Superplastic deformation behavior of Mg alloys: A-review

Faisal Nazeer, Jianyu Long, Zhe Yang, Chuan Li

School of Mechanical Engineering, Dongguan University of Technology, Dongguan 523808, China

Abstract The magnesium alloys are the next-generation lightweight metal materials; however, the production of the complex shape components of thermo-mechanical processed magnesium alloys is very limited in comparison to their die-casted counterparts.Both high strain rate and low-temperature processing are required to save time and cost for the project.Uniaxial tensile testing has been vastly used for determining the plasticity of magnesium alloys at elevated temperatures.Therefore, this review article sheds light on superplastic behaviors under the uniaxial tensile loading of different magnesium alloys.This review also emphasizes low and high-temperature superplasticity, high strain rate superplasticity, deformation mechanism, thermal stability, texture evaluation, and fracture mechanism of extruded/rolled and severe plastic deformed magnesium alloys.At the end of the review, some technical issues have been addressed which can be beneficial for further enhancement in the superplasticity of magnesium alloys.

Keywords: Superplasticity; LTSP; HSRSP; Thermal stability; Texture; Fracture.?Corresponding author.

1.Introduction

One of the primary problems for human development is the excessive emission of CO2which is responsible for global warming.In this view, the use of lightweight metal materials is an excellent approach to increase fuel efficiency in automobile and aerospace applications [1-7].Besides, they are also being used as an alternate material in biomedical implants and also a natural component of bone tissue[8,9].Magnesium(Mg) and its alloys are lightweight materials having density(～1.8 g/cm3) which is 2/3rd of aluminum (Al) (～2.7 g/cm3)and 1/4th of steel (～7.8 g/cm3) and attracted significant interest of researchers owing to high stiffness, excellent corrosion resistance, high damping capacity and good recyclability[7,10-15].However, the potential widespread applications of Mg alloys are restricted by their limited formability at room temperature [16,17].Consequently, Mg alloys need to be processed at elevated temperatures.

It is well known that superplastic forming is an excellent method to address the low formability issue at high temperatures because the non-basal slip can be activated owing to a decrease in critical resolved shear stress (CRSS) in the early stages of deformation and twinning can be exhausted at higher temperature deformation which in turns leads to steady-state flow.Although for large-scale production superplastic forming is not recommended, however, it is logical and more reasonable to use superplastic forming (SPF) for reducing the cost and time of material fabrication.Near-net shape and complex shape components can be formed by SPF.The phenomenon of superplasticity is very interesting for both academic and industrial development owing to its ability to fabricate intricate products from Mg alloys.Besides it is capable to exhibit uniform high elongation without necking or pre-mature failure.

For superplasticity the grain size is a worthy parameter,it is well described that the governing deformation mechanism was grain boundary sliding (GBS) if the grain size was＜15 μm,since this mechanism does not occur for grain sizes higher than about 15 μm.Contrary,the solute drag creep may be dominant deformation in grain size＞15 μm under high-temperature deformation.Besides the other important factor is strain rate sensitivity (mvalue), if the value is near 0.5 then GBS is considered as dominant deformation otherwise if the m-value is＜0.3 the solute drag creep mechanism was dominated mechanism.Another factor under high temperature loading is activation energy (Q-value), different studies reported different Q-values; however, the Q-values do not have any big influence on GBS, while GBS is dependent on the lattice self-diffusion.Therefore, grain size and m-value are important parameters for achieving the superplasticity of Mg alloys.

From a mechanistic point of view, Mg is comprised of a hexagonal closed pack (HCP) crystal structure.The {0002}basal slip and {10-10} prismatic slip have same＜a＞type of burger vector, so their activity cannot accommodate plastic deformation at room temperature [18,19].A precise study reveals that the deformation is accommodated by only basal slip at the early stages of deformation due to low critical resolved shear stress(CRSS)[20].While,the other＜c＞and＜c+a＞non-basal slip activity can be achieved in later stages of deformation owing to high CRSS [21].Therefore, to accommodate deformation, twinning is a distinctive mechanism especially in Mg alloys in the early stages of deformation [22].To date, different types of twinning have been reported which accommodate the strain energy.However, {10-11}＜10-12＞contraction twinning, {10-11}-{10-12} double twinning and low slip activity leads to early failure [23-27].The twinning is strain path and temperature dependent, so the CRSS of the non-basal slip system decreased at the higher temperature.Therefore, Mg alloys are processed at higher temperatures and the cost-effective process is SPF.This practical technique can fabricate large and complex products; however thickness non-uniformity of the details produced by SPF is a serious problem e.g.in manufacturing spherical vessels.In the SPF technique, appropriate micro-structure is required to deform by grain boundary sliding (GBS) which was reported by refs.[16,28-31].Commonly, appropriate micro-structure for GBS phenomenon comprised of highly misoriented fine grains (contained large fraction of high angle grain boundaries),which can be obtained through thermo-mechanical processes such as severe plastic deformation (SPD), equal channel angular pressing (ECAP), friction stir processing (FSP)and high pressure torsion (HPT) [32-34].The products of these processes are only justified for the specific application.Besides, these processes are cost-ineffective, time consuming, and have limited production.The SPD process always produces a weak texture, which enables high elongation in Mg alloys.This aspect is very important, in extruded alloys the texture is strong basal/fiber which can restrict the high elongation.Moreover, extruded Mg alloys can produce a bimodal grain structure after extrusion,while extrusion followed by annealing can provide an equiaxed grain structure.Therefore, based on large size grains, the low m-value, and high n-value, the superplasticity in extruded Mg alloys is difficult to achieve.

Fig.1.SEM analysis of deformed specimen at a strain rate 10?2 s?1 (a) at a temperautre 300 °C (b) 350 °C (c) 400 °C (d) 450 °C [36].

The grain growth happened due to prolonged exposure of Mg alloy at elevated temperature resulting in thermal instability or abnormal grain growth.Therefore, superplasticity was not observed in single-phase alloys or pure metals because they have intrinsically low microstructure thermal stability,while prolonged exposure time can lead to grain growth which restricted the superplasticity.Moreover,some researchers used different elements in cooperation as second phase particles to avoid grain growth through pinning the grain boundaries.For example, Cizek et al.[35]fabricated an Mg-Gd alloy and reported that high-pressure torsion (HPT) processed Mg alloy shown much lower grain growth in comparison to pure Mg.This was attributed to Mg-5Gd particles.Similar results were reported in the study of Bayat-Tork et al.[10].

The high strain rate superplasticity (HSRS) is the keen interest of researchers; this process can save time and energy.Many studies deal with the HSRS of Mg alloys.Most specifically HSRS has been reported in ultra-fine grained or fine-grained microstructure which was prepared by FSP, HPT,ECAP, high ratio differential speed rolling (HRDSR), and indirect extrusion.However, this aspect is very limited.This review article is an attempt to summarize studies related to the superplastic behavior of Mg alloys and their dominant deformation mechanisms.

2.Superplasticity in extruded/rolled alloys

It is commonly perceived that the grain morphology in extruded/rolled Mg alloys is comprised of refined and large equiaxed grains except for those who processed more than 4 passes of rolling.The texture in this type of alloy is a strong basal/fiber basal texture.álvarez-Leal et al.[36]processed ZK30 Mg alloy through an extrusion and reported superplastic behavior with elongation to failure ～360% at a strain rate of 0.001 s?1under temperature 450 °C.Based on the n-value(tends to 2), the average Q-value (130 kJ/mol), and topography of the samples (provided in Fig.1), it was proposed that the GBS was controlling the deformation mechanism.

They reported a bimodal grain structure with a large number of small grains and a few big coarse grains.However,theyobserved a minor increase in grain size (2, 4, and 6 μm) under high temperatures loading (300, 350, 400, and 450 °C).This continuous grain growth under such high temperatures was attributed to microstructure thermal instability.They also reported that the large coarse grain was refined under a superplastic window but existed at low temperatures.Kim et al.[37]processed a fine grain (1.6 μm) Mg-6Zn-0.5Zr alloy by low-temperature indirect extrusion and achieved tensile elongation ～800% at a high strain rate of 0.01 s?1.This high elongation was attributed to the m-value 0.5 and grain morphology (Fig.2).

Fig.2.(a,b) Low magnification EBSD orientation maps, inverse pole figures, and KAM maps of the extruded ZK60 alloy after tensile elongations of (a) 200 and (b) 800% at 0.01 s?1 and 250 °C.The EBSD orientation maps and inverse pole figures refer to the ED [37].

The microstructure also revealed that the grain size was slightly increased ～0.6 and 1 μm at testing temperature 250 °C compared to as-received Mg alloy.The fine MgZn2precipitate contributed to stabilizing the grain structure and therefore facilitated in achieving the high elongations ～800%.For internal strain hardening, the Kernel average misorientation map (KAM) values represented that the higher elongation contained low KAM values.Malik et al.[16]studied the superplastic behavior of fine-grain ZK61 Mg alloy and reported an elongation of ～400% at a temperature of 400 °C and strain rate of 0.001 s?1.

Moreover, the microstructure was thermally stable and attributed to MgZn2phase particle which restricted the grain growth.However, they also reported a slight increase of grain size under high-temperature loading as shown in Fig.3(a-d).Recently, Chaudry and Hyperlink [38]reported that incorporation of Ca can significantly increase the superplasticity of AZ31 Mg alloy.The monolithic AZ31 Mg alloy and AZ31-Ca Mg alloy exhibited an elongation to fracture ～199% and～320%, respectively.They also suggested that the second phase particles((Mg,Al)2Ca)and their distribution have a big influence on the pinning effect and restricted the grain growth.Moreover,the m-value was increased from 0.29 to 0.31 which also influence an increase in the elongation to fracture.Yu et al.[39]studied the superplasticity of an AZT910 Mg alloy(extruded+ 16 passes of rolling) along with RD, 45RD, and TD at a strain rate of 0.001 s?1.They found an exceptional increase in elongation to fracture ～698, 525.8, and 727.5 along with RD, 45RD, and TD, respectively.They believe that the fine grains and spherical Mg17Al12particles were advantageous for the GBS mechanism.Wang et al.[40]processed a fine grain structure ZK60 Mg alloy and show that anisotropy was decreased at high-temperature tensile loading and a superplasticity (maximum elongation ～376-434%) was achieved at a temperature of 275 °C.Similar to other studies they reported the GBS phenomenon at a relatively low temperature 275 °C.Sun et al.[31]also developed an Mg-Gd-Y-Zn-Zr alloy through an extrusion process and reported an excellent superplasticity at 450 °C and a low strain rate of 0.005 s?1.The superplasticity was facilitated by microstructure thermal stability which was dependent on the 14 LHPSO and Mg24Y5phases together with the fragmentation of Mg24Y5particles.In another study, Hua et al.[41]also reported a high elongation of ～410% in an Mg-Zn-Ca-Sn-Mn alloy and proposed GBS and solute drag creep mechanism.Watanabe et al.[42]fabricated a WE43 Mg alloy through a high extrusion ratio and produced precipitates of diameter 200 nm, their alloy exhibited superplasticity with elongation to failure above 1000% at a strain rate of 0.0001 s?1under a temperature of 400 °C.According to the literature, this elongation to fracture is the highest reported in extruded Mg alloys as shown in Table 1.

Table 1The superplasticity of different Mg alloys under different conditions.

From the aforementioned studies, it can be concluded that both low strain rate and high strain rate superplasticity can be achieved through the extrusion/rolling process.However,which deformation mechanism (GBS, solute drag mechanism,or combined effect of GBS and solute drag creep) will dom-

inant for the elongation in the range of ～300-400% is an open question.

Fig.3.OM analysis and grain size statistical distribution under a strain rate 10?2 s?1(a.b) at 350 °C (c,d) 400 °C [16].

For determining the superplasticity, the important parameters are (i) m-value which should be in between 0.4 and 0.5,(ii) high elongation to fracture (iii) Q-value which should be＜110 kJ/mol (near to GB self-diffusion), (iv) grain morphology (v) low KAM values during deformation and (vi) a slight increase in grain size due to high-temperature loading and must exhibit the stability of microstructure.This stability is attributed to the pinning effect of precipitate which significantly restricted the grain growth discussed in Section 5.

3.Superplasticity in SPD processed alloys

In the previous section, we have discussed the superplasticity of extruded/rolled Mg alloys.The literature regarding superplasticity in extruded/rolled Mg alloys are very limited;however, there are many investigations on severe plastic deformed (SPD) Mg alloys.The SPD process provided opportunities for attaining an exceptional decrease of grain size up to a nanometer scale with a homogenous microstructure.Usually, in other metals, the refined grains of several nanometers(nm) lead to low ductility such as; Al and Cu, contrary, in Mg alloys, the fine grains exhibited high ductility due to high slip activity [58-60].Therefore, the superplasticity can easily be achievable in fine grained Mg alloys.Thus, in this section;we will elaborate on the superplasticity of SPD processed Mg alloys.

Xing et al.[61]reported an ultra-fine grained (300 nm)high strength (530 MPa) and superplastic AZ31 multidirectional forged (MDFed) Mg alloy.The uniform elongation was 300% at a temperature of 150 °C.Based on elongation and temperature this superplasticity was referred lowtemperature superplasticity (LTSP).In another investigation,Kai et al.[62]reported an excellent elongation ～800% at a similar temperature of 150 °C in AZ90 Mg alloy which was fabricated through HPT.Miura et al.[43]also fabricated an ultra-fine grain through a combined process of MDFed and rolling on AZ61 Mg alloy and reported a low temperature of 200 °C and a very low strain rate of ～0.000083 s?1superplasticity.This superplasticity was attributed to very fine grain structure and GBS deformation.Kandalam et al.[44]also reported a superplasticity (tensile elongation ～ 470%) in WE43 Mg alloy having grain size (6 μm) after multi-axial forging at a high temperature(375°C)and very low strain rate of (0.0003 s?1).In one another study [55]ultra-fine grained(～340 nm) WE43 Mg alloy was fabricated through ECAP and reported an exceptional increase in elongation ～1230%at a strain rate of 0.01 s?1under temperatures 350-400 °C.It is worth noticing that they have a very high elongation～1000% at temperature 400 °C under a strain rate of 0.1 s?1.Their microstructure analysis revealed that after testing the grain growth did not happen and even plasticity controlled growth of cavity did not occur at high temperature 450 °C.

The ECAP process was also conducted on a ZK10 Mg alloy and obtained a grain size ～5.2 μm after four passes and reported an elongation to fracture 550% and 750% at temperatures 200 °C and 250 °C under a strain rate of 0.0001 s?1,respectively[56].They also reported a transition to a different flow mechanism (viscous glide of dislocation) under a high strain rate with an n-value ～3.3 and Q-value ～112 kJ/mol(the intermediate value between GBS and lattice diffusion).Figuereido and Langdon [57]reported a record of uniform elongation 3050% in an ECAPed processed ZK60 Mg alloy at a relatively low strain rate of 0.0001 s?1.This uniform elongation was attributed to the optimum combination of grain refinement and structural stability.In another study,Figuereido and Langdon [46]conducted an ECAP process on AZ31 Mg alloy reported a grain size ～2.2 μm and obtained elongations＞1000% at a strain rate of 0.0001 s?1under temperatures 350-450 °C.They also conducted an annealing process at 400 °C for 30 min and found a coarse grain microstructure comprised of grain size ～6 μm which suggested that the material was thermally unstable at high temperatures but still they reported superplasticity.

Kai et al.[62]processed Mg-9%Al alloy through the HPT process and obtained a fine grain size.Based on fine grain size they reported elongation of 810% at a very low temperature 200 °C under a strain rate of 0.0005 s?1.They suggested that for grain refinement HPT process is more advantageous than the ECAP process.The HPT process was also conducted on ZK60 Mg alloy by Torbati-Sarraf and Langdon [47].After five revolutions the grain size of extruded Mg alloy (9.4 μm)was significantly reduced to 1 μm.Consequently, a superplastic behavior was reported with an elongation of 535% at a low strain rate of 0.0001 s?1under low temperature 200 °C.Torbati-Sarraf et al.[48]also conducted HPT on ZK60 Mg alloy and produced an ultra-fine grain size ～700 nm.They made a comparison of a superplastic behavior achieved by miniature tensile testing and miniature shear punch testing and concluded that both processes are practical for demonstrating excellent superplasticity.They achieved maximum elongation～940% at a strain rate of 0.0001 s?1under temperature 250 °C.A more complete and precise study of superplasticity on HPT processed AZ91 Mg alloy under different temperatures and strain rates were conducted by Al-Zubaydi et al.[45].According to their study,they achieved the highest value of elongation to fracture 1308% at a temperature of 300 °C under a very low strain rate of 0.0001 s?1.Moreover, the high strain rate superplasticity (at the strain rate of 0.01 s?1)was also achieved at 200 °C and 300 °C having elongation to fracture 590% and 860%, respectively, and reported GBS was the dominant deformation mechanism.Regarding low temperature (150 °C) superplasticity AZ91 Mg alloy also exhibited an elongation of 660% and 760% under a strain rate of 0.001 s?1and 0.0001 s?1, respectively.While for LTSP the governing deformation mechanism was glide dislocation creep accommodated by GBS.Lapovok et al.[63]conducted equal channel angular extrusion (ECAE) on rolled ZK60 Mg alloys and discussed that the ECAE process is beneficial for superplasticity (～1400-2040%) in low strain rate regime of(0.003-0.0003 s?1).Contrary at higher strain rate ECAE and rolled+ECAE both have similar elongation to fracture.

Kim et al.[28]fabricated ZK60 Mg alloy through HRDSR and attained a fine grain structure (grain size 1,2 μm).They reported both low (0.0005 s?1) and high strain rate (0.01 s?1)superplasticity.However the most optimum value ～1000%was recorded at 280 °C under strain rate 0.001 s?1.The possible deformation mechanism was reported to be GBS and slip creep.Kwak and Kim[29]again studied the effect of HRDSR on ZW132 Mg alloy and revealed that the microstructure was thermally stable at 280 °C and an excellent elongation of 1021% was achieved under a strain rate of 0.001 s?1.They observed that the grain coarsening beyond 280 °C leads to deterioration the superplasticity.

MD and Panigrahi [52]fabricated an ultra-fine grain QE22 Mg alloy through FSP and investigated hightemperature tensile testing under various strain rates and revealed that the alloy displayed dual-mode superplasticity.They reported that the alloy exhibited a maximum elongation of 1630% at a higher strain rate of 0.01 s?1under a temperature of 450 °C.While at low temperature 350 °C and low strain rate of 0.003 s?1, the alloy showed an elongation of 850%.A commercially available extruded ZK60 Mg alloy was subjected to the FSP process and obtained a fine grain structure (2.9 μm).The superplasticity (1390%) was achieved at 300 °C under a strain rate of 0.0003 s?1.To reduce the grain size Wang et al.[49]also used the FSP process on AZ80 Mg alloy and reported a maximum elongation of 606%at a temperature of 350 °C under strain rate of 0.0003 s?1.Multi-pass FSP was performed on AZ91 Mg alloy to generate layered microstructure through the thickness and subjected to fix high-temperature tensile testing at 350 °C at strain rates 0.005, 0.001, and 0.0005 s?1.They reported that the alloy exhibited superplasticity at all strain rates, however, maximum elongation of 680% was reported at a strain rate of 0.0005 s?1[53].Recently, Zhou et al.[54]fabricated Mg-Li-Zn alloy with the average grain size ofα-phase 0.61 μm andβ-phase 0.96 μm through FSP.According to their analysis low temperature and high strain rate superplasticity (369%)was achieved at a temperature of 200 °C.Moreover, at a similar low temperature and low strain rate 0.0001 s?1, the alloy exhibited an exceptional increase in elongation to fracture of 1104%.Some other studies also reported superplasticity by using the chip/ribbon technique and flake powder metallurgy(FPM).However, the maximum elongation in FPM is very low while through the chip/ribbon technique the maximum elongation was ～1000% as shown in Table 1.

After a thorough analysis it is concluded that;

1.During high strain rate superplasticity at low temperature dislocation viscous glide followed by GBS is the dominant deformation mechanism.

2.During low and high strain rate superplasticity at high temperature GBS is governing deformation mechanism.

3.The elongation to fracture is also dependent on precipitates and microstructure thermal stability.(Discussed in Section 5).

4.The very high strain rate of (0.1 s?1) superplasticity (elongation to fracture ～1000%) was achieved in WE43 Mg alloy but at a relatively high temperature (400 °C).

4.Deformation mechanisms

Superplasticity is usually observed in uniaxial tensile testing and shear punch testing(SPT).The SPT method is specifically advantageous for situations where the amount of material is available only in small quantities as in the samples produced by SPD processes.Until now these two processes have been extensively conducted for determining the superplasticity of Mg alloys.Torbati-Sarraf et al.[48]reported a comparison between SPT and conventional tensile testing and recommended that both techniques are practically applicable for determining superplasticity.As from the previous section,it is crystal clear that the very fine grain size, uniform microstructure, high m-value, low n-value and thermal stability are required for achieving superplasticity.Under such aforementioned parameters, the obvious deformation mechanism is GBS.álvarez-Leal et al.[64]used friction stir processing (FSP) on coarse-grained WE54 Mg alloy and produced microstructure comprised of ultrafine grains and coarse precipitates.The material exhibited a high elongation of 726%at 400°C under a strain rate of 10?2s?1.Beyond this temperature, they have lost thermal stability and superplasticity has deteriorated.The proposed GBS is the dominant deformation mechanism.However, the high Q-value is associated with very high temperature and the long exposure time during tensile testing leads to an increase in grain size which shows instability and produced cracks, and restricted the further elongation to failure.

Creep usually consists of glide and climb processes which is a sequential process while the solute drag mechanism is a dislocation creep mechanism.The elongation of the grains after deformation is usually happened due to these two processes.The glide is a rate-controlling process while the climb is the slowest of the processes and hinders the movement in the case of dislocation.Ruano et al.[65]reported a solute drag to creep mechanism as the principal deformation mechanism in WE54 Mg alloy.The highest elongation was～312% under a temperature of 450°C at a strain rate of 10?2s?1.The initial grain size was 150 μm, therefore, this achievement is remarkable.The microstructure of deformed specimens appeared strongly elongated.They suggested that this type of microstructure eliminated the GBS as a possible deformation mechanism because uniform microstructure with grain size＜15 μm are the requirements for the GBS mechanism.The m-value was ～3 which suggested that the solute drag creep mechanism is the dominant deformation in WE54 Mg alloy.Hua et al.[41]fabricated a low alloyed Mg-Zn-Ca-Sn-Mn alloy and reported m-value ～0.32 and suggested that superplasticity was co-dominated by GBS and solute drag creep mechanism.They also suggested that co-segregation of Ca and Zn elements at the GB are not favorable for GBS,and thus thermal stability of great importance to achieve superplasticity.Wu and Hsu [66]calculated m-value 0.27 inAZ31 Mg alloy, however, the fine-grain microstructure favors the active operation of GBS deformation.In addition,uniform microstructure i.e., equiaxed grains with uniform distribution impeded the void formation and thus increase the elongation to failure.For low-temperature superplasticity,the GBS or solute drag mechanism is an obvious deformation mechanism.The reason is average m-value,Q-value,n-value and low elongation to fracture which lies in the superplastic regime.Some studies reported low temperature and high strain rate superplasticity, however, according to literature the GBS should be governing deformation mechanism together with m-value in the range 0.4-0.5, while the Q-value and FE are decisive parameters for the appropriate deformation mechanism.Based on Arrhenius type formulas [67]for activation energy, it is obvious that under a low-temperature regime we usually get high Q-value and high n-value which leads to strain hardening in the early stages of deformation and might leads to solute drag creep mechanism.Hence, for very low temperature and high strain rate the prediction/observation of accurate deformation behavior is still an open question.Therefore it is recommended to process the material at the highest temperature where it shows thermal stability.As we know that under high temperature the CRSS values of non-basal slip reduces significantly therefore after yielding in very early stages of deformation dislocation slip and GBS are the preferentially operative deformation mechanisms.While in later stages of deformation smooth flow leads to extensive GBS deformation mechanism.Some other studies also reported solute segregation-assisted superplasticity mechanism along with GBS for achieving superplasticity [41].

5.Thermal stability

Thermo-mechanical processing specially FSP and ECAP can produce very fine grain size.For testing at elevated temperatures in the range of 250-450 °C, the material is required to expose for a long time to achieve this high temperature.This temperature can increase the grain size or change the morphology and volume fraction of precipitates.Therefore the thermal stability and structural stability during deformation are the two main parameters that must be controlled so that the grain size must not go beyond the critical size＞10 μm.The grain size is responsible for achieving low temperature and high strain rate superplasticity and resistance to damage development.Whereas the structural stability leads to an improvement in strain rate sensitivity which is further linked with the resistance against the occurrence of any tensile abnormality and controlled the elongation to fracture.

The precipitates are the best medium to resist the recrystallization process to achieve the required testing temperature.Once after achieving the testing temperature, these precipitates provide a Zener pinning effect and maybe a medium to dynamically recrystallize the grain size during deformation.Kim et al.[37]found MgZn2precipitates at a tensile strain of 0.2 under a strain rate of 0.01 s?1at temperature 250 °C and suggested that these precipitates can contribute to stabilizing the grain structure and promoted the superplasticity.Md and Panigrahi [52]also described that the well-dispersed particle (Mg12Nd eutectics and Mg12Nd2Ag precipitates) along the grain boundaries provided the Zener pinning effect and salute dragging from the segregation of Nd solute atoms at elevated temperatures 650 °C obstacles substantial grain growth during superplastic deformation.Sun et al.[31]reported the thermally stable brittle Mg24Y5and 14H LPSO during superplastic deformation in Mg-Gd-Y-Zn-Zr alloy under temperature 450 °C and suggested that these particles resisted the grain growth and lead to elongation to fracture 972%.Hua et al.[41]also observed solute segregation and precipitates which stabilizes the microstructure (different strains of 5, 100, 200%, at the grip, and near fracture) at elevated temperature which enhanced superplasticity as shown in Fig.4.Al-Zubaydi et al.[45]fabricated HPT processed AZ91 Mg alloy under room temperature.They reported that finer microstructure displayed better thermal stability and promoted superplasticity at a high strain rate during hot deformation.Malik et al.[16]also witnessed MgZn2phase particle at elevated temperature deformation which was the medium to restrict the grain growth and improved the superplasticity in ZK61 Mg alloy.In another study,Mohan et al.[68]fabricated AZ91 Mg alloy which contained a higher amount ofβ-Mg17Al12precipitate and recommended that micro-structure was thermally stable at higher temperatures which leads to high strain rate superplasticity.According to their analysis, the highest possible temperature at which microstructure is thermally stable is better for superplasticity.Wang et al.[49]observed that the coarsening ofβ-Mg17Al12and grains deteriorated the superplasticity in the FSPed AZ80 Mg alloy.This suggested that the change in precipitate morphology is lethal for superplasticity in Mg alloy.Kandalam et al.[44]show that the precipitate was thermally stable at 375 °C but not at 400 °C.Therefore excellent superplastic elongation was achieved at 375 °C and mainly attributed to thermal stability.Yu et al.[39]also revealed that fine recrystallize grain and near-spherical soft Mg17Al12particles are advantageous for the occurrence of GBS.Chaudry and Hamad [38]also linked thermal stability with second phase particles at the grain boundaries at 300 °C in AZ31 Mg alloy and recommended that these particles activate GBS.

According to the above discussion, it is believed that;

1.The binary or ternary alloys can show superplasticity at different temperatures owing to thermal stability which is co-related with precipitates.Thus thermally stable precipitate can produce structural stability which can resist any tensile abnormality and promote superplasticity.This means that not only the fine grain size but different precipitate in different Mg alloys plays an important role in superplasticity.

2.It is obvious that the melting of these precipitates has different temperatures in different Mg alloys.Therefore it is better to process material at the highest possible temperature at which it displayed thermal stability of microstructure and precipitate which can be beneficial for superplasticity.The other reason for the highest possible thermalstable temperature is Q-value and m-value because at high temperature the Q-value appeared very low ～90 kJ/mol(grain boundary diffusion).As we know that this Q-value stands for the tendency of work hardening therefore low Q-value is beneficial for low n-value and high m-value.These all features together with thermal stable precipitate and grain structure can lead to an exceptional increase in elongation to fracture.

6.Texture evaluation

The texture is a significant parameter that determines the ductility in Mg alloys [69].Commonly, a {0001} basal fiber texture and strong {0001} basal texture has been reported in extruded and rolled Mg alloys, respectively [70,71].On the other hand, the FSPed, ECAPed, and HPT exhibited relatively weak, tilted, and random texture in Mg alloys [45,48,52,55,72].Therefore strength and ductility have an obvious effect due to textural changes under room temperature.While the texture development after loading (compression/tensile) has obvious effects owing to twinning, detwinning, the interaction of dislocation with twinning, twintwin interaction, and dynamic recrystallization [7,21,31,73].However, during superplasticity, GBS or solute drag creep mechanisms have been proposed and under such hightemperature loading,twinning can be exhausted.Therefore the effect of resulting texture after the thermo-mechanical process on the superplasticity of Mg alloys is still an open question.Panicker et al.[74]reported that the crystallographic texture has no significant effect on the superplasticity of AZ31 Mg alloy.Similarly, in another study [75]it was also recommended that there is no significant effect of crystallographic texture on the superplasticity of an ECAPed AM60 Mg alloy.Besides they proposed that at lower temperatures some crystallographic changes were owing to a high n-value.Sun et al.[31]fabricated a peak aged Mg-Gd-Y-Zn-Zr alloy and reported random texture distribution, this texture was transformed similar to the extruded texture under high-temperature loading.Recently, Malik et al.[16]proposed that texture was changed during superplasticity in an extruded ZK61 Mg alloy.Similarly, Yang et al.[51]also recommended that the texture was also weakened during superplastic deformation.Moreover, Lin et al.[76]revealed that the texture influence is temperature dependent i.e., ECAPed AZ31 Mg alloy significantly enhanced low temperature superplasticity, contrary the same texture have no momentous effect under high temperature testing.

In textured Mg alloys, {0002} basal slip is preferentially operatively owing to very low CRSS.Contrary to later stages of deformation other non-basal slips can be activated by achieving high CRSS.It is worth mentioning here that the CRSS of non-basal slip is reduced sufficiently and can be operative under high-temperature tensile loading.A reasonable study suggested that the texture can be weakened through dynamic recrystallization [7].Moreover in another study, it was proposed that the large coarse grains always exhibited strong basal texture and refined grains leads to different orientation and a weak basal texture [77].Alvarez-Leal et al.[36]revealed that the coarse grains were refined during hightemperature superplasticity therefore the refinement of grains can be attributed to the textural changing during superplasticity.Recently, Wang et al.[49]demonstrated that the texture during superplastic flow was changed as shown in Fig.5.According to their study, the {0002} basal texture was spread and weakened which was attributed to the GBS phenomenon.Moreover, the effect of non-basal slip activity was reported to be very low which can be seen from {10-10} to {11-20}pole Figs.5.

A more complete study was conducted by Wang et al.[40].They reported that during high-temperature tensile testing, the grip section and gauge sections of the specimen have a different texture.The grip section contained similar grainmorphology and texture of extruded ZK60 Mg alloy as shown in Fig.6(a).However, the gauge section of ED, TD, and 45D revealed that the grains were dynamically recrystallized under high-temperature deformation and the texture was weak and random as shown in Fig.6(b-d).Besides EBSD analysis they also conducted XRD analysis and proved that basal peak intensity was sufficiently reduced under temperature 275 °C.Based on these results they recommended that the GBS control of deformation in the superplastic regime was responsible for texture weakening in the gauge section.

Fig.6.EBSD maps and the corresponding (0002), (1010) pole figures of as-extruded ZK60 alloy after tension at 275 °C: (a) grip section of ED- sample; (b),(c), (d) gauge sections of ED-, TD-, 45D- sample, respectively [40].

Yu et al.[39]comprehensively performed EBSD analysis at different strains (50, 100, 200, and 300%) along with TD under a temperature of 300 °C.They reported that basal texture was rotated towards TD and the texture at 50% strain was significantly weakened and no obvious change in texture intensity was reported in the strain of 100 and 200%.Further,between 200 and 300%, a decrease of intensity and spreading of texture towards TD occurred.The weakened texture at 300 °C is due to the occurrence of the GBS deformation mechanism.MD and Panigrahi [52]recommended that the temperature, exposure time, and strain rate are influential parameters for weakening the texture after superplastic deformation.According to their study, the texture intensity ～8 units was weakened (as-received ～17.6 units) during tensile deformation at 350 °C under a strain rate of 0.003 s?1.Besides for a high strain rate regime of 0.01 s?1at 450 °C, the texture intensity was ～10 units, for similar temperature but at a very lower strain rate of 0.003 s?1texture intensity was in-creased ～29 units.Thus the aforementioned parameters have a big influence on texture.

From the above studies it can be concluded that;

1.Temperature, strain rate, and exposure time put a big influence on texture weakening and hence on governing deformation mechanism.

2.During high-temperature tensile deformation the low yield strength is attributed to＜c + a＞activity and GBS mechanism which weaken the texture intensity.

3.During high-temperature tensile testing the DRX phenomenon leads to obvious changes in texture.The weakening of the texture promotes the GBS phenomenon.

4.The effect of high strain rate superplasticity at lower temperature on texture weakening and further its influence on GBS mechanism is still plausible and needs more studies to evaluate.

7.Fracture mechanisms

The ductile and brittle behavior of the materials can be studied by fracture morphology under tension/compression loading.Usually, the rough cleavages, cracks, and shear planes depicted the brittle nature of the material.Contrary,the pronounced dimples along with tear ridges justify the ductile nature of the material.There are three types of fractures inter, intra and transgranular fractures.In intergranular fracture, the micro-cracks are along the boundaries of two or more grains while in intragranular fracture the micro-cracks are within the grains.If the micro-cracks are across the grains it should be trans-granular fracture.The fracture morphology under high-temperature tensile testing is commonly based on cavities, melting, cracks, and deep dimples.For example, Sun et al.[31]made a relationship between superplasticity and fracture morphology after high-temperature tensile testing of an Mg-Gd-Y-Zn-Zr alloy.They reported that the specimen with the highest elongation 972% was not broken while the other specimens were broken.They reported that the cavities and dimples were dependent on temperature and strain rate.However, their fracture morphologies suggested that cleavage planes, tear ridges, and cavities were present as shown in Fig.7.

Fig.7.Fracture morphology of the samples after HTTT under different conditions of (a) 400 °C, 1 ×10?3 s?1; (b) 400 °C, 5 × 10?3 s?1; (c) 450 °C,1 × 10?3 s?1; (e) 475 °C, 1 × 10?3 s?1; (f) 475 °C, 5 × 10?3 s?1; and(d) is a magnified image of (c) The red arrows indicate the cavities on the fracture surfaces [31].

According to their analysis, the equiaxed grain was 5-8 μm and there was negligible change in grain sizes after 5 test conditions.These grains were distributed on the fracture surface and inside the cavities.Therefore based on equiaxed grains they proposed that the cavities were formed by grain boundary separation.The cavity size and cavity volume fraction were decreased with increasing the temperature at a fixed strain rate of 0.001 s?1.Contrary, at a strain rate of 0.005 s?1, the cavity size was increased but the cavity volume fraction was decreased with increasing the temperature.Therefore it was suggested that cavity size and volume were independent in strain rate and temperature.They also reported that with increasing elongation cleavage planes were very low and inter-granular fracture morphology was dominant(Fig.7(f)).Kim et al.[37]reported that the stress-energy was very low at 250 °C in comparison to 200 °C, this suggested that the cavities might be nucleated, germinated, and propagated but owing to low-stress energy the cleavage planes were not propagated, therefore, leads to the longest elongation.In one another study [78]it was suggested that cavities can be nucleated at the initial stages of superplastic deformation.This means that grain separation started and leads to cavity formation with an increase in strain rate.Once the stress near the big cavity increases to a critical point the trans-granular fracture inside the grains occurred which leads to cleavage planes and earlier failure of the specimen.Similarly, Malik et al.[16]also reported dimples, tear ridges, cracks, and cavities on the fracture surface of specimens under a temperature of 250 °C while cavities at temperature 300 °C.In their study,the volume of the cavity was increased with increasing the temperature under a fixed strain of 0.001 s?1.Moasted et al.[72]also reported SEM fractographs at a fixed strain rate of 0.001 s?1of extruded, ECAPed, and extruded+ECAPed tensile specimens under a temperature of 200 °C.There were a lot of heterogeneous size deep dimples owing to bimodal grain structure in the extruded alloy (Fig.8 (a)).Contrary, the fracture surface is comprised of dense and deep dimples along with tear ridges suggested superplasticity in ECAPed condi-tion (Fig.8(b)).Moreover, a lot of shear planes were reported in extruded+ECAPed indicating a prominent contribution of slip-induced deformation (Fig.8(c)).

Fig.8.SEM fractography of tensile samples broken at 200 °C and the strain rate of 0.001 s?1: samples (a) A0, (b) A4, and (c) B4 [72].

Similarly, Mohan et al.[68]discussed and compared the fracture behavior of FSP AZ31 and FSP AZ91 Mg alloy at a strain rate of 0.01 s?1under temperatures 250 and 330 °C.It is worth mentioning that AZ31 Mg alloy exhibited cleavage planes besides AZ91 Mg alloy displayed cavities formed due to de-cohesion of fine grains during GBS.Raja et al.[53]also discussed the fracture mechanism of FSPed on ascast AZ91 Mg alloy and produced half thickness fine grained(HFG)(Top 1.5 mm FSPed and bottom 1.5 mm as-cast(AC)),section fine grained (SFG) (Top and bottom 1 mm FSPed.Middle 1 mm AC) and full thickness fine grained (FFG)(Full Thickness, 3 mm, FSPed).According to their study, the FSPed regions exhibited the inter-granular fracture while the non-FSPed region displayed dimples and cleavage planes.Notably, the FFG revealed the fiber with inter-granular fracture morphology.The change from dimples and cleavage planes to inter-granular fracture can be attributed to a fine grain structure and it is in good agreement with Refs.[16,31,37].

From the above brief discussion it can be concluded that;

1.For high elongation, cavities could be formed at the early stages of deformation in fnie grains; with increasing strain,the growth of the cavity could be happened due to GBS and decohesion of equiaxed fnie grains.

2.If the grains structure is bimodal then the slip creep mechanism in large coarse grains and GBS in fnie grains leads to both inter-granular and intra-granular fracture.This aspect leads to early crack and loss in superplasticity.

3.For high strain rate and low temperature the strain energy could be high which leads to intra-granular fracture therefore a low elongation to fracture has been reported in Table 1.Contrary to relatively high temperatures and high strain rate the alloys exhibited a high elongation which can be attributed to low-stress energy and the GBS phenomenon.

Table 2Acronym and their full names.

8.Conclusion and recommendations

The superplasticity of different Mg alloys has been reviewed and the following important points can be drawn;

1.The elongation to fracture is found higher in SPD Mg alloys compared to rolled/extruded Mg alloys.The low elongation which is the second important parameter after the m-value is a decisive parameter.However, it is reasonable to take into consideration the geometry of specimens.The low elongation to fracture parameter leads to solute drag mechanism or a combination of solute drag and GBS mechanism while the high elongation and high m-value lead to GBS deformation mechanism.

2.Both low-temperature low strain rate and high-temperature high strain rate superplasticity has been reported while solute drag creep and GBS are the dominant deformation mechanism.During low and high strain rate superplasticity at higher temperature GBS is governing deformation mechanism.The very high strain rate (0.1 s?1) superplasticity (elongation to fracture ～1000%) was achieved in WE43 Mg alloy but at relatively high temperatures(400 °C).

3.The thermal stability is related with precipitates.The thermal stable precipitate can promote structural stability which impedes any tensile abnormality and endorse superplasticity.The melting of different precipitates has different temperatures therefore it is benefciial to process material at the highest possible temperature at which it displayed thermal stability of microstructure and precipitate.The other reason is Q-value and m-value because at high temperature the Q-value appeared very low ～90 kJ/mol, which promotes grain boundary diffusion.

4.Temperature, strain rate, and exposure time put a big influence on texture weakening.During high-temperature tensile testing, the＜c + a＞non-basal slip activity and DRX phenomenon leads to texture weakening and promote the GBS phenomenon.

5.For a high elongation to fracture the cavities are formed at early stages of deformation in fnie grains, while with theincrease in strain the growth of cavity has occurred and attributed to GBS and de-cohesion of equiaxed fine grains.

However, the effect of high strain rate superplasticity at lower temperature on texture weakening and further its influence on GBS mechanism is still plausible and needs more studies to evaluate.Moreover, the volume fraction of precipitate is required to increase by subjecting ECAPed or FSPed Mg alloy to direct aging (150-180 °C for a longer time) such that the grain size must remain very fine.The effect of a large volume fraction of precipitate on thermal stability and GBS is still an open question.The severe thermo-mechanical processes have two disadvantages(i)cost-ineffective(ii)small size production,therefore,it needs to find out some more new low-cost techniques for developing ultra-fine grain structure Mg alloys.

Table 2.

Declaration of Competing Interest

There is no conflict of interest has been reported in this review article.

Acknowledgment

We are very grateful to all the authors who published the research work on the superplastic behavior of different Mg alloys.We are also thankful to the National Natural Science Foundation of China (52005103, 71801046, 51775112), the Guangdong Basic and Applied Basic Research Foundation(2019B1515120095), the Intelligent Manufacturing PHM Innovation Team Program (2018KCXTD029, TDYB2019010),and the Key Project of the CSTC (2019jcyj-zdxmX0013).