Deformation behavior of an extruded Mg–Dy–Zn alloy with long
period stacking ordered phase
Guangli Bi a,n,Yuandong Li a,Xiaofeng Huang a,Tijun Chen a,Jianshe Lian b,
Zhonghao Jiang b,Ying Ma a,Yuan Hao a
a State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals,Lanzhou University of Technology,Lanzhou730050,China
b Key Laboratory of Automobile Materials,College of Materials Science and Engineering,Jilin University,Nanling Campus,Changchun130025,China
a r t i c l e i n f o
Article history:
Received28June2014
Received in revised form
30October2014
Accepted3November2014
Available online11November2014
Keywords:
Mg–Dy–Zn alloy
Microstructure
Mechanical properties
Surface observation
Deformation behavior
a b s t r a c t
Deformation behavior of an extruded Mg–2Dy–0.5Zn(at%)alloy was investigated under uniaxial tensile
test at a range of strain rates of3?10à5–3?10à1sà1and temperature ranging from room temperature
(RT)to3001C.The microstructure of the alloy was mainly composed ofα-Mg,(Mg,Zn)x Dy particle phase
and a large number of long period stacking ordered(LPSO)phases.The tensile testing results indicated
that strain rate sensitivity exponent(m)generally increased with increasing temperature and decreasing
strain rate.Interestingly,m exhibited two different values in logσàlog_εcurves at3001C,indicating
that the plastic deformation mechanism of the alloy occurred to change.The surface and microstructure
observations of the tensile specimens after tension revealed that the deformation behavior of the alloy
was similar to that of the metal matrix composites,where the load transferred from the Mg matrix to the
LPSO phase at low temperatures(RT$2001C)under different strain rates and at3001C under high strain
rates,while the grain boundary sliding accommodated by dislocation slip dominated the whole
deformation behavior at3001C under low strain rates.The alloy exhibited a quasi-superplastic
deformation with an elongation-to-failure of105%at3001C and3?10à5sà1.The quasi-superplastic
deformation was attributed to the stable grains retained by the precipitation of the LPSO phase with
good thermal stability in grain interior at an elevated temperature.
&2014Elsevier B.V.All rights reserved.
1.Introduction
Owing to their light weight,high special strength and stiffness,
magnesium alloys have attracted more attention as one of the
lightest metallic structure materials.At present,most magnesium
alloy products are only casting components,such as gauge panel,
steering wheels and engine blocks applied in automobile industry.
However,few wrought magnesium alloys have been applied
because of their poor formability.It is well known that magnesium
and magnesium alloys have a hexagonal close-packed(hcp)crystal
structure with low symmetry and few slip systems,thus they
always exhibit poor ductility especially at room temperature,
which also limits their practical application[1].Therefore,it is
necessary to understand the deformation behavior of magnesium
alloys to further improve their formability during the hot-working
process.
Recently,a novel long period stacking ordered(LPSO)phase has
been observed in the as-cast and heat treated Mg–RE–Zn(RE
represents rare earth elements)alloys.The LPSO phase is not only
a long-period chemical-ordered structure but also a stacking-
ordered structure[2].The phase contains various types and the
unit cell of each type consists of different stacking sequences,such
as10H(ABACBCBCAB),14H(ACBCBABABABCBC),18R(ABABABCA-
CACABCBCBC)and24R(ABABABABCACACACABCBCBCBC)types
[3].Among them,the18R-and14H-type LPSO structures are
commonly observed in the previous studies.The18R-type LPSO
structure could transform into a14H-type LPSO structure during
heat treatment or hot deformation[4–6].Furthermore,the LPSO
phase is coherent with the magnesium matrix and has a good
thermal stability and high hardness[7–9].Thus,the phase is
considered to be a desirable heat resistant phase to improve
high-temperature properties of magnesium alloys.The previous
investigations indicate that wrought Mg–RE–Zn alloys containing
the LPSO phase exhibit excellent tensile strengths at both room
and elevated temperatures[2,10–13].The microstructure,
mechanical properties and deformation behavior of these alloys
have been widely investigated[14–22].The high mechanical
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Materials Science&Engineering A
n Corresponding author.Tel./fax:t869312973564.
E-mail address:glbi@https://www.doczj.com/doc/c515077117.html,(G.Bi).
Materials Science&Engineering A622(2015)52–60
properties of these alloys are ascribed to the grain re?nement and
the LPSO phase strengthening.The deformation behaviors of the
18R-and14H-type LPSO structures in Mg12ZnY phase are all?1120?
(0001)slip at low temperatures[16,23].Additionally,the previous
literatures have demonstrated that the morphology,scale,amount
and distribution of the LPSO phase signi?cantly affect the defor-
mation behavior of Mg–RE–Zn alloys[6,24,25].
Usually,strain hardening,dynamic recovery and dynamic
recrystallization occur during deformation for the alloys at differ-
ent strain rates and temperatures.The formation of the LPSO
phase could contribute to the complete DRX and inhibit the grain
growth of the alloys at elevated temperatures[11,17].The dis-
persive distribution of the LPSO phase and the complete DRX
could also result in an obtainment of the optimum processing
parameters and the safety deformation conditions in the proces-
sing map[26,27].Moreover,Garc′es et al.[22]studied the
deformation behavior of rapidly solidi?ed Mg97Y2Zn alloy at
temperatures from room temperature to4001C.The results
indicate that the LPSO phase provides the alloy high mechanical
strength below2501C and the non-basal slip controls the whole
deformation above2501C.Recently,O?orbe et al.[28]reported
the high-temperature deformation behavior of the extruded
Mg97à3x Y2x Zn x(x?0.5,1,1.5)alloys at a temperature ranging from
2001C to4001C with a wide range of strain rates.The correspond-
ing experimental results reveal that the deformation of the alloys
is similar to that of a metal matrix composite where the magne-
sium matrix transfers parts of its loads to the LPSO phase at low
temperatures(2001C o T o2501C)and high strain rates e_ε410à4sà1T,but it is controlled by grain boundary sliding (GBS)at high temperatures(3501C o T o4001C)and low/inter-
mediate strain ratese_εo10à6sà1;10à6sà1o_εo10à4sà1T. Additionally,the Mg–RE–Zn alloys containing the LPSO phase exhibit superior superplasticity[12,28–30],since the?ne grains (o10μm)could be obtained and the thermal stable LPSO phase can suppress the growth of grains at high temperatures.For example,Leng et al.[29]found a superplasticity of720%at 4201C and1?10à3sà1in an extruded Mg–9RY–4Zn(RY repre-sents Y-rich misch metal)alloy.Yang et al.[30]achieved a super-plasticity of3570%at4251C and3?10à2sà1in the as-cast Mg–Gd–Y–Zn–Zr alloy fabricated by friction stir processing(FSP).
From the above-reported literatures,it can be seen that most
investigations only focus on the deformation behavior and super-
plasticity of Mg–Y–Zn,Mg–Gd(Y)–Zn and Mg–RY–Zn alloys,but
those of other Mg–RE–Zn alloys have not been referred to until
now.Our previous work has demonstrated the extruded Mg–2Dy–
0.5Zn(at%)alloy exhibits excellent mechanical properties at high
temperatures,its yield tensile strength,ultimate tensile strength
and elongation are245MPa,260MPa and36%at3001C,respec-
tively[11].In this paper,the deformation behavior of the alloy at
different temperatures and strain rates was investigated and the
alloy exhibited a quasi-superplasticity.Microstructure evolution
and surface observation before and after the tensile test were also
discussed to understand the deformation mechanism.
2.Experimental procedure
The investigated alloy with a normal composition of Mg–2Dy–
0.5Zn alloy(at%)was prepared from pure Mg(99.9%),pure Zn
(99.9%)and Mg–20Dy(wt%)mast alloys in a graphite crucible
under an anti-oxidizing?ux.The melts were homogenized at
7501C for0.5h and then were poured into a water-cooling mold
with a diameter of85mm and a length of350mm at7201C.The 3601C and then were aged at1801C for99h.The microstructure, phase structure and composition of the extruded alloy were characterized by optical microscope(OM)(Olym-pusGX71),scan-ning electron microscope(SEM)(JSM-5600)with energy disper-sive X-ray spectroscopy(EDS)and transmission electron microscopy(TEM)(JEM-2100F).Thin foil samples for the TEM observation were prepared using the Ion Polishing System (RES101).Samples for the optical examination were polished and then etched in a solution of4mL nitric acid and96mL ethanol. Tensile tests were carried out in an Instron-type tensile testing machine(Instron1211)at temperatures ranging from room temperature(RT)to3001C and strain rates ranging from 3?10à1sà1to3?10à5sà1for all the specimens tested.Before the tensile test at high temperature,all the tensile specimens were held for6min to make the temperature stay uniform.Three parallel tensile specimens were tested for each date point.Tensile specimens with a gauge dimension of40mm?5mm?1.8mm were cut from the heat treated bars with their length direction parallel to the extrusion direction.
3.Results
3.1.Microstructure
Fig.1shows the microstructure of extruded Mg–2Dy–0.5Zn (at%)alloy aged at1801C for99h.It can be seen that the alloy is mainly composed ofα-Mg,irregular(Mg,Zn)x Dy particle phase and a great number of14H-type LPSO phases.Notably,the
LPSO
G.Bi et al./Materials Science&Engineering A622(2015)52–6053
phase appears with two kinds of morphologies:one is the block shape and the other is the ?ne lamellar shape.The latter one is usually generated by the transformation of 18R-type LPSO phase and the precipitation of α-Mg matrix during the heat treatment and extrusion [11,31,32].The block-shaped 14H-type LPSO phase mainly distributes along the extrusion direction,while the ?ne lamellar-shaped one precipitates in α-Mg matrix and is parallel to each other,as shown in Fig.1and the later Fig.10.In addition,during extrusion,the LPSO phase could be acted as a preferred nucleation site for the dynamic recrystallization (DRX),thus suf ?cient DRX occurs around the block-shaped 14H-type LPSO phase (see Fig.1(a)).After the extrusion,the grain size of the alloy decreases signi ?cantly,the average value of which is about 3μm.3.2.Tensile properties
Fig.2displays the true stress –strain curves of the alloy at different temperatures and strain rates.The ?ow stress and yield strength decrease with increasing testing temperature and decreasing strain rates.The strain hardening exponent n in the empirical Hollomon equation σT ?K εn T is measured using the true stress –strain (σT àεT )data in the uniform plastic deformation stage,where σT is the true stress,εT is the true strain and K is the coef ?cient of hardening.Table 1lists the n -values of the alloy at different temperatures and strain rates.It can be seen from Table 1that the alloy exhibits the high n values from 0.13to 0.18at room temperature (RT),1001C and 2001C under different strain rates,indicating a strong strain hardening effect.In contrast,the strain hardening effect similar to that at RT,the corresponding n -values are 0.15and 0.12at 3?10à1s à1and 1?10à3s à1,respec-tively,while at low strain rates (8?10à4s à1o _ε
o 3?10à5s à1),the strain hardening effect decreases signi ?cantly with decreasing strain rates.The n -value of the alloy decreases to 0.008at 3?10à5s à1.Additionally,it can be seen from Fig.2that hard-ening caused by strain hardening and softening caused by DRX and dynamic recovery (DRC)simultaneously occur during defor-mation.The formation of peak stress indicates that the hardening and softening reach a dynamic balance.After that,softening plays a dominant role in sequent deformation,which causes the forma-tion of a steady soft platform at 8?10à4s à1(see Fig.2(d)).Notably,the serration ?ow instability behavior is observed at RT,1001C and 2001C under different strain rates for the alloy (see Fig.2(a –c)).The behavior of the alloy is known as the Portevin –Le Chatelier (PLC)effect.The PLC effect was also reported in Al alloys and other Mg alloy systems [33,34]and has been thought to be related to the dynamic strain aging (DSA)[33].In this paper,the formation of the DSA mainly arises from the intersection between
Fig.2.True stress –strain curves of the alloy at different temperatures and strain rates.
Table 1
Strain-hardening exponent obtained from the tensile tests.Strain rate (s à1)RT 1001C 2001C 3001C 3?10à10.1830.1700.1620.1581?10à30.1600.1560.1520.1201?10à5
0.150
0.140
0.130
0.008
G.Bi et al./Materials Science &Engineering A 622(2015)52–60
54
investigation on the effect of 14H-type LPSO phase on DSA is in progress.
In order to further understand the deformation behavior of the alloy at different temperatures and strain rates,the strain rate
sensitivity exponent m in the power law relation σT ?K 0_εm T was
measured at the true strain of 3%,where σT is the true stress,εT is
the true strain,K ?is a material constant,_ε
is the strain rate and m is the strain rate sensitivity exponent.From the double-logarithmic
log σT àlog _ε
T curves in Fig.3,the m -values of the alloy were determined to be 0.016,0.012and 0.02at RT,1001C and 2001C,respectively.Interestingly,there exists an in ?ection point in
log σT àlog _ε
T curve at 3001C.The m -value is 0.16at high strain rates but is 0.4at low strain rates.The different m -value indicates a transition of the plastic deformation mechanism.Fig.4shows the variation of elongation-to-failure of the alloy as a function of strain rate at different temperatures.The elongation-to-failure (ε)of the alloy remarkably increases with increasing temperature and decreasing m -value.The εreaches 105%at 3001C under a strain rate of 3?10à5s à1,indicating that the alloy exhibits a quasi-superplastic behavior.The typical tensile specimens before and after tensile testing at RT and 3001C are displayed in Fig.5.It can be seen that the fracture direction of the tensile specimens at RT and different strain rates is tilted about 451from the tension axis,while the obvious necking resistance is observed in the 3.3.Surface observation of specimens deformed in tension
The typical observations of lateral surface and facture surface of the tensile specimens at RT and 3001C under different strain rates are shown in Figs.6and 7,respectively.At RT,a large number of sliding lines intersecting each other present on the lateral surface as shown in Fig.6(a).Some micro-cracks are also observed in the sliding line.It is noted from Fig.6(c)and (g)that the amount of these micro-cracks gradually increases with decreasing strain rates.The macro-cracks form along the sliding line at 3?10à5s à1as shown in Fig.6(k).Simultaneously,some tearing ridges and cleavage surfaces can be seen in the fracture surface,which are typically brittle fracture features.By the high-magni ?cation observation of the fractured surface,the ?ne white particles inlay in the fracture surface (see Fig.6(m)).Micro-cracks initiate,grow and interlink around these ?ne https://www.doczj.com/doc/c515077117.html,-pared with the lateral surface of the fractured tensile specimens at RT,few sliding lines still appear in the lateral surface at high strain rates
(3?10à3s à1o _ε
o 3?10à1s à1)and 3001C,but disappear at low strain rates (3?10à5s à1o _ε
o 8?10à4s à1)as displayed in Fig.7(a),(e)and (i).A great number of embossments with different sizes distribute uniformly on the whole lateral surface at 3001C under a strain rate of 3?10à5s à1(see Fig.7(i)).It can be observed from the high-magni ?cation image that lots of cavities exist around the grain boundaries.A previous literature [35]has demonstrated that these cavities could nucleate by the continuous condensation of vacancies on the grain boundaries that experience a normal tensile stress or by vacancy clustering due to the stress concentration on the grain boundary inclusions produced by strain incompatibility and grain boundary sliding (GBS).These results indicate that GBS signi ?cantly occurs during quasi-superplastic deformation [36,37].The tensile
Fig.3.The variation of ?ow stress as a function of the strain rate at different
temperatures.
Fig.4.The variation of elongation-to-failure of alloy as a function of the strain rate at different
temperatures.
Fig.5.The non-deformed and fractured tensile specimens of the alloy at different strain rates:(A)RT and (B)3001C.
G.Bi et al./Materials Science &Engineering A 622(2015)52–6055
Fig.6.The lateral surface (a),(e)and (i)and the fractured surface (b),(f)and (j)of tensile specimens of the alloy at RT and different strain rates:(a)3?10à1s à1,(e)3?10à3s à1and (i)3?10à5s à1.The high-magni ?cation images of (c),(g)and (k)correspond to the red rectangle frames in (a),(e)and (i),respectively,(d),(h)and G.Bi et al./Materials Science &Engineering A 622(2015)52–60
56
Fig.7.The lateral surface (a),(e)and (i)and the fractured surface (b),(f)and (j)of tensile specimens of the alloy at 3001C and different strain rates:(a)3?10à3s à1,(e)8?10à4s à1and (i)3?10à5s à1.The high-magni ?cation images of (c),(g)and (k)correspond to the red rectangle frames in (a),(e)and (i),respectively,(d),(h)and G.Bi et al./Materials Science &Engineering A 622(2015)52–6057
3.4.Microstructures of specimens deformed in tension
Figs.8and 9display the microstructures of the alloy deformed at 2001C and 3001C under a strain rate of 3?10à5s à1,which present the maximum elongation.Microstructure observations reveal that the alloy occurs to suf ?cient DRX during tensile deformation.Grains of the alloy remain small and equiaxed even at a large strain and high temperature.Grain size of the alloy increases slightly at 3001C compared with that at 2001C.However,the grain size of the tensile specimen in the deformed region for both temperatures is smaller than that in the non-deformed region (see Figs.8(b),(d)and 9(b),(d)).Cavities are also observed at the interface between α-Mg and LPSO phase at 3001C (see Fig.9(a)and (b)).In addition,Fig.10shows the bright ?eld images of the microstructure of the alloy deformed at 3001C under a strain rate of 3?10à5s à1.The block-shaped 14H-type LPSO phases in α-Mg matrix occur to deform and bend.Cracks initiate at the bent surface of the phase and propagate perpendicular to the tensile direction (see Fig.10(a)).Similar experimental results are also observed in other Mg –RE –Zn alloys containing the LPSO phase [17,28].The deformation of the LPSO phase also affects the deformation behavior of the alloy.Simultaneously,the ?ne lamellar 14H-type LPSO phase in α-Mg grain develops and grows due to the diffusion of Dy and Zn atoms [32].Small subgrains form in α-Mg matrix (see Fig.10(b)).The high-magni ?cation image of the 14H-type LPSO phase in Fig.10(c)shows a large number of dislocations around the phase.
4.Discussion
The deformation behavior of the alloy is very complex,which initial microstructure of the alloy does not change during tensile deformation,i.e.,the LPSO phase is still 14H type and its volume fraction is about 36%.In this case,the deformation mechanism of the alloy is similar to that of the metal matrix composites.It is well known that the hardness and Young 0s modulus of the LPSO phase are higher than that of the Mg matrix [38,39].Under the applied stress,especially at high strain rates,the load would transfer from the soft Mg matrix into the hard LPSO phase.As the applied stress exceeds the critical resolved shear stress (CRSS)for basal slip of the LPSO phase and the basal slip system would be activated,the phase self would deform.Hagihara et al.[16]have reported that the dominant plastic mode of the 18R-type LPSO phase is ?1120?(0001)basal slip at RT in the directional solidi ?cation (DS)Mg 12ZnY crystal and the plastic deformation of the phase is highly anisotropic.When the applied stress is loaded parallel to the (0001)plane,where the Schmid factor is negligible,the deforma-tion kinks would be initiated to accommodate the severe strain originating from the applied stress.In addition,Han et al.[40]have also demonstrated that the orientation relationships between the 14H-type LPSO phase distributed along the extrusion direction and the Mg matrix are the (0001)plane of the LPSO phase is parallel to the (0001)plane of Mg matrix,which is denoted as
e0001TLPSO ==e0001Tα,and the (0110)of the LPSO phase is parallel
to the (0110)of the Mg matrix,which is denoted as
e0110TLPSO ==e0110Tαin the extruded Mg –10Gd –2Y –0.5Zn –0.3Zr alloy.When the compressive stress is loaded in a direction parallel to the extrusion direction,the deformation kinks also generate in the deformed microstructure.In the present paper,the block-shaped 14H-type LPSO phase distributes along the extrusion direction and is parallel to the tensile axis (see Fig.10(a)).Thus,it can be considered that the basal plane of the block-shaped
Fig.8.Microstructures of the alloy deformed at 2001C under a strain rate of 3?10à5s à1:(a)deformed region and (c)non-deformed region of the tensile specimens,(b)and (d)are high-magni ?cation images of (a)and (c),respectively.The arrow indicates the extrusion direction (parallel to the tensile direction).
G.Bi et al./Materials Science &Engineering A 622(2015)52–60
58
activating the basal plane slip should be high[16].At high strain
rates,the stress concentration occurs easily because of the
dislocation intersection in the LPSO phase.However,in this case,
it is dif?cult to induce the formation of micro-cracks due to the
rapid deformation rate.In contrast,at low strain rates,the micro-
cracks have enough time to initiate,propagate,coalesce and
eventually form macro-cracks and relax the stress concentration.
The corresponding sliding lines and the typical macro-cracks have
been observed on the surface of tensile specimens(see Fig.6).The
plastic deformation of the block-shaped14H-type LPSO phase
improves the ductility of the alloy.Furthermore,the presence of
the lamellar14H-type LPSO phase in Mg grains inhibits dislocation
movements and thus enhances the tensile strength of the alloy
[22].The strengthening effect of the LPSO phase is more obvious
with increasing testing temperatures due to its good thermal
stability[8,9,11].In addition,the intersection between the phase
and dislocation could promote the occurrence of DRX and the
formation of subgrains,and further stabilize the microstructure of
the alloy at elevated temperatures[28].It is worth mentioning
that the intersection also possibly induces the formation of DSA.
The DSA could enhance the strain hardening effect on the base of
the positive m-value but decrease the ductility of the alloy.
The deformation behavior of the alloy changes at3001C
under different strain rates,which has been indicated by two
different m-values in Fig.3.At high strain rates(1?10–3s–1o_εo3?10à1sà1),the typical dislocation sliding lines still appear on the surface of the tensile specimens(see Fig.7(a)).This
indicates the dislocation slip is the dominant deformation
mechanism same as that at low temperatures.It is noted that
besides the basal dislocation slip,non-basal dislocation slip is also
activated.Thus,the alloy exhibits the high elongation-to-failure deformation mechanism transition of the alloy.In addition to the high m-value(0.4),the formation of embossments and cavities on the surface suggests that GBS dominates the whole plastic defor-mation[36].Additionally,the alloy exhibits a quasi-superplastic deformation behavior,the elongation-to-failure of the alloy is 105%at3001C under a strain rate of3?10–5s–1.During quasi-superplastic deformation,the LPSO phase plays an important role for the present alloy.Those block-shaped14H-type LPSO phases are generally re?ned by cracking and redistributing along the tensile axis with increasing strain(see Fig.9).The re?nement and distribution of the phase could prevent grain coarsening and thus are favorable to the superplastic deformation[28,29].In addition, the local stress concentration caused by GBS would generate at the interface between the phase and the Mg matrix.On the contrary, the formation of those cavities at the interface could relax the stress concentration(see Figs.8and9).The interlinkage of these cavities perpendicular to the tensile axis is restricted by the presence of hard LPSO phase and thus enhances the plasticity of the alloy[20].Furthermore,those lamellar-shaped14H-type LPSO phases in Mg grains occur to bend slightly and intersect intensely with dislocation.The amount of dislocation increases and the dislocation pile-up appears around the LPSO phase.The presence of the phase effectively inhibits the movement of dislocations and further restricts grain growth,especially at3001C.This leads to the formation of subgrains in the Mg matrix(see Fig.10(c)). Hagihara et al.have demonstrated that the LPSO phase could enhance the grain re?nement during extrusion[14].Fine grains are obtained and these grains of the Mg matrix grow slowly, despite the specimen undergoing the quasi-superplastic deforma-tion at3001C under a low strain rate.These?ne and stable grains ($8μm)for the present alloy could satisfy the requirement of
Fig.9.Microstructures of the alloy deformed at3001C under a strain rate of3?10à5sà1:(a)deformed region and(c)non-deformed region of the tensile specimens, (b)and(d)are high-magni?cation images of(a)and(c),respectively.The arrow indicates the extrusion direction(parallel to the tensile direction).
G.Bi et al./Materials Science&Engineering A622(2015)52–6059
5.Conclusions
We investigated the deformation behavior of extruded Mg –2Dy –0.5Zn (at%)alloy at different temperatures (RT $3001C)and strain rates (3?10–1s à1–3?10–5s –1).The alloy is mainly composed of α-Mg,irregular (Mg,Zn)x Dy particle phase and a great number of 14H-type LPSO phases.The LPSO phase appears with two kinds of morphologies:one is the block shape,which mainly distributes along the extrusion direction and the other is the ?ne lamellar shape,which mainly precipitates in the Mg matrix.At low temperatures (RT $2001C)and different strain rates,the alloy exhibits an obvious strain hardening behavior and has a low strain rate sensitivity exponent.The main deforma-tion mechanism of the alloy is similar to that of the metal matrix composites,where the load transfers from the Mg matrix to the block-shaped 14H-type LPSO phase.Interestingly,the alloy exhi-behavior is same as that at low temperatures (load transfer mechanism);at low strain rates (8?10–4s –1–3?10–5s –1),the main deformation behavior of the alloy is GBS accommodated by dislocation slip.
Acknowledgments
This work was ?nancially supported by the National Natural Science Foundations of China (Nos.51301082,51371089,51464031and 51464032).References
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