, whereas
the short-range atomic ordering occurs, if carbon is replaced by nitro-
gen in the steels of the same basic composition.
A reason for this striking difference between carbon and nitrogen
effects is attributed to the nitrogen-enhanced metallic character of in-
teratomic bonds and prevailing covalent bonds due to carbon [19]. Such
a correlation between the electronic structure and atomic distribution
was also observed in the variety of substitutional iron-based alloys
[25], where it was established that the increase in the concentration of
Cr, Mn, Mo, V leads to prevailing localization of electrons at the atomic
sites and assists clustering in the solid solutions, whereas alloying by
Fig. 9. A scheme of small angle neutron scattering measurements and an ef-
fect of nitrogen and carbon on the total cross section of neutron scattering Stot
in the austenitic Cr18Ni16Mn10 and Cr18Ni16 steels. The wavelength of neutrons
is about 0.8 nm; Stot is measured as a weakening of the intensity I0 of the pri-
mary neutron beam.
82 V. G. GAVRILJUK
Ni, Si, Al increases the concentration of free electrons at the Fermi
level and promotes short-range atomic ordering.
Hydrogen in steels cannot affect the distribution of substitutional
solutes because the hydrogenation usually proceeds at ambient tem-
perature, and heating is accompanied by hydrogen degassing. Possi-
bly, the effect of hydrogen on the atomic distribution can be found in
the Ti alloys where solubility of hydrogen at high temperatures is re-
markable.
4. THERMODYNAMIC STABILITY
As well known, carbon negatively affects properties of CrNi and CrMn
austenitic steels because of easy precipitation and difficult dissolution
of chromium carbides. Substitution of carbon by nitrogen increases
thermodynamic stability of austenitic steels, which can be attributed
to the nitrogen-caused short-range atomic ordering. First of all, the
solubility of nitrogen in the iron austenite is increased as compared to
carbon allowing the significant solid solution strengthening. Beside
this, nitrogen in austenitic steels retards the precipitation of interme-
tallic phases during the sensitisation treatments (see, e.g., [26—29]).
On the other hand, nitrogen is more effective in stabilizing the aus-
tenitic structure in relation to martensitic transformation [30]. As
shown in Fig. 10, a, b, after quenching of the chromium steel
Cr15Mo1, the fraction of the retained austenite increases if carbon is
replaced by nitrogen. The combined alloying with carbon nitrogen
provides the highest stability of austenite (Fig. 8, c).
The atomic distribution in austenite is inherited by the as-quenched
martensite and, as a result, the precipitation during tempering is also
shifted to higher temperatures in the same order: carbon nitrogen
carbon nitrogen, which was established in TEM studies [31]. Dila-
tometric curves presented in Fig. 11 illustrate the delay of the 1
st
and
3rd
transformations during tempering, as well as the retardation of the
retained austenite decomposition (2
nd
one) caused by nitrogen- and ni-
trogen carbon. This is expected taking into account the high concen-
tration of free electrons at the Fermi level and the corresponding
short-range atomic order.
Hydrogen in austenitic steels is also found to affect thermodynamic
stability. It is established that hydrogen charging of austenitic steels
causes the transformation (e.g. [32—34]). Very often, this trans-
formation is attributed to the hydrogen-induced defects and stresses as
it occurs during the cold work of austenitic steels. At the same time,
the calculations of the electronic structure show [35, 36] that, with ex-
tremely high hydrogen contents, the cohesive energy in the h.c.p. iron
lattice increases in relation to that in the f.c.c. one in the absence of
any stresses, which suggests that hydrogen makes preferential the
CARBON, NITROGEN, AND HYDROGEN IN IRON-BASED SOLID SOLUTIONS 83
h.c.p. lattice of the -phase.
The common feature in the nitrogen and hydrogen effects on the
thermodynamical stability of austenite is that, at high N and H con-
tents, the h.c.p. lattice becomes more stable than the f.c.c. one and the
h.c.p. -phase exists within the wide concentration range of nitrogen
or hydrogen.
5. CRYSTAL LATTICE IMPERFECTIONS
5.1. Stacking fault energy. As shown first time in [37], the stacking
fault energy (SFE) in pure metals is inversely proportional to the states
density at the Fermi level. Deviations from this rule are observed in
the solid solutions if the constituting elements are located far from
each other in the periodic table. This correlation can be observed for
carbon and hydrogen in austenitic steels: the carbon-caused decrease in
the concentration of free electrons corresponds to increasing SFE [38],
whereas the hydrogen-increased concentration of free electrons corre-
lates with decreasing SFE [39].
Fig. 10. Mössbauer spectra of as-quenched steel Cr15Mo1 alloyed with (% mass)
0.6C (a), 0.62N (b) or 0.29C 0.35N (c). A single line belongs to the iron atoms
in the paramagnetic retained austenite. The sextet consisting of four compo-
nents Fe0 to Fe3 arises from the iron atoms in the ferromagnetic martensite
having a different chromium neighbourhood.
84 V. G. GAVRILJUK
Some uncertainty exists in relation to nitrogen in austenite. Fawley
et al. [40] observed a slightly decreasing effect on the SFE (from 40 to
38 mJ/m2) caused by the variation of nitrogen content within 0.005—
0.05% mass in steel Cr20Ni20. Swann [41] has shown that the addition of
0.12% mass of nitrogen to the steel Cr18Ni13 results in slight lowering
SFE. Dulieu and Nutting [42] obtained nearly the same result in TEM
studies of the steel Cr18Ni10. An increase in the nitrogen content from
0.02% to 0.25% mass produced decreasing the SFE. Stoltz and Vander
Sande [43] found a decrease of the SFE due to nitrogen in austenitic
CrNiMn steels. A feature of their observations was that an increase of
the nitrogen content above 0.24% mass did not affect the SFE.
A non-monotonous SFE concentration behaviour was found in stud-
ies [44], and a direct proportionality between the SFE and concentra-
tion of free electrons was observed. However, the measurements of the
temperature dependence of SFE in steels used in [44] have revealed
that the SFE decreases with increasing nitrogen content at low tem-
peratures [45], which is consistent with the inverse proportionality
Fig. 11. Length change during tempering of martensitic steels Cr15Mo1N0.62,
Cr15Mo1C0.6 and Cr15Mo1N0.35C0.29 quenched in water from 1100C. The temper-
ature ranges of the 1
st
(precipitation of low-temperature carbides or nitrides),
2nd
(decomposition of retained austenite) and 3
rd
(precipitation of stable car-
bide/nitride phases) transformations during tempering are marked.
CARBON, NITROGEN, AND HYDROGEN IN IRON-BASED SOLID SOLUTIONS 85
between SFE and the states density.
5.2. Thermodynamically equilibrium vacancies induced by intersti-
tials. An increase in the equilibrium concentration of vacancies in
presence of interstitial atoms was theoretically predicted by McLellan
for carbon in the iron austenite [46] and Smirnov for a general case
[47]. The first experimental observation of hydrogen-induced supera-
bundant vacancies in hydrides was made by Fukai et al. [48], although
these authors did not discuss their results in thermodynamical terms.
A thermodynamic analysis of hydrogen-induced vacancies in austenit-
ic steels was made in [49], where also the dislocation loops formed in
hydrogen-charged austenitic steels after hydrogen degassing have
been first time observed.
It is remarkable in this relation that similar defects caused by nitro-
gen in austenitic steels were observed by Kikuchi et al. [50] who has
shown that dislocation loops in a CrNi austenitic steel containing
0.5% mass N are formed after ageing at 750C, i.e. after the removal of
nitrogen from the solid solution.
Moreover, according to [51], the diffusion of chromium atoms and,
correspondingly, precipitation of Cr2N nitrides is accelerated by nitro-
gen. Both effects observed by Kikuchi et al. were not interpreted by
these authors as the effect of vacancies. However, they clearly indicate
to the nitrogen-induced superabundant vacancies, which enhance mi-
gration of Cr atoms and, becoming non-equilibrium, form vacancy
clusters.
Superabundant vacancies were never observed in carbon-containing
CrNi austenitic steels, which can be obviously explained by the rather
low solubility of carbon in austenite.
5.3. Interaction between dislocations and interstitials. In the austenit-
ic steels, the enthalpy of binding between dislocations and C, N, H at-
oms is about 0.5 to 0.6 eV [52, 53], 0.7 to 0.9 eV [53, 54] and about
0.1 eV [55, 56], respectively. In ferritic (martensitic) steels, the data
correspond to 0.8 eV [57], 0.8 eV [57] and 0.18 eV [58], respectively.
These results obtained using mechanical spectroscopy suggest that, in
comparison with carbon, the pinning of dislocations by nitrogen atoms
in the -iron is more effective, whereas there is no difference between
pinning of dislocations by carbon and nitrogen in the -iron. The hy-
drogen atoms pin dislocations weaker, and the H-pinning of disloca-
tions in the -iron is a bit stronger than in the -iron.
However, these studies did not take into account a possible change in
the character of the interstitials-dislocation interaction if interstitial
atoms follow dislocations in the course of plastic deformation. As the
metallic character of interatomic bonds decreases the shear modulus
(see [7, 9]), the following consequences are expected for dislocation
properties: (i) the decrease in the start stress of the dislocation sources
(e.g., 2b/L for the Frank—Read source); (ii) a decrease in the specif-
86 V. G. GAVRILJUK
ic energy of dislocations, i.e. their line tension, (b2/4)/log(/5b),
where is the radius of the dislocation curvature; (iii) a decrease in
the distance between the dislocations in the pile-ups, d
(b)/[16(1 )n], where is the applied stress.
The second prediction suggests the enhanced mobility of disloca-
tions, while the third one says about a larger number of dislocations n
in the pile-ups and, consequently, a higher stress at the leading dislo-
cations, l n, where is an applied stress. Consequently, a mi-
crocrack can be opened at lower applied stresses, if the stress at the
leading dislocations cannot be relaxed.
These two aspects of dislocation-interstitials interaction should be
taken into account depending on the relation between the strain rate
and mobility of interstitial atoms. If the interstitial atoms remain es-
sentially immobile during plastic deformation, they pin dislocations
and the effectiveness of pinning is controlled by the corresponding
binding enthalpy.
The situation is strikingly changed if the interstitial atoms can fol-
low dislocations in the course of plastic deformation. In this case, mo-
bility of dislocations is controlled by a change in the interatomic bond-
ing within the clouds of interstitial atoms around the dislocations. The
following measurements using mechanical spectroscopy illustrate this
phenomenon (see Figs. 12 and 13).
In absence of relaxation processes, the main contribution to the in-
ternal friction background is given by the vibrations of dislocation
segments and the value of damping is proportional to the area swept by
Fig. 12. Effect of interstitial elements on dislocation mobility in austenitic
steels measured using strain-dependent internal friction: hydrogen in steel
Cr25Ni20, a is initial state, b–after hydrogen charging, c–after degassing (a);
carbon and nitrogen in steel Cr18Ni16Mn10; measurements are carried out at
elevated temperatures in order to allow the interstitial atoms follow the dislo-
cations (b).
CARBON, NITROGEN, AND HYDROGEN IN IRON-BASED SOLID SOLUTIONS 87
dislocations for one cycle of vibrations [e.g., 59, 60]. At a constant vi-
bration frequency, it is a measure of dislocation velocity.
The effect of C, N and H on the strain dependence of internal fric-
tion is presented in Fig. 12. It is seen that hydrogen charging decreases
the stress needed for plastic deformation and increases velocity of dis-
locations (Fig. 12, a). Hydrogen degassing restores the initial IF curve
except for a small excess caused by new dislocations induced due to hy-
drogen charging.
A higher temperature is needed to satisfy conditions of dislocation
movement accompanied by the migration of carbon or nitrogen atoms
(Fig. 12, b). It is clear from the obtained data that, like hydrogen and
in contrast to carbon, nitrogen in the austenitic steel increases velocity
of dislocations.
The Snoek—Köster (S—K) relaxation provides another possibility to
study mobility of dislocations in N, C, or H-containing ferritic or mar-
tensitic steels. Exactly like, it is the case for IF background and accord-
ing to the both compete models of the S—K relaxation [61, 62], the S—K
relaxation amplitude is proportional to the area swept by dislocations.
Figure 13 represents the S—K relaxation caused by carbon and ni-
trogen in the martensite of steels Cr15Mo1C0.6 and Cr15Mo1N0.62. In spite
of a significantly smaller fraction of martensite in the nitrogen steel
after quenching from 1100C (48.9% against 72.6% in the carbon
Fig. 13. Effect of carbon and nitrogen on dislocation mobility in martensite of
steel Cr15Mo1 measured using the Snoek—Köster relaxation. The fractions of
martensite are equal to 72.6% and 48.9% in the carbon and nitrogen steels,
respectively.
88 V. G. GAVRILJUK
steel), the strength of S—K relaxation caused by the nitrogen-
dislocation interaction is much higher.
As follows from the studies by Takita and Sakamoto [63] (see Fig.
14), the strength of hydrogen S—K relaxation in the -iron is higher by
one order of the magnitude than that of the -relaxation in the hydro-
gen-free iron, which is caused by dislocations of the same type as the
S—K relaxation in presence of hydrogen.
The presented experimental data clearly show that nitrogen and hy-
drogen in austenitic and martensitic steels enhance dislocation mobili-
ty, whereas carbon decreases it. This unusual behaviour of dislocations
moving with the carbon, nitrogen or hydrogen atmospheres can be un-
derstood based on the above-mentioned data on the electronic structure
of corresponding solid solutions. The nitrogen and hydrogen atoms in
the dislocation atmospheres enhance the metallic character of the in-
teratomic bonds, which are weaker than covalent bonds promoted by
carbon atmospheres around the dislocations.
6. MECHANICAL PROPERTIES
All of the interstitial elements increase the yield strength of steels ex-
cept for the hydrogen- and nitrogen-caused softening in the appropri-
ate range of temperatures and strain rates. This unusual phenomenon
caused by similar effects of hydrogen and nitrogen on the electronic
structure will be discussed in the section devoted to fracture. Present-
ed below is the analysis of carbon and nitrogen effects on the low-
temperature and grain-boundary strengthening of austenitic steels.
6.1. Strengthening at low temperatures. An abnormal increase of the
Fig. 14. The -relaxation in the absence of hydrogen and hydrogen-caused
Snoek—Köster relaxation in the -iron, according to Takita and Sakamoto (ac-
cording to [63]).
CARBON, NITROGEN, AND HYDROGEN IN IRON-BASED SOLID SOLUTIONS 89
yield strength by nitrogen in the austenitic steels at low temperatures
was first time observed by Nilsson and Thorwaldsson [64]. It was as-
cribed to the nitrogen-caused splitting of the dislocation core in the
several slip planes like it occurs in the b.c.c. lattice and a theory of
such a b.c.c. behaviour of nitrogen austenitic steels was proposed in
[65, 66]. However, more precise measurements were carried out by
Nylas and Obst [67] who revealed a three-stage character of the (T)
curve in consistency with Seeger’s theory for f.c.c. crystals with low
stacking-fault energy smaller than 100 mJ/m2. According to this theo-
ry, because of the absence of strong Peierls barriers for dislocation slip
in the f.c.c. crystal lattice, the flow stress is determined by the inter-
section of dislocations and generation of point defects.
Seeger [68] analysed the following three processes which are respon-
sible for the temperature-dependent flow stress in f.c.c. crystals below
the temperature of self-diffusion: (1) interaction between gliding edge
dislocations and the forest of dislocations threading the slip plane;
(2) interaction between gliding screw dislocations and this forest;
(3) thermally activated generation of vacancies by jogs formed during
the intersection of screw dislocations. At a given temperature, the
plastic flow will be controlled by one of them requiring the lowest
stress for the movement of dislocations (Fig. 15, a).
In accordance with this theory, the temperatures of the start of
thermal activation
)1(
0
T and
)2(
0
T for processes (1) and (2) are extremely
high. As the process (2) is a prerequisite for the process (3), the one
that requires the higher shear stress will control the slip of screw dis-
locations. Superimposing the curves representing the above three pro-
cesses results in three sections of the straight lines (see Fig. 15, b).
Just such a mechanical behaviour of austenitic nitrogen steels was ob-
served by Nyilas et al. [67]. Therefore, in accordance with Seeger’s the-
ory, depending on the temperature, the operative mechanism for the
flow stress is represented by part 1 corresponding to the cutting of
edge dislocations through dislocation forest, part 2 from the cutting of
screw dislocations through dislocation forest or part 3 controlled by
the generation of vacancies by jogs at screw dislocations.
The effect of carbon and nitrogen on the temperature dependence of
the yield strength in austenitic steels is presented in Fig. 16. It follows
from the comparison with Fig. 15, b that the third and first stages of
Seeger’s scheme, namely the generation of vacancies and the cutting of
edge dislocations through the dislocation forest, control the plastic
flow at temperatures below 100 K.
The increase of the yield strength in the temperature range of the
third stage occurs if nitrogen or carbon is added to the austenitic
CrNiMn steel, and the effect of nitrogen is much higher. Thus, the in-
tersection of screw dislocations and, namely, the generation of vacan-
cies by the jogs discern the low-temperature mechanical behaviour of
90 V. G. GAVRILJUK
three studied steels. The generation of vacancies by jogs requires, as a
preceding stage, the formation of a constriction in the extended dislo-
cations. The more split dislocations, the higher stress has to be applied
to form the constriction. Therefore, the nature of the different me-
chanical behaviour of the studied steels in stage 3 controlled by differ-
ent splitting of dislocations at low temperatures. In other words, car-
bon and nitrogen change the temperature dependence of the stacking
fault energy to a different degree.
This suggestion was confirmed by the low-temperature TEM studies
[69]. It was obtained that, in contrast to carbon, nitrogen strikingly
decreases the SFE at low temperatures, which results in a larger ap-
plied stress for the formation of constriction in the intersecting screw
dislocations.
A reason for the nitrogen-caused change in the temperature depend-
ence of the SFE lies in the increased states density at the Fermi level
Fig. 15. Flow stress versus temperature for f.c.c. crystals with a low value of
stacking fault energy, according to Seeger’s theory [68]: if only one thermally
activated process is operating (a); for three controlling mechanisms of plastic
deformation: cutting of edge (1) and screw (2) dislocations through the dislo-
cation forest and generation of vacancies by jogs in screw dislocations (3), as
predicted to occur in the crystals with low stacking-fault energy (b).
CARBON, NITROGEN, AND HYDROGEN IN IRON-BASED SOLID SOLUTIONS 91
(see Fig. 1). With decreasing temperature, the temperature-caused
blurring of the Fermi level is decreased and, consequently, the states’
density increases, which is accompanied by decreasing SFE in accord-
ance with the inverse proportionality between the states density at the
Fermi level and the SFE [37]. The low-temperature increase of the yield
strength in the carbon austenitic steels is rather small because of the
weak temperature dependence of the SFE resulted from prevailing co-
valent bonds and the low states’ density at the Fermi level.
One can suppose that, like nitrogen, hydrogen is expected to strik-
ingly increase the yield strength at low temperatures because of strong
increase in the states density at the Fermi level in hydrogen-charged
austenitic steels.
6.2. Strengthening by grain boundaries. Nitrogen is found to increase
more effectively the grain-boundary strengthening in austenitic steels
as compared to carbon (see Fig. 17). Three main theories, namely those
of planar slip [72], cold work [73] and grain-boundary dislocation
sources [74], were proposed for the interpretation of the Hall—Petch
equation T 0 kd1/2
where T is the yield strength, 0 is the friction
stress of the lattice, d is the grain size and k is a coefficient character-
ising the transfer of the slip through the grain boundary. As follows
from Fig. 17, nitrogen increases the coefficient k.
Nitrogen austenitic steels are a unique object for testing the above-
mentioned theories of grain-boundary strengthening (see about details
[53]). Main experimental data are the following.
Unlike carbon, nitrogen assists the planarity of slip in austenitic
steels, which was first time shown in [75]. Therefore, if planar slip
Fig. 16. Effect of nitrogen and carbon on the temperature dependence of the
yield strength of austenitic steel Cr18Ni16Mn10. Stages 1 to 3 controlling dif-
ferent mechanisms of plastic flow are shown in consistency with Seeger’s the-
ory [68].
92 V. G. GAVRILJUK
controls the value of k, it has to be decreased with increasing nitrogen
content because a critical stress at the head of the pile-ups needed for
the slip transfer to adjacent grains could be achieved at smaller applied
stresses. Such an idea is in contradiction with numerous experimental
data.
The cold work theory [73] explains the Hall—Petch equation in terms
of increased dislocation density at the grain boundaries with decreas-
ing the grain size, which makes more difficult the slip transfer
through the grain boundary (see also experimental data in [76]). Ni-
trogen increases the cold-work hardening in austenitic steels [77, 78],
which is consistent with the increase of k expected according to the
cold work theory.
Nevertheless, no contribution of nitrogen to the cold-work harden-
ing occurs in steel with basic composition of Cr18Ni16Mn10 [53], and, at
the same time, as shown in Fig. 17, the coefficient k in this steel in-
creases with increasing nitrogen content. Therefore, the increase in
the density of dislocations near the grain boundary with decreasing
grain size is not critical for the grain-boundary strengthening, at least
for the yield strength.
The theory of the grain-boundary dislocation sources attributes the
Fig. 17. Effect of nitrogen and carbon on grain boundary strengthening of
steel Cr18Ni16Mn10. Data of Norström [70] for steel Cr18Ni14Mo3 alloyed with
nitrogen and of Köstler and Sidan [71] for steel Mn18Cr4 alloyed with carbon
are shown for the comparison.
CARBON, NITROGEN, AND HYDROGEN IN IRON-BASED SOLID SOLUTIONS 93
Hall—Petch relationship to the long-range internal stresses caused by
the dislocations located within the grain boundaries. The only experi-
mental evidence supporting this theory is that the grain-boundary seg-
regation of solute atoms contributes to the grain-boundary strengthen-
ing. However, the experimental data about a weak segregation of ni-
trogen [79] and remarkable segregation of carbon [80] at the grain
boundaries in austenitic steels are not consistent with the supposed de-
cisive role of the grain-boundary dislocation sources.
Possibly, the idea proposed by Cottrell [81] as far back as in the fif-
ties that the coefficient k is controlled by the stress needed for unpin-
ning the dislocation sources in the adjacent grain still remains the
most fruitful for the interpretation of the Hall—Petch equation. A
larger binding enthalpy of dislocations to nitrogen than to carbon in
austenitic steels is consistent with this Cottrell’s idea except for the
only remark that the slip planarity is not necessary condition for the
grain-boundary strengthening.
Hydrogen is notoriously known by its strong affinity with the grain
boundaries. In view of hydrogen-caused embrittlement, its role in
grain-boundary strengthening is not worth of detailed studies.
6.3. Fracture. Carbon decreases the toughness of austenitic steels;
however, it does not cause the ductile-to-brittle transition. The excel-
lent combination of strength and toughness in nitrogen austenitic
steels was firstly shown in [82]. At the same time, the ductile-to-brittle
transition occurs in the impact tests at the nitrogen contents higher
than 0.8% mass [83, 84]. Numerous observations of the fracture sur-
face reveal a surprising similarity between the low-temperature im-
pact embrittlement of nitrogen austenitic steels and hydrogen brittle-
ness (see Fig. 18 and [85—87]). In contrast, no ductile-to-brittle transi-
tion occurs in carbon austenitic steels.
As shown by Tomota et al. [87], the brittleness of nitrogen austenitic
steels under low-temperature impact loading is resulted from the ni-
trogen-enhanced highly localized plasticity and shortage of time for
the relaxation of stresses. Similarly, the hydrogen-enhanced localized
plasticity is one of the most reliable hypotheses of hydrogen embrit-
tlement (e.g., [3]). Based on the similar effect of nitrogen and hydrogen
on the electronic structure of austenitic steels, one can state that the
N- and H-caused macrobrittleness is of the same physical nature and
originates from the increase in the concentration of free electrons and
the excessive electron density within the nitrogen and hydrogen at-
mospheres around the dislocations [56].
Unlike hydrogen, nitrogen atoms in austenite are immobile at room
temperature, and this is why the common tension tests do not reveal
the nitrogen-caused embrittlement. However, at high strain rates, the
interstitial atoms are transported by dislocations even at cryogenic
temperatures like those that it was shown in [88] using autoradiog-
94 V. G. GAVRILJUK
raphy.
The nature of slip localization due to nitrogen and hydrogen is of
different type. The slip in the nitrogen austenite is localized due to
short-range atomic order [19]. Hydrogen cannot change the distribu-
tion of substitutional solutes, at least at the room temperature hydro-
genation. Therefore, some other reason has to exist for the hydrogen-
caused localisation of slip within the separate bands. A mechanism re-
lated with the hydrogen-induced vacancies has been proposed to be re-
sponsible for the hydrogen-caused slip localisation [6, 56]. It is similar
to the voids sheeting analysed by Ashby et al. [89]. Like the mi-
crovoids, the hydrogen-induced vacancies within the hydrogen atmos-
pheres around dislocations decrease the load-bearing area in the slip
plane, which results in slip localization.
7. CONCLUSIONS
1. Change in the atomic interactions determines the structure and
Fig. 18. Hydrogen- and nitrogen-caused fracture of austenitic steels: hydro-
gen-caused slip bands resulted in cracking at a twin boundary (after [85]) (a),
mirror surfaces of steel Cr18Mn18N0.6 after pseudo-brittle fracture during the
low-temperature impact test (after [87]) (b).
CARBON, NITROGEN, AND HYDROGEN IN IRON-BASED SOLID SOLUTIONS 95
properties of the iron-based solid solutions containing carbon, nitro-
gen or hydrogen. Carbon decreases the concentration of free electrons,
whereas nitrogen and hydrogen increase it.
2. Carbon increases and hydrogen decreases the stacking fault energy
of austenitic steels. With increasing nitrogen content, stacking fault
energy is being changed non-monotonously.
3. Carbon promotes clustering of solute atoms in austenitic solid solu-
tions, whereas nitrogen assists the short-range atomic ordering.
4. All these elements increase the thermodynamically equilibrium con-
centration of vacancies in the -iron solid solution and assist migration
of substitutional solutes.
5. Dislocations in austenitic steels are more effectively pinned by ni-
trogen than carbon. Pinning of dislocations by hydrogen is insignifi-
cant. At the same time, if the clouds of interstitial atoms can follow
dislocations in the course of deformation, the nitrogen and hydrogen
atoms enhance plasticity due to excessive free electron concentration
in the dislocation atmospheres, i.e. prevailing metallic character of in-
teratomic bonds and, consequently, decreased line tension of disloca-
tions.
6. Abnormal strengthening of nitrogen austenitic steels at low temper-
atures is explained based on the nitrogen-increased concentration of
free electrons resulting in the striking temperature dependence of
stacking fault energy. A similar effect is expected for hydrogen-
charged austenitic steels and not for carbon ones.
7. The pseudo-brittle fracture of nitrogen- and hydrogen-containing
austenitic steels occurs at the conditions of dislocation slip accompa-
nied by the migration of nitrogen or hydrogen atoms. This phenome-
non is attributed to the local H- and N-increased concentration of free
electrons and superabundant vacancies leading to the enhanced micro-
plasticity and localization of plastic deformation.
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| id | nasplib_isofts_kiev_ua-123456789-112467 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1024-1809 |
| language | English |
| last_indexed | 2025-12-07T17:37:52Z |
| publishDate | 2016 |
| publisher | Інститут металофізики ім. Г.В. Курдюмова НАН України |
| record_format | dspace |
| spelling | Gavriljuk, V.G. 2017-01-22T13:31:22Z 2017-01-22T13:31:22Z 2016 Carbon, Nitrogen and Hydrogen in Iron-Based Solid Solutions: Similarities and Differences in their Effect on Structure and Properties / V. G. Gavriljuk // Металлофизика и новейшие технологии. — 2016. — Т. 38, № 1. — С. 67-98. — Бібліогр.: 89 назв. — англ. 1024-1809 PACS: 61.66.Dk, 61.72.J-, 61.72.Lk, 61.72.Nn, 62.20.mj, 62.20.mm, 64.75.Nx, 71.55.Ak DOI: 10.15407/MFiNT.38.0067 https://nasplib.isofts.kiev.ua/handle/123456789/112467 Interstitial N, C and H atoms in iron-based solid solutions are compared in terms of their effect on the structure and properties. Electronic structure and stacking fault energy, atomic distribution, interaction of interstitial atoms with dislocations and vacancies, mobility of dislocations, mechanisms of deformation and fracture are compared based on theoretical calculations and experimental observations. As shown, nitrogen and hydrogen increase the electron density of states at the Fermi level of f.c.c. iron, whereas carbon decreases it. Correspondingly, the concentration of free electrons increases within the nitrogen and hydrogen iron-based solid solutions and decreases in the carbon ones. A correlation is revealed between the character of interatomic bonds and the short-range atomic order in the studied solid solutions: nitrogen assists short-range atomic ordering in the spatial distribution of alloying elements, whereas carbon promotes their clustering. As consequence, nitrogen increases thermodynamical stability of austenitic steels, whereas carbon makes steel sensitive to precipitation of carbides from the solid solution that deteriorates corrosive characteristics. The most impressive is a correlation between the change in the electronic structure and properties of dislocations. In contrast to prevailing covalent bonds in carbon steels, the enhanced metallic character of interatomic bonds, as caused by nitrogen, increases mobility of dislocations that results in excellent plasticity and fracture toughness. However, the same effect caused by hydrogen is a cause of the hydrogen embrittlement through the hydrogen-enhanced localized plasticity. A unique similarity with hydrogen embrittlement becomes apparent in the course of impact loading of austenitic nitrogen steels, where, due to the absence of sufficient time for relaxation of stresses, the nitrogen-enhanced localized plasticity occurs resulting in a pseudo-brittle fracture. The different is only the mechanism for localization of plastic deformation: the shortrange atomic ordering caused by nitrogen and the increased concentration of superabundant vacancies due to hydrogen dissolution. Проаналізовано вплив елементів втілення N, C і H в твердих розчинах на основі заліза на їхню структуру і властивості. На основі теоретичних розрахунків і експериментальних результатів порівнюються електронна структура і енергія дефектів пакування, розподіл атомів у твердих розчинах, взаємодія атомів втілення з дислокаціями та вакансіями, рухомість дислокацій, механізми пластичної деформації та руйнування. Встановлено, що Нітроґен і Гідроґен збільшують густину електронних станів на рівні Фермі ГЦК-заліза, в той час як Карбон зменшує її. Відповідно, концентрація вільних електронів підвищується в твердих розчинах Нітроґену і Гідроґену на основі γ-заліза і зменшується при розчиненні Карбону. Виявлено кореляцію між характером міжатомового зв’язку і близьким атомовим порядком в аустенітних сталях: Нітроґен сприяє близькому атомовому упорядкуванню в розподілі леґувальних елементів, в той час як розчинення Карбону супроводжується їх кластеризацією. Як наслідок, Нітроґен підвищує термодинамічну стабільність аустенітних сталей, а Карбон робить сталь чутливою до виділення карбідів із твердого розчину, що погіршує корозійні властивості. Найбільш вражаючою є кореляція між електронною структурою і властивостями дислокацій. На відміну від переважаючих ковалентних зв’язків у вуглецевих сталях, посилений Нітроґеном їхній металічний характер підвищує рухливість дислокацій, наслідком чого є висока пластичність і в’язкість руйнування. Але аналогічний вплив Гідроґену є причиною водневого окрихчування через посилену Гідроґеном локалізовану пластичність. Унікальна схожість з водневим окрихчуванням має місце, якщо аустенітна азотиста сталь піддається ударному навантаженню. Внаслідок недостатнього часу для релаксації напружень, посилена Нітроґеном локалізована пластичність призводить до псевдокрихкого руйнування. Відмінним є лише механізм локалізації пластичної деформації: близьке атомове упорядкування, спричинене Гідроґеном, і збільшення концентрації надлишкових вакансій у випадку розчинення Гідроґену. Выполнен анализ влияния элементов внедрения N, C и H в твёрдых растворах на основе железа на их структуру и свойства. На основе теоретических расчётов и экспериментальных данных сравниваются электронная структура и энергия дефектов упаковки, распределение атомов в твёрдых растворах, взаимодействие атомов внедрения с дислокациями и вакансиями, подвижность дислокаций, механизмы пластической деформации и разрушения. Установлено, что азот и водород повышают плотность электронных состояний на уровне Ферми ГЦК-железа, в то время как углерод уменьшает её. Соответственно, концентрация свободных электронов увеличивается в твёрдых растворах азота и водорода на основе γ-железа и уменьшается при растворении углерода. Найдена корреляция между характером межатомных связей и ближним атомным порядком в аустенитных сталях: азот способствует ближнему атомному упорядочению в распределении легирующих элементов, в то время как растворение углерода сопровождается их кластеризацией. Как следствие, азот увеличивает термодинамическую стабильность аустенитных сталей, а углерод делает сталь чувствительной к выделению карбидов из твёрдого раствора, что ухудшает коррозионные свойства. Наиболее впечатляющей является корреляция между электронной структурой и свойствами дислокаций. В отличие от превалирующих ковалентных связей в углеродистых сталях, усиленный азотом их металлический характер увеличивает подвижность дислокаций, следствием чего является высокая пластичность и вязкость разрушения. Однако аналогичное влияние водорода является причиной водородного охрупчивания стали из-за усиленной водородом локализованной пластичности. Уникальное сходство с водородной хрупкостью имеет место, если аустенитная азотистая сталь подвергается ударному нагружению. Вследствие недостаточного времени для релаксации напряжений, усиленная азотом локализованная пластичность приводит к псевдохрупкому разрушению. Различным является лишь механизм локализации пластической деформации: ближнее атомное упорядочение, обусловленное азотом, и повышение концентрации избыточных вакансий в случае растворения водорода. en Інститут металофізики ім. Г.В. Курдюмова НАН України Металлофизика и новейшие технологии Дефекты кристаллической решётки Carbon, Nitrogen and Hydrogen in Iron-Based Solid Solutions: Similarities and Differences in their Effect on Structure and Properties Карбон, Нітроґен і Гідроґен в твердих розчинах на основі заліза: схожість та відмінність їх впливу на структуру і властивості Углерод, азот и водород в твердых растворах на основе железа: сходства и различия их влияния на структуру и свойства Article published earlier |
| spellingShingle | Carbon, Nitrogen and Hydrogen in Iron-Based Solid Solutions: Similarities and Differences in their Effect on Structure and Properties Gavriljuk, V.G. Дефекты кристаллической решётки |
| title | Carbon, Nitrogen and Hydrogen in Iron-Based Solid Solutions: Similarities and Differences in their Effect on Structure and Properties |
| title_alt | Карбон, Нітроґен і Гідроґен в твердих розчинах на основі заліза: схожість та відмінність їх впливу на структуру і властивості Углерод, азот и водород в твердых растворах на основе железа: сходства и различия их влияния на структуру и свойства |
| title_full | Carbon, Nitrogen and Hydrogen in Iron-Based Solid Solutions: Similarities and Differences in their Effect on Structure and Properties |
| title_fullStr | Carbon, Nitrogen and Hydrogen in Iron-Based Solid Solutions: Similarities and Differences in their Effect on Structure and Properties |
| title_full_unstemmed | Carbon, Nitrogen and Hydrogen in Iron-Based Solid Solutions: Similarities and Differences in their Effect on Structure and Properties |
| title_short | Carbon, Nitrogen and Hydrogen in Iron-Based Solid Solutions: Similarities and Differences in their Effect on Structure and Properties |
| title_sort | carbon, nitrogen and hydrogen in iron-based solid solutions: similarities and differences in their effect on structure and properties |
| topic | Дефекты кристаллической решётки |
| topic_facet | Дефекты кристаллической решётки |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/112467 |
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