The Sources and Affecting Factors of Creep Threshold Stress of Magnesium Based Composite
Tensile creep experiments were carried on AZ91D magnesium alloy and aluminum silicate short fiber reinforced AZ91D magnesium matrix composite. The creep threshold stress of AZ91D composite depends on the Al atoms solute atmosphere, the bearing and transferring force of short fiber. Threshold stress...
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Інститут проблем міцності ім. Г.С. Писаренко НАН України
2016
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| Cite this: | The Sources and Affecting Factors of Creep Threshold Stress of Magnesium Based Composite / J. Tian // Проблемы прочности. — 2016. — № 1. — С. 197-208. — Бібліогр.: 13 назв. — англ. |
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Tian, J. 2020-12-03T21:17:18Z 2020-12-03T21:17:18Z 2016 The Sources and Affecting Factors of Creep Threshold Stress of Magnesium Based Composite / J. Tian // Проблемы прочности. — 2016. — № 1. — С. 197-208. — Бібліогр.: 13 назв. — англ. 0556-171X https://nasplib.isofts.kiev.ua/handle/123456789/173433 539.4 Tensile creep experiments were carried on AZ91D magnesium alloy and aluminum silicate short fiber reinforced AZ91D magnesium matrix composite. The creep threshold stress of AZ91D composite depends on the Al atoms solute atmosphere, the bearing and transferring force of short fiber. Threshold stress decreases with increasing temperature, increases with the increase of short fiber volume fraction and the load transfer coefficient , but the extent of increase decreases with the increase of the amount of β-Mg₁₇Al₁₂ precipitation phase. The load transfer coefficient α characterizes the bearing and transferring capabilities of short fibers. As a result, the mathematical model of threshold stress in the magnesium matrix composite is obtained. The work was supported by the National Science Foundation of Guangdong Province (Grant No. 10151170003000002). en Інститут проблем міцності ім. Г.С. Писаренко НАН України Проблемы прочности Научно-технический раздел The Sources and Affecting Factors of Creep Threshold Stress of Magnesium Based Composite Источники и факторы влияния пороговых напряжений в композите на основе магния при ползучести Article published earlier |
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The Sources and Affecting Factors of Creep Threshold Stress of Magnesium Based Composite |
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The Sources and Affecting Factors of Creep Threshold Stress of Magnesium Based Composite Tian, J. Научно-технический раздел |
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The Sources and Affecting Factors of Creep Threshold Stress of Magnesium Based Composite |
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The Sources and Affecting Factors of Creep Threshold Stress of Magnesium Based Composite |
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The Sources and Affecting Factors of Creep Threshold Stress of Magnesium Based Composite |
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sources and affecting factors of creep threshold stress of magnesium based composite |
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Tian, J. |
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Інститут проблем міцності ім. Г.С. Писаренко НАН України |
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Источники и факторы влияния пороговых напряжений в композите на основе магния при ползучести |
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Tensile creep experiments were carried on AZ91D magnesium alloy and aluminum silicate short fiber reinforced AZ91D magnesium matrix composite. The creep threshold stress of AZ91D composite depends on the Al atoms solute atmosphere, the bearing and transferring force of short fiber. Threshold stress decreases with increasing temperature, increases with the increase of short fiber volume fraction and the load transfer coefficient , but the extent of increase decreases with the increase of the amount of β-Mg₁₇Al₁₂ precipitation phase. The load transfer coefficient α characterizes the bearing and transferring capabilities of short fibers. As a result, the mathematical model of threshold stress in the magnesium matrix composite is obtained.
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The Sources and Affecting Factors of Creep Threshold Stress of Magnesium Based Composite / J. Tian // Проблемы прочности. — 2016. — № 1. — С. 197-208. — Бібліогр.: 13 назв. — англ. |
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2025-11-26T20:33:26Z |
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2025-11-26T20:33:26Z |
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1850773778181652480 |
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UDC 539.4
The Sources and Affecting Factors of Creep Threshold Stress of Magnesium-
Based Composite
J. Tian
School of Mechanical Engineering, Dongguan University of Technology, Dongguan, China
841608534@qq.com
Tensile creep experiments were carried on AZ91D magnesium alloy and aluminum silicate short fiber
reinforced AZ91D magnesium matrix composite. The creep threshold stress of AZ91D composite
depends on the Al atoms solute atmosphere, the bearing and transferring force of short fiber.
Threshold stress decreases with increasing temperature, increases with the increase of short fiber
volume fraction and the load transfer coefficient �, but the extent of increase decreases with the
increase of the amount of �-Mg17Al12 precipitation phase. The load transfer coefficient �
characterizes the bearing and transferring capabilities of short fibers. As a result, the mathematical
model of threshold stress in the magnesium matrix composite is obtained.
Keywords: magnesium matrix composite, creep, threshold stress, AZ91D, load transfer
coefficient.
Introduction. Currently, a lot of researches have been conducted on the high
temperature creep properties and behaviors of metal matrix composites [1, 2]. Many
researchers underlined that there is a threshold stress and the creep is not generated when
the applied stress is below that limit value [3, 4].
So far, there is not an available creep theory which can explain all the characters of
creep. The core issues of researches are the mechanisms of dislocation over the second
phase and the sources of threshold stress. The sources and physical meaning of creep
threshold stress and the way of quantitative expression have not been determined. Some
researchers believe that the creep threshold stress may come from the interactions between
mobile dislocations and precipitated phase in matrix [5, 6]. A one-dimensional micro-
mechanical model, which is based on the law for interfacial sliding, has been developed for
thermomechanical deformation of continuous fiber reinforced metal matrix composites [7].
Based on the recent studies on (coherent precipitates Al3Sc + non-coherent dispersion
Al2O3 reinforcing phase) / aluminum alloy composite strengthening, the synergetic effect of
the two ways is discovered. Furthermore, the total threshold stress is the sum of stress and
back stress. The former is generated when mobile dislocations field is imposed by coherent
precipitates to the mobile dislocation [8]. Dispersion strengthened Al-Mg alloys are same
with pure Al-Mg alloy in that creep deformation is related to dislocation climb and
dislocation – solute drag. The threshold stresses in the two materials are related to the
function of mobile dislocation density, dislocation velocity and solute concentration in the
dislocation core [9]. However, the creep studies focus on the dispersion strengthening,
particle reinforcement or long fiber reinforced metal matrix composites. There are few
studies about the high temperature creep of short fiber reinforced metal matrix composites.
For example, the assessment of back stress and load transfer approaches for rationalizing
creep of short fiber reinforced aluminum alloys has been demonstrated [5]. Chmelik et al.
[2] studied the creep properties and behaviors of an unreinforced AZ91 magnesium alloy
and a similar alloy reinforced with short alumina fibers. And his results showed the
introduction of short alumina fibers into an AZ91 magnesium alloy improved the creep
resistance because of the introduction of a threshold stress that serves to reduce the
effective stress acting on the material [10]. Sklenièka et al. [1] conducted creep tests on an
© J. TIAN, 2016
ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 197
AZ91–20 vol.% Al2O3 short fiber composite and on an unreinforced AZ91 matrix alloy,
and the results showed the creep resistance of the reinforced material considerably
improved compared with the matrix alloy, and the creep strengthening arose mainly from
the effective load transfer between plastic flow in the matrix and the fibers. However, the
sources of threshold stress of metal matrix composites reinforced with short fiber have not
been explained. Nowadays, the research and development of magnesium matrix composites
play a more and more important role in automotive and aerospace industries. Therefore, it
is necessary to conduct a systematic, experimental and theoretical analysis on the creep
behavior and mechanism of short fiber reinforced magnesium matrix composite. Here, the
constant stress tensile creep tests are conducted on two kinds of metal materials in different
conditions of temperatures and stresses. One is aluminum silicate short fiber reinforced
AZ91D composite with different fiber volume fraction, the other is AZ91D magnesium
alloy. The nature of the threshold stress and the affecting factors of aluminum silicate short
fiber reinforced AZ91D composite are investigated intensively. It strives to obtain the
reasonable explanation and insights of the source of creep threshold stress in aluminum
silicate short fiber reinforced AZ91D composite, which is currently still a controversial
issue. The goal of this paper is to guide the practical application of the material and
enriching the creep theory of short fiber reinforced magnesium matrix composites through
the findings.
Experimental Procedure. The metal materials were prepared by squeeze casting
method with aluminum silicate short fiber and AZ91D magnesium matrix alloy (Mg-
9%Al-1%Zn-0.3%Mn, the percentage is mass fraction). The AZ91D magnesium matrix
composite is proved to possess good bonding interface and mechanical properties, and be
able to reinforce composite effectively. The preform of aluminum silicate short fiber is
composed of randomly distributed aluminum silicate short fiber (Al2O3-SiO2, the diameter
is not more than 5 �m and the length is less than 80 �m). The volume fractions of short
fiber in the three metal materials after squeeze casting are 20, 25, and 30%, respectively.
For the convenience of expression, such as reinforced AZ91D magnesium matrix composite
with 25% aluminum silicate short fiber is written as 25%Al2O3-SiO2(sf)/AZ91D, where sf
indicates the short fiber. AZ91D magnesium matrix composites and AZ91D alloy were
both tested with the tensile creep experiments to the final creep rupture. The experiments
were conducted using the GWT105 lasting test machine at 473, 523, and 573K, with the
applied stresses of 30 to 100 MPa. The creep curve was drawn through linear variable
displacement transducer (LVDT) data acquisition system.
Results and discussion.
High-Temperature Creep Curve. The curves of strain � and strain rate �� of AZ91D
alloy versus time t at T � 473 K, � � 60 MPa are depicted in Fig. 1. The curves of strain
� and strain rate �� of 25%Al2O3-SiO2(sf)/AZ91D composite versus time t at T � 473 K,
� � 70 MPa are depicted in Fig. 2. It is clear that these two creep curves performed as
typical metal creeps in three stages: after the generation of an initial deformation in an
instant at the first phase, strain increases rapidly, while strain rate decreases gradually. The
first stage is deceleration creep stage. When the strain rate decreases gradually to a stable
value, the second stage of creep begins, which is called steady creep stage. The character of
the stage is that the creep strain increases almost linearly with time. The creep time of the
steady creep stage accounts for a large proportion of the entire creep time. The third stage is
creep damage stage, the strain rate increases significantly until the fracture of the material.
Temperature and load are the two main factors affecting the creep process. The steady-state
creep time of AZ91D alloy at T � 473 K and � � 60 MPa is about 12 h, while the
steady-state creep time of AZ91D composite at T � 473 K and � � 70 MPa is about 100 h.
The steady-state strain rate of the composite is significantly lower than the alloy of one
order of magnitude. It is visible that the creep resistance of the composite is much higher
than that of the alloy under the same temperature and similar external stress.
J. Tian
198 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1
Indexes of Creep Threshold Stress and True Stress. If there is creep threshold stress
� th in a metal material at fixed temperature, the strain rate equation of the metal material
can be expressed with Eq. (1), which is given in [11]. Formula (1) can be rewritten as
formula (2):
� ( ) ,� � �� �A th
n (1)
� ( ),/� � �1
1
n
thA� � (2)
where A and A1 are constants, which are related with material and temperature, n is the
creep exponent, � is the applied stress, � th is the threshold stress, and � �� th is called
as effective stress of the metal material. Because of the aluminum silicate short fiber
reinforced AZ91D magnesium matrix composite with different fiber volume fractions and
AZ91D magnesium alloy, n is valued as 3, 5, and 8 to make �
/� �1 n � diagram. In the
diagrams with good linear relationships, where the corresponding values of n are indexes
of true stress, the true threshold stress can be obtained at different temperatures. The linear
relationship between the composite and matrix alloy becomes the best when n� 3 in Figs. 3
and 4. Then, the creep threshold stress � th can be obtained in Table 1.
The Sources and Affecting Factors of Creep Threshold Stress …
ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 199
Fig. 1. The creep and strain rate curves of AZ91D magnesium alloy at T � 473 K and � � 60 MPa.
Fig. 2. The creep and strain rate curves of 25%Al2O3-SiO2(sf)/AZ91D composite at T � 473 K and
� � 70 MPa.
200 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1
T a b l e 1
Creep Threshold Stress Values of Materials
Material T , K Threshold stress �th , MPa
AZ91D 473
523
573
473
41
33
31
50
20%Al2O3-SiO2(sf)/AZ91D 523
573
473
46
37
53
25%Al2O3-SiO2(sf)/AZ91D 523
573
473
49
39
57
30%Al2O3-SiO2(sf)/AZ91D 523
573
53
42
a
b
Fig. 3. The �
/� �1 3 � plots of AZ91D alloy (a) and 25%Al2O3-SiO2(sf)/AZ91D composite (b).
J. Tian
The Sources of the Creep Threshold Stress. Non-equilibrium solidification, the
segregations of Mg solid solution Al-poor (magnesium dendritic crystals) and Al-rich
(solidification of remaining liquid) regions are caused in the casting-state structure of AZ91
alloy. Al-rich area is composed of white intermediate phase �-Mg17Al12 or black
�-Mg17Al12+�-Mg eutectic phase. The phase of AZ91D in cast state is mainly composed of
�-Mg and �-Mg17Al12 [12]. When the creep is generated at high temperature, the solute Al
in supersaturated �-Mg solid solution will firstly segregate in the grain boundary to form
the Al atoms air mass. Then, the non-continuous precipitation of lamellar or rod-like
�-Mg17Al12 phase will occur in the form of particle. The coarsening softening of
�-Mg17Al12 will decline creep resistance of AZ91D alloy. At the moment, only �-Mg
matrix will continue to prevent the creep. �-Mg matrix is solid solution, so the Al atoms
solute atmosphere, which is formed by the segregation of solute Al in the �-Mg solid
solution in the grain boundary, will follow dislocation motion through diffusion. Because
the diffusion rate is slower than slip rate of dislocation slow motion of dislocation, the Al
atom solute atmosphere will drag the movement of dislocation. The drag attraction of
mobile dislocation generates the threshold stress. The threshold stress of AZ91D magnesium
alloy is 41 MPa at T � 473 K. It is visible that solute Al atoms atmosphere generates great
pinning on mobile dislocation, which is the source of threshold stress of AZ91D alloy.
ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 201
a
b
Fig. 4. The �
/� �1 3 � plots of 20%Al2O3-SiO2(sf)/AZ91D (a) and 30%Al2O3-SiO2(sf)/AZ91D (b)
composites.
The Sources and Affecting Factors of Creep Threshold Stress …
Most of the nanoparticles in the surface of aluminum silicate short fiber reinforced
AZ91D composite are MgO particles, while a small amount of the particles are MgAl2O4
particles [12]. It is obvious that there is a layer of nanoparticles on the fiber surface of the
composite under high temperature creep. Most of the nanoparticles are uniform MgO
particles with a layer thickness of about 0.2 �m [12]. The reinforcement of the composite is
achieved, i.e., the bearing and mass loading capabilities of short fiber are enhanced. The
threshold stresses of AZ91D alloy are 41 and 31 MPa, at T � 473 and 573 K, respectively.
While at the same temperature, the threshold stresses of 25%Al2O3-SiO2/AZ91D composite
are 53 and 39 MPa, respectively. It can be seen that the threshold stresses of 25%Al2O3-
SiO2/AZ91D composite are larger than matrix with 12 and 8 MPa, at T � 473 and 573 K,
i.e., the threshold stresses increase by 29.3 and 25.8% compared with the matrix. The
increased value can only be derived from the effect of aluminum silicate short fiber
reinforcement.
Besides the �-Mg and �-Mg17Al12 phases which can be observed in the matrix, there
are AlPO4, MgO, and Al2O3 SiO2 phases in AZ91D composite [12]. �-Mg and
Al2O3 SiO2 are two major phases in AZ91D composite, and the mobile dislocation
movement is mainly affected by these two phases. Other phases can be considered to take
effects through Al2O3 SiO2. It can be considered that, mobile dislocation motion is only
affected by these two factors after an influence coefficient is introduced to describe the
effects of other phases. Creep is generated when mobile dislocations get rid of the two
effects and start to move. And now the sum of the two forces is threshold stress. During the
process of high temperature creep, solute atmosphere will attract the mobile dislocation,
which is a part of the source of the threshold stress of AZ91D composite. The solute
atmosphere is formed by the segregation of Al atoms in grain boundary in the
supersaturated �-Mg solid solution. Because the elastic modulus of AZ91D matrix is
smaller than the one of aluminum silicate short fiber, the strain of matrix is larger than that
of short fiber under external loads, and dislocations in matrix will undoubtedly be dragged
by short fiber. The drag force on mobile dislocation, which is generated from the
inconsistency of the strains of matrix and short fiber, is the bearing and transferring force of
short fiber, i.e., another part of the source of the threshold stress in AZ91D composite.
The Impacts of Load Transfer Factor and Material Impacting Factor. When creep
is generated in AZ91D alloy and mobile dislocations leave from the solute atmosphere, a
kind of stress will form. Friedel [13] called the threshold stress that exists in alloy as
breakaway stress � b , and the stresses at creep in the alloy can be expressed by means of
the following expressions:
� b
mW c
b kT
�
2
35
, (3)
W G Vm a��
�
�
�
�
�
��
1
2
1
1�
�
�
| | ,� (4)
where Wm is the binding energy of dislocations and solute, c is the solute concentration
(Al atom concentration, c� 0.08), b is the Burgers vector of dislocation, b� �32 10 10. m,
G is the shear modulus of Mg, G T� �192 10 864. . , k is the Boltzmann constant,
k � �13806505 10 23. J/K, T is the absolute temperature, �Va is the volume difference
caused by the difference of Mg and Al atomic radius, �Va � �82 10 30. m3, and � is
Poisson’s ratio of Mg, �� 0.34. However, Eq. (3) overestimates the value of stress � b ,
and Eq. (3) cannot estimate the variations of the elastic interaction energies of dislocations
and solute atmosphere. The variations of the energies are caused by the variations of solute
202 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1
J. Tian
atoms concentrations, while the variations of the concentrations are caused by variations of
the distances between line dislocations and solute atoms. Therefore, the value of � b
calculated by Eq. (3) increases more than 2-fold. Hence, both the concentration of Al atom
and the coefficient c in Eq. (3) will be reduced. If the concentration of aluminum atom
changes from 0.08 to 0.06 and multiplying Eq. (3) changes by a factor of 1/2, it can be
obtained that � b � 42 MPa, at T � 473 K. The result is close to the experimental stress of
AZ91D alloy, � b � 41 MPa. Therefore, Eq. (3) can be rewritten as Eq. (5) to obtain the
empirical formula of threshold stress of AZ91D alloy (breakaway stress):
� b
mW c
b kT
�
1
2 5
2
1
3
, (5)
where c1 is the reduced solute concentration (Al atom concentration, c1 � 0.06).
With Eq. (5), the creep threshold stresses of AZ91D alloy at T � 523 and 573 K are
obtained as 35 and 32 MPa. The results are very similar to the experimental results of
� th � 33 MPa (T � 523 K) and � th � 31MPa (T � 573 K), which confirms that formula (5)
possesses considerable accuracy.
The other part of the threshold stress in AZ91D metal materials is the loading and
transferring force of short fibers, which is affected by many internal factors such as the
orientation, aspect ratio and the distribution of short fibers, the MgO particles on surfaces
and the interface of short fibers and matrix. Thus, the calculation of force is quite
complicated. In order to estimate the force, one can simply calculate it by multiplying the
load transfer capacity with a material influence coefficient H. H comprehensively reflects
the internal factors which affect the force. And H can be obtained through the
experimental data.
When AZ91D composite creeps under certain temperature T and certain stress �, the
load transfer capacity of aluminum silicate short fiber reinforcement is equal to the product
of the load transfer factor � and the external stress �, which is ��. The loading and
transferring forces of short fibers are equal to the product of load transfer capacity and
material influence coefficients H. Thus, the mathematical model of threshold stress of
AZ91D metal materials can be expressed with Eq. (6):
� ��th
mW c
b kT
H�
1
2 5
2
1
3
. (6)
Because of the varying lengths, fibers are randomly distributed in approximate
two-dimensional form. Strict dealing with load transfer is a highly complex issue in
mechanics. As a simplified experimental method, a load transfer factor � has been
involved, which varies from 0 to 1. �� 0 indicates no load transfer, while �� 1 indicates
that load fully transfers to short fiber. Thus, load transfer coefficient � characterizes the
bearing and transferring load capacity of short fiber. The empirical formula of composite
strain rate can be expressed as Eq. (7) [11]:
�
�
( ) ,
�
�
�c
b
n� �1 (7)
where ��c is the strain rate of composite and ��b is the strain rate of matrix.
With the strain rate values of matrix and 25%Al2O3-SiO2(sf)/AZ91D composite within
the experimental range, the diagram of load transfer factor corresponding to temperature
ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 203
The Sources and Affecting Factors of Creep Threshold Stress …
and stress can be plotted by applying Eq. (7) in Fig. 5. It is obvious that load transfer factor
� is more sensitive to temperature than stress. Fitting the space points with quadratic
surface equation, the empirical formula of the load transfer factor � of Al2O3-SiO2(sf)/
AZ91D composite, which varies with temperature and stress, can be obtained as Eq. (8):
� � � ��
� �
� � � � �116 10 3 10 2 10 2 10 4 06 105 2 4 2 4 3 2. .T T T
1441. . (8)
Of course, as for the Al2O3-SiO2(sf)/AZ91D composites with different volume
fractions, the load transfer factors are also affected by the volume fractions. The diagrams
of fiber volume fraction versus the load transfer factor of Al2O3-SiO2(sf)/AZ91D composite
are plotted at same temperature and different loads in Fig. 6. At the same temperature and
low external loads, the impact of fiber volume fraction on load transfer factor is not
obvious. However, the effect becomes more obvious with the increase of external load.
Similarly, at same loads and lower temperatures in Fig. 7, the impact of fiber volume
fraction on load transfer factor is not obvious, while the effect becomes more obvious as
the temperature increases. The main reason is that the influences of temperature and load
on load transfer factor are greater than the influence of volume fraction. Temperature and
load affect the rate of increase of load transfer, which increases with the increase of volume
fraction.
204 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1
Fig. 5. The � �� �T plot for 25%Al2O3-SiO2(sf)/AZ91D composite.
Fig. 6. Load transfer factor dependence of fiber volume fraction for Al2O3-SiO2(sf)/AZ91D composite
at the same temperature and different loads.
J. Tian
With the creep threshold stresses of 25%Al2O3-SiO2(sf)/AZ91D composite, which are
obtained in the test range, by formula (6) and (8), the diagrams of material impact factor
versus temperature and stress can be drawn in Fig. 8. Parameter H is more sensitive and
dependent on temperature than stress. Fitting the space points with quadratic surface
equation will obtain the empirical formula of material impact factor of 25%Al2O3-
SiO2(sf)/AZ91D composite versus temperature and stress, which is expressed as Eq. (9):
H T T T�
� �
� � � � �125 10 6 10 3 10 35 10 206 104 2 3 2 3 2. . .� � 1 2712�
. . (9)
The threshold stresses of 25%Al2O3-SiO2(sf)/AZ91D composite can be determined by
formulas (6), (8), and (9). The theoretical values can be calculated with the formulas. The
experimental values of threshold stresses are also listed in Table 2. The theoretical values of
the threshold stresses are similar to the experimental values, indicating that utilization of
formula (6) as the empirical formula of threshold stress of composite is highly accurate.
Influence of Volume Fraction of Short Fiber on Creep Threshold Stress. At same
temperature, the threshold stress increases with the increase of aluminum silicate short fiber
volume fraction, while the increase is substantially small (Table 1). The diagrams of fiber
ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 205
Fig. 7. Load transfer factor dependence of fiber volume fraction for Al2O3-SiO2(sf)/AZ91D composite
at the same load and different temperatures.
Fig. 8. The H T� �� plot for 25%Al2O3-SiO2(sf)/AZ91D composite.
The Sources and Affecting Factors of Creep Threshold Stress …
volume fraction versus threshold stress of Al2O3-SiO2(sf)/AZ91D composite at different
temperatures are depicted here (Fig. 9). The increase rates of threshold stress versus fiber
volume fraction are basically the same at different temperatures. Based on the above
analysis, it may be attributed that the sources and affecting factors of threshold stresses of
Al2O3-SiO2(sf)/AZ91D metal materials with different fiber volume fractions are the same.
With the increase in the number of aluminum silicate short fiber, the total drag forces
increase. The drag forces are generated from inconsistency of strains of matrix and short
fiber on the mobile dislocation. The increase of bearing and transferring capacities of short
fiber increases the creep threshold stresses of metal materials. However, with the increase
of the amount of fiber and the density of dislocation, the precipitates easily precipitate at
the dislocations. Therefore, with the increase of fiber volume fraction, the amount of
precipitation of �-Mg17Al12 precipitates increase. The precipitates are easy to coarsen and
soften, so that the threshold stress of the composite is not greatly improved with increased
fiber volume fraction.
Temperature Influence on Creep Threshold Stress. According to the previously
obtained experimental data, the plot of threshold stress versus temperature is depicted in
Fig. 10. The threshold stresses of AZ91D alloy and AZ91D composite both decrease with
increasing temperature. The rates of threshold stresses decreasing with increasing
temperature for metal materials with different fiber volume fractions are basically the same.
The tangent lines of the three curves parallel at the same temperature, indicating that the
sources and affecting factors of threshold stress are the same. However, the rate of
threshold stress decreasing with increasing temperature for composite is significantly lower
than that of alloy, indicating that the creep resistance of composite is significantly higher
than that of alloy. The curve of the composite is upward convex, while the curve of the
matrix alloy is downward convex, which indicates that the sources and affecting factors of
206 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1
T a b l e 2
Comparison of Theoretical and Experimental Values of Threshold Stress
of 25%Al2O3-SiO2(sf)/AZ91D Composite
T , K Theoretical values of �th , MPa Experimental values of �th , MPa
473 54.26 53
523 51.55 49
573 41.65 39
Fig. 9. Creep threshold stress of Al2O3-SiO2(sf)/AZ91D composite versus fiber volume fraction.
J. Tian
threshold stresses of the composite and alloy are different. The empirical formulas of
threshold stresses of AZ91D alloy, 20%Al2O3-SiO2(sf)/AZ91D, 25%Al2O3-SiO2(sf)/AZ91D,
and 30%Al2O3-SiO2(sf)/AZ91D metal materials varying with temperature changes, can be
obtained by curve fitting, respectively, such as formulas (10), (11), (12), and (13):
� th T T� �
�12 10 13552 41353483 2. . . , (10)
� th T T��
��1 10 0916 1595393 2 . . , (11)
� th T T��
��12 10 11152 20601483 2. . . , (12)
� th T T��
��14 10 13144 25149063 2. . . . (13)
The mathematical models of threshold stresses varying with temperature fit with
experimental data. Compared with the previously derived mathematical models of the
empirical formulas (6), (8), and (9), the calculation is simple and convenient. In engineering
applications, the models can be employed to simply estimate the threshold stresses of metal
materials at different temperatures. Therefore, the high temperature creep properties of
material can be predicted.
C o n c l u s i o n s
1. The improvement of creep resistance of composite can be attributed to the effective
load bearing and transferring capabilities, and the load transfer coefficient � characterizes
the bearing and transferring capabilities of short fibers, and � is more sensitive to
temperature than stress and � increases with the increased volume fraction of short fibers.
2. A portion of the creep threshold stress of Al2O3-SiO2(sf)/AZ91D composite is a
kind of drag force, which is generated by the Al atoms atmosphere on mobile dislocation.
The atoms atmosphere is formed by the segregation of Al in �-Mg solid solution in grain
boundary. Another portion of the stress is another kind of drag force, which is generated by
the inconsistent strains of matrix and short fiber on the dislocation, i.e., bearing and
transferring force of short fiber.
ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 207
Fig. 10. Creep threshold stresses of AZ91D alloy and Al2O3-SiO2(sf)/AZ91D composite versus
temperature.
The Sources and Affecting Factors of Creep Threshold Stress …
3. Temperature and the volume fraction of short fiber are two main factors which
affect the threshold stress. Threshold stress decreases with increasing temperature, increases
with increasing volume fraction of short fibers, which are affected by the material impact
factor H and load transfer coefficient �.
Acknowledgment. The work was supported by the National Science Foundation of
Guangdong Province (Grant No. 10151170003000002).
1. V. Sklenièka, M. Svoboda, M. Pahutova, et al., “Microstructural processes in creep of
an AZ91 magnesium-based composite and its matrix alloy,” Mater. Sci. Eng. A,
319–321, 741–745 (2001).
2. F. Chmelik, P. Lukáè, M. Janeèek, et al., “An evaluation of the creep characteristics of
an AZ91 magnesium alloy composite using acoustic emission,” Mater. Sci. Eng. A,
338, Issues 1–2, 1–7 (2002).
3. J. Olbricht, A. Yawny, M. L. Young, and G. Eggeler, “Mechanical and micro-
structural observations during compression creep of a short fiber reinforced AlMg
metal matrix composite,” Mater. Sci. Eng. A, 510–511, 407–412 (2009).
4. J. Èadek, H. Oikawa, and V. Ðustek, “Threshold creep behaviour of discontinuous
aluminum and aluminum alloy matrix composites: An overview,” Mater. Sci. Eng. A,
190, Issues 1–2, 9–23 (1995).
5. A. Yawny, G. Eggeler, “Assessment of back stress and load transfer approaches for
rationalizing creep of short fiber reinforced aluminum alloys,” Mater. Sci. Eng. A,
387–389, 905–909 (2004).
6. T. L. Dragone, W. D. Nix, “Steady state and transient creep properties of an
aluminum alloy reinforced with alumina fibers,” Acta Metall. Mater., 40, No. 10,
2781–2791 (1992).
7. R. A. Karnesky, L. Meng, and D. C. Dunand, “Strengthening mechanisms in
aluminum containing coherent Al3Sc precipitates and incoherent Al2O3 dispersoids,”
Acta. Mater., 55, 1299–1308 (2007).
8. D. Sherby and E. M. Taleff, Influence of grain size, solute atoms, second-phase
particles on creep behavior of polycrystalline solids, Mater. Sci. Eng. A, 322, 89–99
(2002).
9. R. Fernández and G. Gonsález-Doncel, “Threshold stress and load partitioning during
creep of metal matrix composites,” Acta Mater., 56, No. 11, 2549–2562 (2008).
10. Y. Li and T. G. Langdon, “Creep behavior of an AZ91 magnesium alloy reinforced
with alumina fibers,” Metall. Mater. Trans., 30, No. 8, 2059–2066 (1999).
11. J. S. Zhang, High Temperature Deformation and Fracture of Materials, Science
Press, Beijing (2007).
12. J. Tian, W. F. Li, L. F. Han, and J. H. Pen, “Microstructure, interface and fabricating
process of Al2O3-SiO2/AZ91D composites prepared by squeeze infiltration,” Hot
Work. Technol. Sinica, 38, 53–56 (2009).
13. J. Friedel, Dislocations, Pergamon Press, Oxford (1964).
Received 03. 08. 2015
208 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1
J. Tian
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/ENU (Use these settings to create Adobe PDF documents best suited for high-quality prepress printing. Created PDF documents can be opened with Acrobat and Adobe Reader 5.0 and later.)
>>
/Namespace [
(Adobe)
(Common)
(1.0)
]
/OtherNamespaces [
<<
/AsReaderSpreads false
/CropImagesToFrames true
/ErrorControl /WarnAndContinue
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/IncludeNonPrinting false
/IncludeSlug false
/Namespace [
(Adobe)
(InDesign)
(4.0)
]
/OmitPlacedBitmaps false
/OmitPlacedEPS false
/OmitPlacedPDF false
/SimulateOverprint /Legacy
>>
<<
/AddBleedMarks false
/AddColorBars false
/AddCropMarks false
/AddPageInfo false
/AddRegMarks false
/ConvertColors /ConvertToCMYK
/DestinationProfileName ()
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/Downsample16BitImages true
/FlattenerPreset <<
/PresetSelector /MediumResolution
>>
/FormElements false
/GenerateStructure false
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/MultimediaHandling /UseObjectSettings
/Namespace [
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(CreativeSuite)
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]
/PDFXOutputIntentProfileSelector /DocumentCMYK
/PreserveEditing true
/UntaggedCMYKHandling /LeaveUntagged
/UntaggedRGBHandling /UseDocumentProfile
/UseDocumentBleed false
>>
]
>> setdistillerparams
<<
/HWResolution [2400 2400]
/PageSize [612.000 792.000]
>> setpagedevice
|