Abdul Mlik,Yngwei Wng,b,*,Cheng Hunwu,b,Fisl Nzeer,Muhmmd Abubker Khn

a School of Material Science and Engineering,Beijing Institute of Technology,Beijing 100081,China

b National Key Laboratory of Science and Technology on Materials under Shock and Impact,Beijing 100081,China

Abstract From the mechanistic point of view,magnesium alloys are lightweight materials and are receiving increasing attention in the past several years in various fields Prof.Liang Zhen from Harbin Institute of Technology,China and the United State military are showing keen interest in the development of magnesium alloys as ballistic resistant material.However,their use is still limited owing to low ductility,low formability,and average mechanical properties.The magnesium alloys components must withstand the shockwave under hypervelocity ballistic impact.The ballistic testing can produce gradient variations of the strain and stress-energy away from the crater,and useful for the development of these alloys in the military and aerospace industry.Therefore,the present review article shed light on the post deformation analysis of the Mg alloys subjected to the different projectiles under ballistic impact,and the underlying mechanisms were discussed.In the end,some important issues regarding the ballistic impact and further studies in this fiel were proposed.? 2020 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Ballistic behavior;Mg alloys;Deformed zones;Texture;Hardness.

1.Introduction

The cost of the military and aerospace vehicles is roughly proportional to fuel consumption.Therefore,the uses of lightweight materials bring the reduction of the cost.Instead of the weight-saving criterion,the excellent mechanical properties,such as;high strength,high hardness,high damping capacity,and high strength to weight ratio is also required against the resistance of any specifie threat.Metals and their alloys used in these applications must withstand high shockwave and dynamic impact loading in the stringent environment and against deleterious sources such as;different armor penetration and micrometeoroid collision.

During the past decades,different materials such as;steel[1,2],aluminum[3-5],and titanium[6,7]were subjected to ballistic impact.Among them,steel and titanium are considered potential ballistic resistance materials;however,the density of these materials is very high.Al alloys considered lightweight material and the best replacement of steel and Ti alloys.The broad simulative,experimental and post deformation analysis has been thoroughly investigated after ballistic impact against different types of armor projectiles[3-5].Mishra et al.[8]proposed a convenient approach to calculate the ballistic efficien y(η),and the valueη>1 is recommended for ballistic safety.In our previous study,we have found ballistic efficien y～1.5 by using the same approach in Al-7055 alloy against tungsten core projectile[3].Moreover,a comprehensive analysis of deformed microstructure and mechanical phenomena arising due to hypervelocity impact has been well documented for Al,Ti,and steel.The detailed insights of the important phenomena such as grain refinemen or recrystallization,microhardness evolution,different types of twinning,dislocation slip,phase transformation,and amorphization have been reported previously[3,4,9,10].

Among these prospective materials,Mg is considered as the lightest structural material having density～1.78gcm?3[11-13]which is approximately 77% lower than of steel and 35% lower than that of Al[11].Till to date,Harbin Institute of Technology[14-17]and US military industries and their researchers are showing their keen interest in ballistic resistance of Mg-based alloys[18].Mg-based alloys provided a unique combination of strength,low density,high specifi damping capacity,and high shock absorbency and shock mitigation which is 100 times greater than Al alloys[18].Hence Mg alloys are suitable and can be a good replacement to reduce the weight and fuel consumption of conventionally used armor materials.However pure Mg has disadvantages such as fir hazards,low strength,and low hardness[19].Therefore,during the past several years a lot of investigations of different series of Mg alloy have been conducted on the sheet formation,high-temperature mechanical behavior[20-23],reducing the anisotropic mechanical behavior[24,25],high strain rate deformation[26-32],texture evolution and its effect on mechanical behavior[33-37],corrosion resistance[38-41]and increasing the strength of the alloy by incorporating different rare-earth elements[42-47].Additionally,different thermo-mechanical processes and techniques have been employed to enhance strength and ductility such as equal channel angular pressing(ECAP)[48-50],friction stir welding[51,52],extrusion[26,53-55],double extrusion[56],rolling[57-59],extrusion following by rolling[60],precompression[36,61-65]and hot isostatic pressing(HIP)[66].

It is believed that the mechanical behavior of the Mg alloys is strain rate path-dependent.The different mechanical response has been characterized by loading along the different direction of hexagonal closed packed(hcp)crystal structure[26,27].In our previous review article,we have focused on the dynamic compressive mechanical behavior of the Mg alloys and suggested that the materials which exhibited twinning and slip induced deformation are strain-rate sensitive,while materials which are only dependent on twinning does not show any significan strain rate sensitivity[11].Contrary,hypervelocity ballistic impact is a useful method to study the microstructure evolution under dynamic loadings as it can produce asymmetric deformation owing to a very high strain rate～106s?1.Additionally,it provides gradient variation of the thermal activation,strain rate,and strain at different locations away from the crater wall.The asymmetric deformation and microstructure evolution at different locations away from the crater wall can provide hints of the different deformation processes and can be useful to study the different deformation processes.Hitherto,limited literature has been reported on the ballistic behavior and microstructure evolution of the Mg alloys under hypervelocity impact by using different types of projectiles.In-situ analysis of deformation behavior is also impractical by existing technical capabilities owing to the short interval of shock wave impact and high strain rate.Therefore,post deformation behavior has been broadly used to understand the deformation process.Thus,this review article is emphasized in the post deformation behavior of Mg alloys after ballistic impact.Additionally,this article suggested some technical issues regarding the ballistic safety against standardized projectiles for the Mg alloy so that their use can be widespread in military and aerospace applications.

2.Macroscopic observations of ballistic impacted alloys

It is commonly observed that high shockwave impact produced severe plastic deformation adjacent to the crater.Zou et al.[14,15,17]studied the comprehensive post deformation analysis of AM60B alloy under different velocities(0.5kms?1,4kms?1,and 5kms?1)and different projectiles(GCr15 and 2017Al ball)as shown in Table.1.They reported that the rigid GCr15 projectile was kept intact at a low velocity(0.5km/s)and as a result hemisphere crater was formed.Moreover,no obvious voids or cracks were nucleated at the bottom of the crater as shown in Fig.1(a,b).In terms of depth of penetration(4.31mm)and diameter of the crater(5.04),it was suggested that the radial penetration ability of the GCr15 rigid projectile was much lower and almost all energy carried by the projectile was absorbed by the target.However,for the same material under hypervelocity impact(4kms?1and 5kms?1),an orange peel shaped morphology near to the hemispherical crater were formed as provided in Fig.1(c-f)[14,15,17].Notably,they witnessed the thunderflas phenomenon due to shock wave pressure～45GPa and the temperature rising～2300K which leads to melting and vaporization of both targeted plate and 2017 Al projectile.Interestingly,the diameter and depth of crater were about four times larger than the GCr15 projectile under both velocities as shown in Fig.1(e,f)which exhibited that the radial penetration ability of the 2017 Al projectile was high in comparison to GCr15 projectile.They also reported numerous cracks adjacent to the crater and mainly attributed to the adiabatic shear band which is considered as the precursor of the failure of the material.Recently,Zhang et al.[68]performed the ballistic test at a speed of 900ms?1on an Mg-3Al-1Zn alloy target material against a GCr15 steel projectile of 3mm diameter.The crater was formed of almost similar diameter,while the depth of the crater was～8.7mm.Their results also displayed that the radial penetration ability of the projectile is much lower and well consistent with the study of Zou et al.[15].In another study,Shi et al.[67]investigated the effect of T12 steel projectile on a 8mm thick plate of Mg-Gd-Y-Zr alloy,in their experiments two tests were conducted at a velocity of 400 ms?1.They revealed that one of the bullets was completely perforated while the other one was not fully perforated.However,they did not provide any detail about the front crater or the rear face of the Mg sheets.The investigation by Nishida et al.[72]provided that the crater formation of LPSO type high strength Mg alloy was slightly circular cone at a velocity of 2kms?1,contrary it was hemispherical at a velocity of 5kms?1.Moreover,the fracture behavior exhibited that the LPSO Mg alloy has a brittle nature.

Table 1The Mg alloys that were subjected to ballistic impact under different projectiles and different velocities.

Table 2The diameter of crater and depth of penetration against parabellum and NATO projectile[70].

Fig.1.Macroscopic observation of top view and a cross-sectional view of the formation of the crater in AM60B Mg alloys target impacted by different velocities and different projectiles(a,b)velocity of 0.5kms?1[15](c,d)velocity of 4kms?1[16](e,f)velocity of 5kms?1[17](the images were reconstructed).

Zhang et al.[71]also investigated the ballistic behavior of a GCr15 steel projectile on an Mg-Al-Mn alloy and found that the projectile was kept intact and embedded in the crater,the radial penetration ability of the projectile was much lower and there were no cracks at the broad side and bottom of the crater.This response is quite similar to the study of Zou et al.[14].Jones et al.[73]showed that the ballistic behavior of AZ31B alloy is quite comparable to that Al-5083 alloy.They also suggested that a high strength with no loss of ductility is highly desirable.

Generally,it is accepted that the crater formation and depth of penetration are dependent on the absorption energy and hardness of the material.It can be anticipated that the incorporation of some useful high absorption energy element(lead(Pb),gold,rare earth elements)may provide substantial resistance against the ballistic impact.A similar study was thoroughly investigated by Abdullah et al.[70].They investigated the effect of absorption energy by varying the weight percent(1,5 and 10%)of the Pb and failure behavior under ballistic impact at velocities(935ms?1and 976ms?1)against two different projectiles(9mm×9mm)parabellum and(5.56mm×45mm)NATO projectile.According to their results,the parabellum projectile was not perforated,while NATO projectile was fully perforated through a 25mm thick sheet.

Fig.2.The macro-views of the impacted sheets of AZ31+1%Pb,AZ31+5%Pb,AZ31+10%Pb,and AZ31B Mg alloys(a-d)front view against 9mm×19mm Parabellum projectile(e-h)front view against 5.56mm×45mm NATO projectile(i-l)rear view against 5.56mm×45mm NATO projectile[70].

However,they revealed that the crater diameter and depth of penetration was varied in considered Mg alloys under parabellum projectile impact.Therefore,they recommended that the AZ31B+1%Pb provided the most suitable absorption energy during impact.A smaller diameter(13mm)was observed in AZ31B+1%Pb Mg alloy,but the depth of penetration(10mm)was high in comparison to the AZ31B Mg alloy as shown in Fig.2(a-d),which means that this composition effectively absorbed the energy and transfers it to the material.Moreover,the reported that hardness of AZ31B Mg alloy was greater than all other considered Mg alloys therefore depth of penetration in AZ31B Mg alloy was smaller as listed in Table.2.Contrary,a complete perforation was occurred in AZ31B and Pb incorporated Mg alloys under NATO projectile impact as shown in Fig.2(e-l).The front view demonstrated the same crater diameter of about 7mm in all Mg alloys while the rear view provided the evidence of fully perforated targeted material.

From the foregoing discussion,it can be concluded that the increase in velocity and type of the projectile has a substantial effect on the crater formation and radial penetration ability under ballistic impact.Apart from this,the high absorption energy owing to high ductility,high strength,thickness,and hardness are the noteworthy parameters for the resistance of the projectile.However,the resistance against different types of projectiles and their nose shape,size and radius of the conical tip on the formation of crater and damage patterns(Petal formation,bending,stretching and bulge,and deviation of the projectile)on the front face and rear face in different Mg alloys(solution heat-treated,aged,and over-aged alloys)is still an open question.Different types of Al,Cu,Ti covers of 2-3mm in thickness can be used in front of the Mg sheet to increase the ballistic resistance.Moreover,the ballistic resistance against any projectile can also be improved by making special designs of armor panels to customize the combination of strength and toughness as proposed by Mishra et al.[8].

Fig.3.Schematic illustration of the distribution of the deformed microstructure away from the crater in AM60B target impacted at a velocity of(a)4kms?1 and zones are(I)UFGZ,(II)HDDTZ(III)LDDTZ and(b)at a velocity of 5kms?1 and zones are(I)UFGZ,(II)UFGTZ(III)HDDTZ and(IV)LDDTZ[16,17].

3.Post deformation analysis

3.1.The gradient variations away from the crater

The response of the Mg alloys under different loading conditions is pretty different,i.e.,the deformation is uniform/symmetric under quasi-static loading;while under dynamic loading,it is non-uniform/asymmetric.Recently,Li et al.[74]subjected a bimodal and fine-graine AZ31 Mg alloy under quasi-static compression and reported twin induced deformation in bimodal grains,besides no abnormality in grain size and dynamic recrystallization,occurred during deformation which suggested that quasi-static deformation was a uniform/symmetric deformation.In our previous investigations,we have observed adiabatic shear band,recrystallization,and rise in temperature under dynamic loading and different grains structure and orientations after high strain rate compression which was the evidence of asymmetric deformation[26,27].Contrary,under the ballistic impact the deformation is much more complicated than quasi-static[75],and dynamic compression[27]owing to a short interval of time for matrix/target to response against the shockwave impact.It is obvious that the shock wave propagates at bulk sound velocity and consider as the precursor of the plastic deformation.Besides,it produces gradient variations in strain,strain rate,and temperature rise away from the crater wall.Murr et al.[76]shed light on the microstructure evolution under hypervelocity impact on different materials and reported the general feature such as twinning,micro-bands,and recrystallization in different thick metal targets.Zou et al.[16,17]thoroughly explained and characterized the deformation zones distribution along with radial penetration and at the bottom of the crater under hypervelocity impact on AM60B Mg alloy target material and denoted the different zone by;(1)low-density deformation twinning zone(LDDTZ),(2)High-density deformation twinning zone(HDDTZ)(3)ultrafin grain twinning zone(UFGTZ)(4)ultrafin grain zone(UFGZ),the variation and the width of these zones are presented in Fig.3(a,b).It is worth noticing that the deformation zones have different variations in sizes that can be attributed to the difference in the speed of the projectile.Moreover,the deformation zones widths are small on the broad side of the crater in comparison of the bottom of the crater.

Similarly,Zhang et al.[68]describe the gradient variations in microstructure as shown in Fig.4(a-g).It can be observed from Fig.4(a)that the morphology of the grains and their size were varied from left to right and different zones have been marked with blue lines.The UFGZ was consisted of～0.4mm,while Fig.3(a,b)demonstrated that the zone is～0.8mm away from the broad side of the crater.Additionally,the deformation was confine up to 6.5mm away from the crater,while in AM60B Mg alloy the deformation is confine～10-11mm away from the crater.This difference can be attributed to a different type of target material,speed,and initial processing history of the alloy.Based on the different deformation mechanisms and grain structures,as provided in magnifie micrographs in Fig.4(b-g),Zhang et al.[68]also described these zones as UFGZ,UFGTZ,HDDTZ,and LDDTZ zones.In another study,Zhen et al.[14]represented the UFGZ and UFGTZ as transformed band zones and LDDTZ,HDDTZ as deformed band zones,and proposed twin induced rotational dynamic recrystallization phenomenon responsible for the formation of UFGZ.

Hence,the gradient variation in the strain and the resulting different deformation zones in Mg alloys under ballistic impact are strongly dependent the grain size,strength,speed,and type of the projectile,initial processing history of the alloy such as hot-rolled sheet,extruded sheet and as-cast alloy and texture of the alloy(discussed in next section).

3.1.1.Low-density deformation twinning zone(LDDTZ)

The Mg alloys have low symmetry hcp crystal structure with a slightly less than the ideal c/a ratio and are greatly different than symmetric face-centered cubic(FCC)structure materials.The numbers of activated slip systems in these alloys are inadequate to accommodate the plastic deformation.The most common slip systems are{0002}<110>basal slip and{100}<110>prismatic slip,both have the sametype of burgers vector and can accommodate the strain when loading is parallel to the basal plane[77].Contrary,the slip along the c-axes is comparatively difficul to be activated.Previous researchers briefl calculated the CRSS for{0002}<110>basal slip,{100}<110>prismatic slip,and{112}<113>pyramidal slip systems approximately 0.5MPa,10-45MPa,and 35-80MPa,respectively[78-80].From this data,it can be concluded that the activity ofslip system is difficul owing to very high CRSS.However,Agnew et al.[81-83]and other researchers[79,84,85]provided the evidence ofunder quasi-static loading.Additionally,To fulfil the von Mises[86]criterion,twinning provided a key role to accommodate the strain.A significan number of investigations have been well presented on different types of twinning activity previously[87-94].Besides,Zhang and Joshi[87]systematically explained the critical resolve shear stresses(CRSS)～2.5MPa for the{102}<101>extension twinning activity.While in another study CRSS of about～30-100MPa were reported for{101}<102>contraction twinning activity[95].However,twin activity is also path-dependent i.e.,the compressive loading perpendicular to the c-axes or the tensile loading parallel to the c-axes is advantageous of twin activity.Consequently,twinning activity together with different slip system plays a significan role in accommodating the plastic strain.

Fig.4.The gradient variations of microstructure in Mg-3Al-1Zn alloy under the ballistic impact of a GCr15 Projectile at velocity 900ms?1(a)montage of the OM micrograph away from the crater(b-g)the magnifie OM micrograph of different zones marked on Fig.4(a)away from the crater[68].

Shi et al.[67]found primary and secondary twinning and twin-twin interaction as shown in the EBSD map and BC map in Fig.5(a,b)at the broad side of the crater.Besides,they radially observed{103}twinning which required very high CRSS for nucleation.This shows that the stress induced by shock wave impact on the broad side of the crater in Mg-Gd-Y-Zr is very high and the strain produced has been accommodated by all types of twins which were active during deformation.Besides,in their studies,non-uniform orientations also seemed to be prevalent in the grains that were attributed to lattice rotation accommodated by local strains.Another Z shaped twinning having width～100-200nm in TEM analysis was also observed in their investigation and mainly denoted with twin-twin interaction.In another study Zhang et al.[68]found LDDTZ～2.5mm away from the crater and EBSD scanned map and grain boundary map for this zone has been reconstructed and provided in Fig.5(c,d).The{102}tensile twinning was dominant in this region.While no other secondary or twin-twin interaction was observed in their study.This type of twinning is preferentially active owing to low CRSS(～2-5MPa)and accommodated the strain.{102}tensile twinning is commonly observed in many Mg alloy under high strain rate compression though SHPB and mainly ascribed as the predominant deformation behavior owing to very low CRSS[96-98].Contrary,the contraction,and double twinning required high CRSS and this zone is far away from the crater so it can be predicted that the stress-energy was small in magnitude in this region.Besides,the twinning is difficul to be operative in a fin grain structure.Therefore,it is expected that other types of twinning may not be operative owing to very high CRSS.

Fig.5.The microstructure away from the crater representing Low density deformed twinning zone(LDDTZ)(a,b)in Mg-Gd-Y-Zr alloy[67](c,d)in Mg-3Al-1Zn alloy[68].

Another reason is the strong basal texture,when the Mg alloy subjected to loading along c-axes of the hcp crystal structure the Schmid factor comes with a very low value for contraction and double twinning,hence as a result very low Schmid factor(τ=CRSS/SF)value leads to very high CRSS for contraction and double twinning.Thus,contraction and double twinning in this region was not operative and mainly attributed to the texture and the anticipated high CRSS owing to grain size(14.5μm).Zou et al.[17]also fin cross twinning in LDDTZ and recommended that the twinning was only nucleated in the grains having preferred orientation for twin nucleation;the activation of cross twinning is obvious because the as-received grain size in this study was varied in the range of 200-400μm which was more advantageous for nucleation of twinning.

3.1.2.High-density deformation twinning zone(HDDTZ)

Fig.6(a,b)represents the deformed microstructure away from the crater of an AM60B Mg alloy under a velocity of 5kms?1[16].The number of twin lamella per grain was increased in comparison to LDDTZ and even the grain boundaries were difficul to be observed in Figs.6(a-inset(6a)).Asgari et al.[99]recommended weak and random grain orientations/texture in cast Mg alloys which are different than typically strong basal textured thermomechanical processed Mg alloys.Similarly,the alloy used by Zou et al.[16]was an as-cast alloy and as-received microstructure consisted of large size grains>200μm.Therefore,the dense cross twinning produced in preferred oriented grains and predicted that the grains have c-axes approximately perpendicular to the normal direction(ND)of the AM60B alloy.Xin et al.[100]reported different{102}<101>extension twin variants under tensile loading.Similarly,different cross twinning can be evidence of the activity of different twin variants as presented in Fig.6(a).Additionally,the TEM micrograph explained the different twin in HDDTZ of the AM60B Mg alloy as marked with the red and yellow dotted lines in Fig.6(b).The HDDTZ is found～6.6mm away from the perforation path on the broad side of the crater and～11.8mm at the bottom of the crater as shown in Fig.3(b).While Zhang et al.[68]reported the same area～1.5mm away from the board side of the crater(Fig.4).They have observed only{102}tensile twinning in this region as evident by the EBSD map and GB map in Fig.6(c,d).Moreover,Shi et al.[67].found tensile twinning,double twinning,and radially observed{103}twinning on the bottom side of the crater.They also observed boundaries with the orientation relationship of 78.3°/<102>and recommended that these boundaries belong to{101}tensile twining.These unusual twin boundaries are the cause of the impeding and accumulation ofbasal dislocations in the twined areas,while,further complex deformation might induce the different local orientations relationships in comparison to the ideal twin/matrix relationship.Similarly,Li et al.[101]observed twin-like domains in deformed pure Mg specimen with a misorientation of 57°/<100>which do not satisfy any of the pre-define known twin relationship.Moreover,{102}?{101}and{102}?{102}double twinning modes and some unambiguous orientations,which do not fulfil the distinctive twin/matrix orientation relationship were noticed in the severe complexed deformed Mg alloys[102-104].A more precise study was conducted by Mao et al.[105]and found various twin patterns such as parallel twins and double twins under the sudden impact on an AZ80 Mg alloy.Contrary,they found some unusual twins with boundaries 68.8°/<110>,56°/<100>and 77°/<110>which was further systematically assigned with{101}?{101},{102}?{102}doubly twinning and{102}twinning,respectively.Barnet et al.[106]revealed that the localized shear bands or deformed zones might be evolved from the doubly twins consequently these structures comprised of very high shear strain.Xue at al.[67]reported primary and secondary twins in stainless steel under dynamic deformation.However,the deformation behavior in Mg-3Zn-1Al alloy[68]is different based on twin types as shown in Fig.6(c,d).Contrary,in another study Zou et al.[15]illustrated the same area as deformed zones and proposed that these zones can be distinguished based on twin volume fraction per grain.However,their study did not reveal the types of twins.Asgari et al.[107]also suggested that the twinning fraction is attributed to the grain size and twinning interface energy can change the CRSS of the twinning.In Mg alloys,twin interface energy is very large for very fin grain size[108],contrary to the large size grains it sufficientl reduces.Therefore,a lot of cross twinning(may be primary and secondary)has been observed in Fig.6(a)and only{102}tensile twinning was witnessed in Mg-3Al-1Zn alloy which requires low CRSS even in small grain size of the range 10-20μm.

Fig.6.The microstructure evolution of(a)the HDDTZ of AM60B alloy under a velocity of 5kms?1(inset of Fig.6(a)is LDDTZ of the same alloy)(b)TEM analysis of the HDDTZ showing secondary twins[17](c)EBSD map of the Mg-3Al-1 Zn alloy(d)GB map of the same zones showing tensile twinning at 1.5mm away from the crater[68].

From Fig.6(c)it can also be deduced that the EBSD map displayed inhomogeneity of crystal orientations(different colors of grains)after ballistic impact in comparison to rolled as received strong basal texture(same red color grains).These gradient orientations are related to the abundant twinning and lattice rotation induces by local strains,moreover,a big fraction of dislocations are essential to manage the lattice curvature.Once these dislocations impeded in the deformed structure,a lot of low angle grain boundaries(LAGBs)were produced,and hence inhomogeneity might be occurred in the deformed region(which was being under observation).

3.1.3.Ultrafin grain twinning zone(UFGTZ)and ultrafin grain zone(UFGZ)

Recently,many thermomechanical processes have been developed and homogeneous fin grain size(～1μm)of Mg alloys has been achieved and mainly ascribed with the continuous and discontinuous dynamic recrystallization which was assisted by dislocation slip during high-temperature processing and by achieving activation energy(Q～135Kj/mol).Contrary,ballistic impact always carried out under room temperature and UFGTZ and UFGZ have been developed adjacent to the crater as shown in Fig.7(a,b).These are special zones because the maximum strain and stress-energy have been induced in these regions.It is reported that twinning induced rotational dynamic recrystallization was thought to be the main mechanism that is responsible for the formation of UFGTZ and UFGZ[14-17].Yin et al.[109]reported dynamic recrystallization in AZ31 Mg alloy owing to the differently oriented twinning intersection with each other and proposed twinning induced dynamic recrystallization model.Besides,Xue et al.[110]found that the deformation twinning has a significan role in the formation of nanostructures in stainless steel under dynamic loading.The micro twins,micro bands,along with dynamically recrystallized grains were observed away from the crater in stainless steel targets and Cu targets by Murr et al.[111]and by Kennedy et al.[112].

It is obvious that the severity of plastic deformation increased toward the edge of the crater.Therefore,Zhang et al.[68]revealed the UFGTZ and UFGZ adjacent to the crater as shown in Fig.7(a).The magnifie view of the UFGTZ is provided in Fig.7(b).They reported that the twin’s laths were embedded in big grains that were surrounded by the dynamically recrystallized grains in UFGTZ.They also reported that the cross intersected and parallel deformation twins subdivided the parent grains into the smaller grains as shown in Fig.7(a,b).This mixed region of twinning and ultrafin grains is providing evidence that these ultrafin grains are associated with twinning behavior.

Zou et al.[17]reported that high-density dislocation cells inside the ultrafin grains(UFGs)and recommended that the dislocation slipping play a significan role in reducing the grains size.They also shed light on the non-homogeneous deformation which was the cause of the uneven distribution of the temperature field Therefore,this uneven distribution of temperature together with dislocation slipping trigged the non-uniform deformation and formed UFGTZ and UFGZ as shown in Fig.7(d and d-inset).During impact proceeding,twins in recrystallized grains were induced by residual stresses in recrystallized microstructure as shown in Fig.7(dinset).The UFGTZ was not reported in the Refs.[16,67].However,fin grains are considered to be the cause of the dynamic recrystallization mechanism owing to the shock wave,and strain energy stored in the material adjacent to the crater as shown in Fig.7(c,d).Zhang et al.[68]revealed that the boundaries of the recrystallized grains are straight and the distributions of DRXed grains(Fig.7(c))are providing clear evidence of twinning induced grain refinement Similarly,dynamically recrystallized grains have been reported in different alloys under dynamic loadings in such as;Al alloy[113,114],Ti[115],TC4 alloy[116],and steel[117,118].Contrary,the cracks,and shear bands have been reported near the crater by Shi et al.[67].

Zou et al.[17]and Zhang et al.[68]systematically explain the formation of the UFTGZ and UFGZ.According to their studies,firstl,the dislocations slipping together with{102}<101>extension twinning were preferentially operated to accommodate the stress concentration in preferentially oriented grains.Secondly,with the impact proceeding,the secondary twinning trigged in localized zones owing to the increase in stress concentration.Additionally,high order twins together with primary and secondary twins lead to the fragmentations of the twin laths.Thirdly,with the further increase in the strain energy,the slipped dislocation was impeded and piled up on the twin boundaries of the fragmental twin laths and results in the formation of the polygonal subgrains.During the formation of the UFGZ,the twins in the ultrafin grain zone were induced and formed UFGTZ,which is mainly attributed to the residual stress and shock wave impact.Finlay,sub-grains assisted with a temperature rising,and further increase in strain energy results in the rotation of the polygonal grains to form the random oriented dynamic recrystallized grains with straight boundaries as schematically illustrated in Fig.8.

Fig.7.The microstructure evolution after the ballistic impact of Mg-3Zn-1Al alloy(a)low magnifie SEM micrograph representing UFGTZ and UFGZ(b)the magnifie SEM micrograph representing UFGTZ[17](c)The SEM micrograph of UFGZ[17](d)the OM micrographs representing UFGZ and inset of the Fig.7(d)is the OM micrograph of UFGTZ of AM60B Mg alloy[68].

3.2.ASB induces deformation

Adiabatic shear bands(ASBs)are the result of thermal accumulation in localized areas owing to localized strain and has been frequently observed in materials subjected to dynamic loading.The ASBs are considered as the precursor of the material failure.However,the Mg alloys are high thermal conductivity material therefore;the adiabatic shear band is not commonly observed.Apart from ASB,the double twinning and contraction twinning leads to crack in Mg alloys as they are comprised of high shear stresses.Yang et al.[119,120]systematically explained the adiabatic shear susceptibility under high strain rate compression in peak aged ZK60 Mg alloy and suggested that the formation of ASB was higher and 2.5 times faster than the quenched specimen.They also recommended that higher ASB susceptibility was mainly attributed to high strength and precipitates.In a previous investigation,it was reported that the rise in temperature inside the ASB was～334°C which leads to grain refinemen inside the ASB,while the localized strain was also higher than the normal strain exhibited by the stress-strain curve of Mg-Zn-Zr alloy[121].Wu et al.[122]and He et al.[123].reported that width and rise in temperature inside the ASB increased with increasing the strain rate.Besides,Xu et al.[124,125]determined the critical strain and strain rate in Ti alloys for the development of the deformed bands and transformed bands.They also recommended that both types of bands can be developed together at different strain and strain rate levels.This implies that ASB nucleation and development can easily be understandable under a ballistic impact.Zou et al.[15]thoroughly explained the nucleation of ASBs in AM60B alloy under ballistic impact and reported ASB in terms of transformed bands which were consisted of very fin grains and the strain inside the transformed band was much higher in comparison of deformed bands as shown in Fig.9(a,c).In another study,Zhang et al.[71]revealed that the nucleation of narrow ASBs at the twin boundary regions was primarily ascribed to the highly localized shear stress concentration at twin boundaries owing to impeding and piling up of the dislocations.They also revealed that the ASB was composed of very fin recrystallized grains together with the presence of Mg17Al12nano precipitates.Lots of ASBs have been observed in UFGZ(Fig.9(b)),which was attributed to a localized increase in stress-energy and high strain rate induced during projectile impact.Similarly,Shi et al.[67]and Zhang et al.[68]reported the nucleation of ASB adjacent to the crater.

Fig.8.fl wchart of the different mechanism in the formation of ultrafin grain zone adjacent to the crater through twinning induce rotational dynamic recrystallization mechanism.

Fig.9.The formation of ASB/transformed band near the edge of the crater(a,b)OM analysis(c)TEM analysis[15].

Fig.9(c)elaborated the TEM image of the transformed zone of AM60B alloy after ballistic loading;the grains are very fin and nano-size,suggesting that the transformed bands have absorbed the high fraction of strain,induced during shockwave impact.However,the depth analysis of ASB has been presented in AM60B alloy but the texture inside the ASB is still plausible and needs more attention,apart from the texture analysis the precipitates evolution inside the ASB and dislocation density inside the nano-size grains are also hot topics which are needed to thoroughly explain the ASB behavior in Mg alloys under ballistic impact.

3.3.Textural changes

The deformation behaviors in Mg alloys are highly dependent on the texture of the materials.Generally,the random orientation of the c-axes distribution has been reported in as-cast Mg alloys[99],Contrary in wrought Mg alloys(hot-rolled or extruded)strong basal texture or fibe texture are developed such that the a-axes distribution has been found almost parallel to the extrusion or rolling direction[60,126].Wang et al.[127]thoroughly explained the different types of texture(fibe texture,recrystallization texture,strong basal texture)and proposed that hexagonal materials are different owing to low slip systems which leads to different mechanical behavior.Besides,the twinning dependencies on the path loading leads to the anisotropic mechanical behavior in wrought Mg alloys regardless of grain size[128].Zhang et al.[68]systematically explained the texture evolution after ballistic impact as shown in Fig.10(a-f).They have used a hot rolled strong basal textured Mg-3Zn-1Al alloy and reported that the c-axes distributions were parallel to the normal direction(ND)of the sheet.After ballistic impact texture was greatly affected owing to different deformed zones.According to their studies,the deformation was confine within 6.5mm of the crater therefore texture at this zone was found almost same with as-received texture as shown in Fig.10(a).Zhu et al.[129].and Jiang et al.[130]reported that the intensity of the basal texture changed during the recrystallization process.While in another study it is recommended that the big size grains displayed strong basal texture[131].Many studies related to the development of the texture after high strain rate compression has been reported during the last several years[11,26,96,132-134].Among them,Li et al.[133]reported that the〈a〉axes of the grains were distributed almost parallel to the extruded direction(ED)were changed perpendicular to the ED after high strain rate compression and mainly attributed to a large fraction of{102}<101>extension twinning activity.Therefore,owing to twin induced deformation a large fraction of the grains were aligned 86°with original matrix and formed c-axes distribution along the RD of the sheet,and corresponding IPF figur also illustrated that grain orientations are closed to{100}//ND(Fig.10(c-d)).Additionally,they reported that nearly 1.5mm away from the crater the MUD values dropped to 9.4 and 2.1,respectively as shown in Fig.10(e,f).In another study Shi et al.[67]revealed that owing to frequently observed{103}compressive twinning and double twinning changed the{0002}orientations of the grains by 50°from ND to loading direction(LD)of the sheet at the broad side of the crater as shown in Fig.10(g,h)and reported a slightly lower texture intensity(8.910)as compared to the undeformed texture intensity(11.103).While the texture in the bottom of the crater was found almost the same but the intensity of the texture was increased(13.880)due to compressive loading;however the stresses in this region are not high enough as shown in Fig.10(i).

Thus,it is worth noticing that the more nearer the crater the more weaken the texture owing to the nucleation of the dynamically recrystallized grains.Additionally,twin induced deformation owing to compressive loading and grain refine ment owing to twin-twin interaction together with dislocation slips leads to weakening the texture away from the crater and mainly attributed to twin induced dynamic recrystallization.

Fig.10.Changes in texture(a-f)in Mg-3Al-1Zn alloy after ballistic impact at 900ms?1[68](g-i)Mg-Gd-Y-Zr under a ballistic impact of a T12 steel projectile at a velocity 400ms?1[67].

4.Hardness evolution and stress-strain analysis

Vickers microhardness profile of different deformed zones and inside of ASB of different Mg alloys are reconstructed and presented in Fig.11.Zhang et al.[68]reported the highest hardness in UFGZ and then continuous decrease up to 12mm away from the perforation path as shown in Fig.11(a),which is the evidence of gradient variations of microstructure due to the different strain and strain rate levels.Additionally,the hardness of the UFGZ is approximately two times of the original hardness of the Mg-3Zn-1Al alloy.Similarly,Zou et al.[16,17]show variations in hardness at different zones as shown in Fig.11(c,e).This increase is attributed to the grain refinin through rotational dynamic recrystallization and strain hardening.Zhen et al.[14]and Zou et al.[15]also reported an increase in hardness in ASB as shown in Fig.11(b).Contrary,prior literature recommended that dislocation density increased with increasing the pressure(loading)in different metals and alloys[135,136].In a previous investigation,we have characterized the ASB through focus ion beam(FIB)sampling method and found that the grain structure inside the ASB was consisted of both very fin and elongated grains owing to dislocation accumulation combined with the rise in temperature[114].Similarly,Zhang et al.[71]suggested that the ASB consisted of very fin grains containing very high dislocations.Thus,according to the hall Petch grain boundary strengthening mechanism(σy=σ0+Ky/d1/2),the smaller the grain size,the more the grain boundary concentration and hence higher the strength and hardness of the material.

Fig.11.The hardness profil of(a)Mg-3Zn-1Al alloy away from the perforation path[68](b)The hardness profil in ASB and other zones[15](c,e)The variations of the hardness away from the crater of AM60B alloy[16,17](d,f)the stress-strain analysis of the AM60B alloy at high strain rate compression[16,17].

Similarly,the increase in hardness was witnessed in UFGZ and other zones and mainly attributed to shock wave impact which further produced(1)the dislocation accumulation at grain boundaries(2)gradient deformed microstructure comprised of different twins in a polycrystalline material.

Zou et al.[16,17]thoroughly explained the deformed zones based on the stress-strain analysis under high strain rate compression through a split Hopkinson pressure bar as presented in Fig.11(d,f).The stress-strain analysis exhibited different behavior in different zones.The yield strength(YS)of the original specimen roughly calculated～100MPa,while it increased continuously in LDDTZ,HDDTZ,and UFGZ under both 2500s?1and 5000s?1strain rates.The former research elaborated that twinning induces deformation should be suppressed in grain size<3μm.Therefore strain hardening is a big challenge in Mg alloys[137,138].It is also recommended that the twin induced deformation increases the ultimate tensile/compression strength while the deformation only mediated by the slip induced deformation displays less strain hardening.Similarly,in AM60B alloy at a strain rate,2500s?1the specimen(red stress-strain curve in Fig.11(d))exhibited an increase in strain and decrease in strain hardening owing to increase in thermal softening and deformation was mediated bydislocation[26,133].However,at 5000s?1the same UFGZ specimen exhibited a small magnitude of strain hardening which can be attributed to texture modificatio during high shock wave impact.In our previous investigation,we have induced a large volume fraction of extension twinning in ZK61 Mg alloy which leads to the remarkable increase in YS along with ED,TD,and ND of the alloy and mainly attributed to twinning induce grain refinemen and texture changes[132].Similarly,the HDDTZ exhibited high YS owing to twin induced grain refinement Apart from the Hall Petch grain refinemen mechanism,texture plays a significan role in the YS of the alloy.Wang et al.[139]suggested that when the c-axes of the hcp crystal structure are perpendicular to the loading direction then{0002}<110>basal slip provided zero effect and deformation will be mediated by the pyramidal slip planes and hence increase the YS of the Mg alloy.Similarly,it is anticipated that after grain refinin the preferred orientations of the grains are supporting the pyramidal slips and hence increased the YS of the alloy in different deformed zones as shown in Fig.11(d,f).It is noted that the YS of the AM60B alloy increased in a continuous manner from base alloy towards the crater,which is also evidence of the gradient variations of the microstructure.

Conclusively,different deformed zones exhibited an increase in YS and hardness which can be attributed to the gradient variation of strain and strain level,texture,and grain refinemen hall Petch effect.However,a detailed investigation of the texture evolution and then its impact on the stress-strain analysis is still an open question and needs more investigations to reveal the deformation mechanism which can be useful for the improvement of ballistic impact or resistance against the stringent environment.

5.Conclusions and future recommendations

The present review article focused on the macroscopic view and the deformation behavior of Mg alloys under the ballistic impact.Several finding and recommendations have been discussed below.

The increase in the speed of the projectile and its type has a substantial effect on radial penetration ability and depth of the penetration.The strain-induced by projectile produced gradient variations of microstructure away from the crater.The deformation zones after impact were characterized into the following four zones;(1)low-density deformation twinning zone,(2)high-density deformation twinning zone,(3)ultrafin grain twinning zone,(4)ultrafin grain zone.The ultrafine-grai zone is thought to be the result of twin induced rotational dynamic recrystallization.The different types of twins were dominated in the deformed region mainly of primary and secondary types of twins.The unusual twin with lattice orientation different than predefine twin/matrix was observed and mainly considered as{102}tensile twining.Owing to the gradient variations,the more nearer the crater the more recrystallized grains and weaker texture were developed.The deformed microstructure towards the crater showed an increase in yield strength and hardness in a continuous manner,which is also the evidence of gradient variations of the microstructure.

The use of magnesium alloys in ballistic applications is achievable,but several of its properties are required to be improved such as;hardness,absorption energy,ductility,and strength.The bullet types such as;standardized 7.62mm armor-piercing projectile(API)tungsten core,steel core,soft and hard steel blunt projectile,and their effects on the crater formation,depth of penetration and radial penetration ability are still open questions.While the post deformation behavior including the texture analysis and stress-strain analysis of the deformed zones are hot topics and needed to be explored.Additionally,the special designs of armor panels and front covers of different materials can enhance the strength and toughness,therefore,the studies on these hot issues must be considered to further explore the ballistic behavior of Mg alloys.

Declaration of Competing Interest

The authors have no conflic of interest.

Acknowledgment

This project is financiall supported by the National Natural Science Foundation of China.(Grant no.51702015).