Microstructure evolution during high strain rate tensile deformationof a fine-grained AZ91 magnesium alloy

Fang Chai, Datong Zhang n, Weiwen Zhang, Yuanyuan LiNational Engineering Research Center of Near-net Shape Forming for Metallic Materials, South China University of Technology, Guangzhou 510640, PR China

a r t i c l e i n f o

Article history:Received 30 July 2013Received in revised form8 October 2013Accepted 13 October 2013Available online 21 October 2013

Keywords:AZ91 magnesium alloySubmerged friction stir processingMicrostructureDynamic recrystallization

a b s t r a c t

A fine-grained AZ91 magnesium alloy prepared by submerged friction stir processing is subjected to hightemperature tensile test at 623 K and 2�10�2 s�1 to intermediate strains of 270%, 510%, 750% and failurestrain of 990%, and microstructure evolution of the experimental material during tensile test isinvestigated. The initial grain size is about 1.2 μm. Microstructures within the gauge region are muchfiner than those of grip region, and the grain aspect ratios remain approximately 1.0 in the wholesuperplastic deformation. With the tensile strains increasing, the average size of β-Mg17Al12 particlesincreases, and the density of the β-Mg17Al12 particles decreases. Due to the pinning effect of β-Mg17Al12particles and the occurrence of DRX, the fine microstructures are maintained in the whole superplasticdeformation process. Grain boundary sliding is the main deformation mechanism, and cavities areformed in the triple junctions of grains and around the second phase particles during deformation. Theexcellent high strain rate superplasticity of the AZ91 magnesium alloy is mainly attributed to its initialfine microstructure and good thermal stability.

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1. Introduction

Magnesium alloys have wide applications in the automobileand transportation industries because of their low density andhigh specific strength [1–3]. However, magnesium alloys exhibitpoor room temperature formability due to the limited number ofindependent slip systems in the hexagonal close-packed (HCP)crystal structure [4,5]. Superplasticity refers to the ability of acrystalline material to exhibit high uniform elongation whenpulled in tension at elevated temperatures, which has beendemonstrated to be an effective way of shaping magnesium alloysinto complex components [6]. In recent years, the development ofmagnesium alloys for superplastic forming has been paid con-siderable attention.

There are two basic requirements for achieving superplasticity,i.e. fine grain size and high microstructure stability [7]. AZ91magnesium alloys, as one of the most widely-used magnesiumalloys in industry, possess good castability, high room temperaturestrength and good thermal stability [6]. Numbers of researchershave reported superplasticity in AZ91 magnesium alloys. Mabuchiet al. investigated superplastic behavior of a fine-grained AZ91alloy processed by equal channel angular extrusion (ECAE), andthe results showed that the as-ECAE materials exhibited lowtemperature superplasticity, with an elongation of 661% at 423 K

and 6�10�5 s�1 [8]. Kim et al. reported that Mg–9Al–1Znmagnesium alloy with ultrafine-grained microstructures producedby different speed rolling exhibited excellent superplasticity andmicroformability at relatively low-temperatures and low strainrates [9]. However, the optimum superplastic strain rates obtainedin those AZ91 magnesium alloys are relatively low for superplasticforming, which increase the forming time in components manu-facturing. High strain rate superplasticity (HSRS) is the capabilityof a material to undergo extensive plastic deformation without theformation of a neck prior to failure at high strain rates. In HSRS,the values of tensile elongations over 200% are generally obtainedat strain rates above 10�2 s�1 [10]. Kim et al. pointed out thatexcellent low temperature superplasticity (at temperatures lessthan 0.5Tm, where Tm is the melting temperature, LTSP) inmagnesium alloys processed by ECAE was reported extensively,but success in achieving HSRS in magnesium alloy was rarelyreported [11]. They suggested that it was mainly due to a greatnumber of non-equilibrium grain boundaries in magnesium alloywhich retarded superplastic strain rate. The development of HSRShas great potential since it can make use of these materials inrapid superplastic forming operation, which cannot only increaseproductivity, but also improve product quality without undesir-able oxidation [12]. Therefore, preparation of materials with HSRSplays a significant role in expanding the application ofmagnesium alloy.

Compared with ECAE, friction stir processing (FSP), a novelsevere plastic deformation technique based on friction stir welding(FSW) developed by The Welding Institute (TWI) in UK, can result

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n Corresponding author. Tel.: þ86 20 87112272; fax: þ86 20 87112111.E-mail address: dtzhang@scut.edu.cn (D. Zhang).

Materials Science & Engineering A 590 (2014) 80–87

in a complete recrystallized microstructure with a higher ratio ofhigh-angle boundaries [13–15]. Consequently, the optimum super-plastic strain rate can be increased in FSP material. However, theheat accumulation during FSP may lead to grain growth anddeteriorate the mechanical properties to some extent [16]. Mostrecently, of particular interest is to combine the FSP techniquewith rapid cooling to prepare much finer microstructure. Based onthis consideration, submerged friction stir processing (SFSP) hasbeen developed, which means that the entire processing is carriedout underwater [17]. Su et al. reported that the grain size ofcommercial 7075Al alloy could be refined when combining FSPwith a mixture of water, methanol, and dry ice [18]. Hofmann et al.demonstrated that SFSP is an improved method of creatingultrafine-grained bulk materials [17]. However, almost all theprevious studies of SFSP focus on the grain refinement andmechanical properties at room temperature, and researches onsuperplastic behavior of the SFSP material are rarely reported. Inour previous work, we prepared a fine-grained AZ91 magnesiumalloy with an average grain size of 1.2 μm by SFSP, and the SFSPAZ91 magnesium alloy exhibited large elongation of 990% at2�10�2 s�1 and 623 K, indicating excellent HSRS could beachieved in the SFSP AZ91 magnesium alloy [19]. Until present,excellent HSRS has been found in some thermal resistant magne-sium alloy prepared by different techniques, such as powdermetallurgy processed ZK61 [20], high-ratio differential speedrolling ZK60 [11] and FSP Mg–Zn–Y–Zr alloys [21,22]. Due to theexistence of second phases containing Zr or rare earth elements,microstructure thermal stability of these fine-grained magnesiumalloys can be improved to achieve HSRS. On the other hand,elongations of Mg–Al–Zn alloys, which are the most widely-usedmagnesium alloys in industry, are generally below 200% when thetensile strain rate is higher than 10�2 s�1 [6,23–25]. The mainreason for the poor HSRS in Mg–Al–Zn alloy may lie in their poormicrostructure stability compared with the heat resistant magne-sium alloys mentioned above. Aimed at developing HSRS Mg–Al–Zn alloys, microstructure evolution during superplastic deforma-tion at high strain rate needs to be investigated for material designand microstructure optimization. In this study, microstructureevolution of AZ91 magnesium alloy processed by SFSP is investi-gated in detail at strain rate of 2�10�2 s�1 and 623 K throughmicrostructure examination of the specimens deformed to differ-ent intermediate strains. Based on the experimental results,microscopic deformation mechanism of the SFSP AZ91 alloy isfurther investigated.

2. Experimental procedures

Cast AZ91 magnesium alloy billets with a composition of9.08Al–0.60Zn–0.27Mn–0.014Si–0.002Fe–0.012Ce (wt%) wereused in the present investigation. The AZ91 casting alloy wascomposed of α-Mg dendrites and coarse eutectic β-Mg17Al12 phase,and the average grain size of α-Mg matrix was measured to be72 μm, as reported in our previous work [19]. Plates with athickness of 6 mm were machined from the cast billets andsubjected to SFSP at a rotation speed of 800 rpm and a transversespeed of 60 mm/min. A 5.6 mm diameter, 5 mm length cone-threaded pin and a concave shoulder 16 mm in diameter wereused in SFSP. The SFSP experiment was carried out on FSW-3LM-003 welding machine equipped with a cooling system. The platewas completely submerged in water and the flow rate of the waterwas 29 ml/s during the processing.

Tensile specimens with a gauge of 5 mm length, 3.5 mm widthand 2 mm thickness were electro-discharged machined from theprocessed zone with the tensile axes parallel to the processingdirection. The surface of the tensile specimens were polished using

a fine sandpaper and the final thickness of the tensile specimenswas retained about 1.5 mm. Constant velocity tensile tests werecarried out at 623 K and at strain rate of 2�10�2 s�1. Thespecimens required 15 min (including heating and stabilizationtime) to reach thermal equilibrium before tensile tests. Thetemperature variation during the tensile tests was not more than71 K. Additionally, in order to investigate the microstructureevolution with strains during the test, the SFSP AZ91 magnesiumalloys were examined at intermediate strains of 270%, 510%, 750%and failure strain of 990%, and the test time was 3, 5, 7 and 9 min,respectively. The intermediate tensile specimens were quenchedin water quickly. Microstructures in the gauge region (A) and gripregion (B) were examined, respectively (shown in Fig. 1). Materialsin the gauge region experienced superplastic tensile deformation,while materials in the grip region only experienced static heatingunder the same thermal condition.

Microstructures of the experimental materials were examinedby optical microscopy (OM, Keyence, VHX-600, Japan) and scan-ning electron microscopy (SEM, Nova Nano 430, FEI, USA)equipped with energy dispersive spectroscopy (EDS, Inca 300,Oxford, UK) and transmission electron microscopy (TEM, JEM-2100, JEOL, Tokyo, Japan). TEM observation was carried out usingan operation voltage of 200 kV. For optical and SEM microstruc-tural investigations, the specimens were polished up to a 0.5 μmdiamond paste, and etched using a solution of 5 g picric acid, 10 mlacetic acid, 10 ml distilled water and 80 ml ethanol. Tensilefracture morphologies of the deformed specimens were observedby SEM. Microstructures of the tensile specimens within the gaugesection and in the grip were observed by OM and SEM. The sizes ofthe α-Mg grains and the β-Mg17Al12 particles were measured usingthe mean linear intercept technique, and the average grain sizeand particle size mentioned below were analyzed on about 500grains and about 200 particles, respectively.

3. Results

Microstructures of AZ91 magnesium alloy after SFSP are shownin Fig. 2. As shown in Fig. 2a, the microstructures are greatlyrefined into equiaxed and uniform grains as a result of dynamicrecrystallization (DRX) during SFSP. Since the grain is too fine to bediscerned clearly by OM, a TEM image is shown in Fig. 2b. Theaverage grain size of magnesium grains is about 1.2 μm. In general,a small grain size of less than 10 μm is required to attain super-plasticity in metals [26]. Furthermore, as reported by Watanabeet al., a fine-grained AZ91 alloy with grain size about 1 μmprepared by ingot metallurgy route was supposed to possess HSRS[27]. This was in consistence with our results in a previous study[19], i.e. the AZ91 magnesium alloys prepared by SFSP exhibitedexcellent HSRS with an elongation of 990% at 2�10�2 s�1 and623 K. Moreover, from Fig. 2c, the β-Mg17Al12 particles pinningon grain boundaries can be seen clearly (shown by arrows).

Fig. 1. Appearance of AZ91 magnesium alloy elongated to 990% at 623 K and2�10�2 s�1.

F. Chai et al. / Materials Science & Engineering A 590 (2014) 80–87 81

The particles have a spherical or ellipsoidal shape, and the averagesize is about 250 nm. The nominal stress–nominal strain curve ofthe SFSP AZ91 magnesium alloy deformed at 623 K and2�10�2 s�1 is shown in Fig. 3. It can be seen that the nominalstress increases rapidly with nominal strain up to about 0.6, andthen decreases gradually with strain increasing. More details onsuperplastic deformation behaviors of the SFSP AZ91 alloy havebeen reported in a previous study [19].

Fig. 4 shows the OM image within the gauge section (position Ain Fig. 1) and grip region (position B in Fig. 1) of the SFSP AZ91tensile specimens at 623 K and 2�10�2 s�1 at strains of 270%,510%, 750% and 990%. Fig. 4a and b shows the micrographs after anelongation of 270% at the gauge and grip section, respectively. Anobvious grain growth is observed at both the gauge and gripsection. Compared with the initial grain size of 1.2 μm, the averagegrain size is 2.8 μm and 3.1 μm for the gauge and grip region,respectively. Microstructures in the grip region are slightly largerthan that in the gauge section. When the elongation increases to510%, microstructure at the grip experiences remarkable growth,

while microstructure at the gauge section does not changesignificantly. The average grain size at the grip and gauge regionis about 4.5 μm and 3.2 μm, respectively (Fig. 4c and d). At thestrain of 750%, the average grain size in the gauge and grip sectionis measured as 4.1 and 5.2 μm (Fig. 4e and f), indicating that graingrowth is still obvious under both dynamic and static conditions.When the tensile specimen fractured (at the strains of 990%), thegrains continue to grow, and the average grain size at the gaugeand grip region is about 5.5 μm and 6.2 μm, respectively (Fig. 4gand h). The corresponding average grain size (d) and the grainaspect ratio (r¼dL/dT, where dL is the grain diameter along thelongitudinal direction (tensile direction) and dT is the transversedirection) during the high temperature tensile deformation aredepicted in Fig. 5. It can be found that the grains are coarseningduring the superplastic deformation both at the gauge and gripsection in comparison to the initial SFSP AZ91 specimens. How-ever, compared with the grip section, the grains at the gaugeregion grow much slower. Even finally fractured, the microstruc-tures of the SFSP AZ91 tensile specimen are relatively fine. Inaddition, the grain aspect ratios in the gauge region are all about1.0 during the whole superplastic deformation, i.e. the grains arenearly equiaxed and not elongated. This suggests that grainboundary sliding is the predominant deformation mechanismduring the superplastic deformation.

Fig. 6 shows the SEM microstructures of the SFSP AZ91magnesium alloy along the tensile direction deformed to inter-mediate strains of 270%, 510%, 750% and failure strain of 990%,respectively. At the strain of 270%, the fine β-Mg17Al12 particles,which are spherical or angular in shape, are distributed homo-geneously on the matrix. The average size of the β-Mg17Al12particles is about 1.72 μm. With the strains increasing, theβ-Mg17Al12 particles are gradually coarsening, and the average β-Mg17Al12 particle size increases to about 2.65 μm when the tensilespecimen is fractured. Furthermore, the density of the β-Mg17Al12particles decreases as the strains increases. Fig. 7 depicts thevariation of the particle size and density with tensile test time of

Fig. 2. (a) OM and (b) TEM image of the SFSP AZ91 magnesium alloys; (c) TEM image and (d) SEM image showing particles in SFSP specimen.

Fig. 3. The nominal stress–nominal strain curve of AZ91 magnesium alloy preparedby SFSP deformed at 623 K and 2�10�2 s�1.

F. Chai et al. / Materials Science & Engineering A 590 (2014) 80–8782

Fig. 4. OM image of the SFSP AZ91 magnesium alloy within the gauge section and near the grip region along the tensile direction at strains of 270% (a: gauge section, b: gripregion); 510% (c: gauge section, d: grip region); 750% (e: gauge section, f: grip region); 990% (g: gauge section, h: grip region) (the horizontal is tensile direction in thispaper).

F. Chai et al. / Materials Science & Engineering A 590 (2014) 80–87 83

3, 5, 7 and 9 min, respectively. When the tensile test time of thetensile specimens is 3, 5, 7 and 9 min, the corresponding β-Mg17Al12 particle density per square millimeter is about 120,000,80,000, 60,000 and 30,000, respectively. In addition, some cavitiesare observed in Fig. 6a and c, which are shown by arrows. Asshown in the figure, most of cavities exist in the triple junctions ofgrains.

It is accepted that grain boundary sliding makes a substantialcontribution to the deformation process of the superplastic flow[28,29]. The surfaces of the deformed specimens are observed toinvestigate grain boundary sliding. The surfaces of the specimenswithin the gauge region along the tensile direction deformed todifferent strains are shown in Fig. 8. Grain boundary sliding can beseen in all the tensile specimens deformed to four different strains,and the evidence of grain boundary sliding is more obvious athigher strains. At a strain of 270%, the microstructure is still fineand remains equiaxed (Fig. 8a). With the strains increasing, thegrain sizes become coarser. When the specimen is strained to

failure, the topography of the deformed specimen shows that grainboundary sliding occurs significantly (Fig. 8d).

4. Discussion

4.1. Grain size of the SFSP AZ91 magnesium alloy for HSRS

Superplasticity is the ability of the material to exhibit highuniform plastic deformation prior to failure, often without theformation of a neck. To achieve good superplasticity, there areseveral requirements for the microstructure: (1) equiaxed grains(equiaxed ratio less than 1.4); (2) fine microstructure (grain sizeless than 10 μm); (3) double-phases microstructures and (4) goodthermal stability [30]. In general, superplastic elongations aredefined as elongation higher than 200% and HSRS refers tothe occurrence of these elongations at strain rates not lowerthan 10�2 s�1 [10,31]. The superplastic flow takes place bygrain boundary sliding and the associated accommodationprocess [29,32]. A strain rate for superplastic flow is described as

Fig. 5. Variation of the average grain size and grain aspect ratio of the initial anddeformation at 623 K and 2�10�2 s�1.

Fig. 6. SEM micrographs of the SFSP AZ91 magnesium alloys showing the β-Mg17Al12 particles with the gauge region deformed to intermediate strains at 623 K and2�10�2 s�1: (a) 270%; (b) 510%; (c) 750% and (d) 990%.

Fig. 7. Variation of the β-Mg17Al12 particle size and density with tensile test time.

F. Chai et al. / Materials Science & Engineering A 590 (2014) 80–8784

follow [33]:

_εsp ¼10DgbGb

kTbd

� �2 sG

� �2ð1Þ

where Dgb is the coefficient for grain boundary diffusion, G is theshear modulus, b is the Burgers vector, k is Boltzmann's constant, Tis the absolute temperature, d is the spatial grain size and s is theflow stress. Due to the inverse dependence on grain size with apower of 2, Eq. (1) demonstrates that superplastic flow occurs athigher strain rates when the grain size is reduced. Figueiredo et al.concluded that there are two possible strategies for achievingHSRS in magnesium alloys: (1) improving thermal stability of themicrostructure and (2) preparing finer initial grain sizes byintroducing additional grain refinement [34].

Most recently, Kim et al. developed a model showing that thedeformation behavior of the fine-grained magnesium alloys atelevated temperature could be well-depicted by using the con-stitutive equation as follows [35]:

_ε¼ A1Dn

ef f

d2sE

� �2þA2

Def f

b2sE

� �5ð2Þ

where A1 and A2 are the material constants, E is the elasticmodulus for pure magnesium [36], d is the true grain size, Dn

ef f isthe effective diffusivity (¼DLþχfgbDgb, where DL is the latticediffusivity for pure magnesium [37], χ is a constant (¼1�10�2)[36], Dgb is the grain boundary diffusivity for pure magnesium[36], fgb is the fraction of atoms associated with grain boundaries(¼πδ/d, where δ, the width of grain boundary, is assumed to be2b)), Def f is another effective diffusivity (¼DLþα50(s/E)2Dp, whereDp is the pipe diffusivity for pure magnesium (¼Dgb) [37]), fp is thefraction of atoms associated with dislocations (¼50(s/E)2 [37])and α is a constant (¼0.016 [38]). This model makes it possible tocalculated the optimum grain size for superplastic deformation ata certain strain rate. For many fine-grained Mg–Al–Zn alloys,A1¼7.59�108 and A2¼6.67�108 [36]. As for AZ91 magnesiumalloy, the grain size predicted by Eq. (2) at 623 K and 2�10�2 s�1

is 1.08 μm, which is reasonably close to the experimental value of1.20 μm determined by TEM analysis (Fig. 2). In Kim's research ofan ultrafine-grained Mg–Zn–Zr alloy, the grain size predicted byEq. (2) at temperatures below 523 K was 1.74 μm, which was alsoclose to that measured by EBSD analysis (¼1.93 μm) [11]. Wata-nabe et al. also reported that the grain size of PM ZK61 alloypredicted by Eq. (2) and measured by TEM were 0.51 μm and0.65 μm, respectively [20]. Therefore, there exists small deviationbetween the predicted and the measured grain size. In this paper,the relatively good fit of the experimental data indicates that theSFSP AZ91 magnesium alloy can achieve an excellent elongation athigh strain rate. As a comparison, Cavaliere et al. produced a FSPAZ91 magnesium alloy with an average size of �4 μm, and theductilities of the alloy were lower than 100% at a strain rate of1�10�2 s�1 and 498 K [24]. In our previous study, an elongationof �180% was attained at 2�10�2 s�1 in FSP AZ91 alloy with anaverage size of �3 μm [39]. The grain size of these AZ91 alloysmay be too coarse for the superplastic deformation to take place athigh strain rate, so their HSRS behaviors are not as good as thatreported in this study. Mohan et al. reported that an ultrafine-grained AZ91 alloy with an average grain size of 0.5 μm preparedby FSP exhibited excellent HSRS, with elongations above 200% atall the experimental temperature and a strain rate of 2�10�2 s�1

[40]. Watanabe et al. prepared a fine-grained (1.7 μm) AZ91magnesium alloy by hot extrusion, and large elongations of over300% was obtained at a high strain rate of 1�10�2 s�1 [27]. It isconsidered that a grain size of about 1 μm or less is necessary forAZ91 magnesium alloy to achieve excellent HSRS within a certaintemperature range.

4.2. Microstructure stability of the SFSP AZ91 magnesium alloyfor HSRS

Except for a fine microstructure, microstructure stability isanother prerequisite for achieving HSRS. As shown in Fig. 2c, thecoarse β-Mg17Al12 networks in the as-cast AZ91 alloy disappear

Fig. 8. SEM images showing surface morphologies of the specimens along the tensile direction deformed at 623 K to a strain of (a) 270%; (b) 510%; (c) 750% and (d) 990%.

F. Chai et al. / Materials Science & Engineering A 590 (2014) 80–87 85

after SFSP, and most of the β-Mg17Al12 particles are preferentiallylocated at grain boundaries. Owing to the pinning effect of theβ-Mg17Al12 phase particles on grain boundary movement, thermalstability of the SFSP AZ91 alloy can be improved. Fig. 4a, c, e and fshows the microstructures of the SFSP specimens within the gaugeregion along the tensile direction at different strains. At the firststage of deformation, the grain size increases to 2.8 μm. Withtensile test time increasing, the microstructure of the SFSP AZ91alloy grows coarse slightly. Even finally fractured at an elongationof 990%, the microstructure is still fine and the average grain size isabout 5.5 μm, which is within the grain size range for attainingsuperplasticity. That is to say, the fine-grained microstructure canbe retained during the high temperature tensile test. Fig. 7 showsthe variation of the β-Mg17Al12 particle size and density withtensile test time. Two things are clear. First, the amount ofβ-Mg17Al12 particles decreases with strain increasing. Second, asthe superplastic deformation proceeds, there is an increase inparticle size. Feng et al. conducted differential scanning calorime-try (DSC) analyses on FSP AZ91 magnesium alloy, and the resultsshowed that there was a broad exothermic peak around 523 K andbroad endothermic peak around 643 K for the FSP AZ91 samples[41]. The endothermic peak might be associated with the dissolu-tion of β-Mg17Al12 particle, and this may be the reason for theamount of the β-Mg17Al12 particles decreasing in the SFSP AZ91specimens tested at 623 K. Moreover, the average size of the β-Mg17Al12 particles increases from 1.72 μm at the strain of 270% to2.65 μm when failed (0.25 μm in the initial condition) duringsuperplastic deformation. The average particle growth rate is�0.0025 μm/s. Yang et al. studied the enhanced superplasticityin FSP Mg–Gd–Y–Zr alloy, and they observed the average size ofparticles increased from 1 μm before deformation to 3 μm attemperatures (688 and 698 K) and a strain rate of 1�10�3 s�1

after superplastic deformation; the coarsening rate of the particleswas about 0.0003 μm/s [42]. Although the particle coarsening rateof β-Mg17Al12 particle is much higher than that of rare-earthcontaining particle, microstructure of SFSP AZ91 alloy is still fineenough during high temperature tensile test. The relatively fine β-Mg17Al12 particles pinning at the grain boundary play a significantrole in providing thermal stability to the SFSP AZ91 microstructureunder high temperature conditions. Furthermore, it is well knownthat the best condition for superplasticity is the material with amicro-duplex microstructure [43]. As shown in Fig. 6, the SFSPAZ91 alloy with the grains of the magnesium matrix and β-Mg17Al12 present on a micrometric scale and equiaxial in shapecan be seen as a micro-duplex microstructure, which is respon-sible for its HSRS.

DRX is also one of the main reasons for maintaining the fine-grained structure during superplastic deformation. As shown in

Fig. 5, microstructures in the grip and gauge sections of the tensilespecimens, which are heated at the same temperature for thesame period of time, are different with each other. Compared withthe grip regions, α-Mg grains in the gauge sections are much finer,indicating that the microstructure evolution mechanisms aredifferent. Materials in the gauge region may experience twocompeting processes, i.e. grain growth caused by the high tem-perature and DRX caused by tensile deformation. In general,superplastic materials with fine-grained microstructures prior totensile test show obvious strain hardening due to grain growth,grain elongation and grain clustering effects, and no apparentstrain softening behavior in the initial stage of superplasticdeformation can be seen [44,45]. However, the SFSP AZ91 magne-sium alloy shows apparent strain softening behavior at a nominalstrain 40.6 as shown in Fig. 4, suggesting that DRX appearsduring the superplastic deformation. Tan et al. reported a similarphenomenon in Mg–3Al–1Zn alloy sheet [46]. One the other hand,materials in the grip regions experience grain growth only sinceDRX cannot take place in static condition. Therefore, microstruc-tures in the grip regions are coarser than those in the gaugeregions. Due to the competition between DRX and the concurrentgrain growth processes, the grain in the gauge region does notchange significantly. This is the other reason for the excellent HSRSin the SFSP AZ91 magnesium alloy.

4.3. Failure mechanism of the SFSP AZ91 magnesium alloy duringHSRS

Fig. 9a and b shows the SEM images of the SFSP AZ91magnesium alloy showing cavities during high temperature tensiletest at strains of 270% and 990%, respectively. In Fig. 9a, it can beseen that some second phase particles are divorced from thematrix under the tensile stress, and cavities are nucleated aroundthese particles. It was reported that small particles have a smallconstrained plasticity zone, thus cavities are ready to appeararound these particles [47]. Lü et al. also reported that the body-centered cubic (BCC) structure of Mg17All2 was incompatible withthe HCP structure of magnesium matrix and Mg17All2 itself wasrelatively soft, which led to the fragility of the Mg/Mg17All2interface and even in the Mg17All2 particles [48]. Moreover, atthe failure strain of 990%, some cavities are observed in the triplejunction of grains (Fig. 9b), which also can be seen in a strain of510% (Fig. 6c). Stress concentration is easily triggered at the triplejunction of grains, so cracks and cavities are formed in theseregions. Cavaliere et al. also reported that the tensile testedspecimen of FSP AZ91 magnesium alloy was characterized bytriple-junctions fracture [24].

Fig. 9. SEM images of the SFSP AZ91 magnesium alloy showing cavities during tensile test along the tensile direction at 623 K and 2�10�2 s�1 at strains of (a) 270% and(b) 990%.

F. Chai et al. / Materials Science & Engineering A 590 (2014) 80–8786

In our previous study, the elongations of FSP Mg–Al–Zn alloys,including FSP AZ31 magnesium alloy with a single-phase micro-structure [23] and FSP AZ91 magnesium alloy [39], are almostbelow 200% at high strain rates, while the results in the SFSP AZ91magnesium alloy show the effectiveness of using the FSP techni-que with water cooling to attain excellent HSRS [19]. According tothe investigation on microstructure evolution of the SFSP AZ91alloy during HSRS, the initial fine-grained microstructure preparedby SFSP, the good thermal stability caused by the second phaseparticles, and the balance between grain growth and DRX areresponsible for the excellent HSRS.

5. Conclusions

Microstructure evolution of the fine-grained AZ91 magnesiumalloy prepared by SFSP during superplastic deformation at highstrain rate of 2�10�2 s�1 is investigated. The results are summar-ized as follows:

1. Through SFSP, both α-Mg grains and β-Mg17Al12 phase aresignificantly refined. The average grain size of SFSP AZ91 alloyis about 1.2 μm, which is close to the calculated grain size toachieve HSRS within a certain temperature range.

2. After failure at a strain of 990%, the average size of α-Mg grainsand β-Mg17Al12 particles is about 5.5 μm and 2.6 μm, respec-tively. The grain aspect ratios remain approximately 1.0 indifferent straining stages. Due to the pinning effect ofβ-Mg17Al12 particles and the occurrence of DRX, the finemicrostructures are maintained in the whole superplasticdeformation process.

3. The excellent HSRS of the SFSP AZ91 alloy is attributed to theinitial fine-grained microstructure and the good thermalstability.

4. During superplastic deformation, cavities are mainly formed attwo kinds of positions, i.e. in the triple junctions of grains andaround β-Mg17Al12 particles.

Acknowledgment

This work was sponsored by the Fundamental Research Fundsfor the Central Universities (No. 2012ZZ0051).

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