Dynamic tensile properties and microstructural evolution of extruded EW75 magnesium alloy at high strain rates

Jinhng Yu,Bo Song,Dabiao Xia,Xun Zng,Yuaning Huang,Norbrt Hort,Pingli Mao,Zhng Liu

a School of Mechanical Technique,Wuxi Institute of Technology,Wuxi 214121,China

b School of Materials and Energy,Southwest University,Chongqing 404100,China

c National Engineering Research Center for Magnesium Alloys,Chongqing University,Chongqing 400044,China

dMagIC-Magnesium Innovation Centre,Helmholtz-Zentrum Geesthacht,Geesthacht 21502,Germany

eSchool of Materials Science and Engineering,Shenyang University of Technology,Shenyang 110870,China

Received 27 June 2019;received in revised form 21 January 2020;accepted 12 February 2020 Available online 28 May 2020

Abstract The dynamic tensile properties and microstructural evolution of an extruded EW75 magnesium alloy deformed at ambient temperature and different high strain rates(from 1000 to 3000 s-1)along extrusion direction(ED)were investigated by Split Hopkinson Tension Bar(SHTB).The corresponding deformation mechanisms,texture evolution and microstructure changes were analyzed by optical microscope(OM),electron backscatter diffraction(EBSD)and transmission electron microscope(TEM).The results show that the extruded EW75 magnesium alloy along ED exhibits a conventional positive strain rate sensitivity that the dynamic f ow stresses increase with increasing strain rate.Texture measurements show that after dynamic tension,the initial weak texture of extruded EW75 magnesium alloy tansforms to a relatively strong＜10-10＞//ED texture with increasing strain rates.The microstructural analysis demonstrates that dislocation motion are main deformatin mode to accommodate dynamic tensile deformation at high strain rates.In addition,the interactions of dislocation-dislocation and dislocation-second phase lead to the increase of f ow stress and strain hardening with increasing strain rate.? 2020 Published by Elsevier B.V.on behalf of Chongqing University.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:EW75 magnesium alloy;High strain rates;Split Hopkinson test;Texture;Microstructure.

1.Introduction

Magnesium and its alloys are desirable structural materials for automotive,aerospace,defense and other energy intensive industries due to their low density(1.7g/cm)and high specifi strength[1-4].Automobile component is the one of the main applications for magnesium alloys to meet the stringent requirements for light weight design[5].In the last decades significan works have been performed in the investigation on mechanical properties of magnesium alloys,but most them have been focused under quasi-static loading[6-9].During service,magnesium alloys may be subjected to rigorous loading such as shock,impact and vibration,where high strain rates are commonly achieved.When the speed of vehicle crash reaches 60 km/h,the strain rate of the material exceeds 103s?1[10].The understanding on mechanical behavior at high strain rate is also very crucial to improve and optimize the manufacturing processes of magnesium alloys such as high-speed machining[11-12],high speed rolling[13-18]and high-speed extrusion[19-20].

Fig.1.Extruded EW75 magnesium alloy bar and its specimen:(a)Square bar;(b)Schematic of dynamic specimen.

Dynamic loading at high strain rates could exhibit remarkable influence on mechanical behavior and deformation mechanisms of materials comparing with quasi-static conditions.Previous researches on the dynamic tensile properties of Mg alloys mainly focus on Mg-Al alloys[10,21-27].Feng et al.[10]studied the dynamic tensile properties of AZ31B alloy with strain rates ranging from 700 s?1to 3×103s?1.They found that compared with the quasi static tension,the tensile strength and fracture elongation at high strain rates is larger.Zhou et al.[21]characterized the dynamic tensile properties of twin-roll cast-rolled and hot-rolled AZ31B alloy with strain rates ranging from 10?3s?1to 375 s?1.The results show that the yield strength and ultimate tensile strength monotonically increase with increasing strain rate,while the strain hardening exponent proportionally decreases with increasing strain rate.Kurukuri et al.[22-23]researched the dynamic tensile properties of a commercial rolled AZ31B alloy varying at the strain rates ranging from 10?3to 103s?1.Their results demonstrate that the dynamic tensile loading induced a positive strain rate sensitivity.

The deformation mechanism of Mg alloys under dynamic loading was also investigated.Mao et al.[24]characterized the dynamic tensile properties of an extruded AM30 magnesium alloy with strain rates ranging from 102s?1to 103s?1.The results show that the dependence of yield strength on strain rate in tension along extrusion direction is considerably pronounced.Ulacia et al.[25-26]analyzed the dynamic tensile properties of AZ31 alloy sheet with strain rates varying between 10?3and 103s?1.During tension tests along the in-plane directions,the prismatic slip is the main deformation mechanism,leading to alignment of the＜10-10＞directions with the tensile axis and to a spread of the basal pole toward the in-plane direction perpendicular to the tensile axis.Kurukuri et al.[22]found that the yield strength with tensile loading along rolling direction is controlled by non-basal slip and strongly rate sensitive.While the yield strength under transversal direction tensile loading is controlled by twinning and rate insensitive.Magnesium alloys have a positive strain rate sensitivity and mainly behave with a non-basal slip under high strain rates.As known,Mg-RE(rare earths)alloys have been attracted more and more interesting owing to their high-performances,including high strength at high temperature,excellent creep resistance and good corrosion resistance,etc.However,the previous studies still focused on the dynamic deformation mechanism of non-rare earth(RE)containing magnesium alloys.High strain rate deformation of Mg-RE alloys has received relatively few attentions.Additionally,RE crystallographic texture could be formed in such alloys after thermomechanical processing through alloying RE elements[27],which can influenc the deformation mechanism largely.

This paper is aimed at investigating the dynamic tensile deformation mechanism and microstructural evolution of extruded EW75 magnesium alloy at high strain rates.Split-Hopkinson tension bar(SHTB)apparatus,a very useful and widely used tool to study the dynamic behavior of various engineering materials[28-30],was utilized to characterize the dynamic tensile properties of an extruded rare earth magnesium alloy EW75 at ambient temperature.The detailed microstructure and texture evolution were analyzed by optical microscope(OM),electron back-scatter diffraction(EBSD)and transmission electron microscope(TEM).

2.Experimental procedures

A square bar of extruded EW75(Mg-5.0Y-7.0Gd-1.3Nd-0.5Zr)alloy was used as shown in Fig.1(a)which was provided by Beijing General Research Institute for Non-ferrous Metals.After being heat treated at 535 °C for 24 hours,the extrusion was carried out at 400 °C and with an extrusion rate of 1～2 m/min to produce square bars with a size of 17 mm×20 mm.The corresponding extrusion ratio is 20.Specimens for dynamic tensile tests were machined using electrical discharge machining(EDM)from square bars along the extrusion direction(ED).The size of dynamic tensile specimens is shown in Fig.1(b).

The Split-Hopkinson Tensile Bar(SHTB)was utilized to perform dynamic tension experiments at ambient temperature with strain rates from 1000 to 3000 s?1.SHTB system is shown in Fig.2.During the dynamic tensile test,the specimen was put between the incident bar and transmitter bar.Lubrication was used to reduce friction effects.Standard GB/T 30069.1-2013(Metallic Materials-Tensile testing at high strain rates-Part 1:Elastic-bar-type systems)was used for dynamic tensile testing.Three specimens were tested for each data point.True stress,true strain and strain rate of each impact can be calculated according to the one-dimensional stress wave theory.More details of experimental device and methods have been described in the literatures[31-32].

Fig.2.Schematic diagram of SHTB system.

Metallographic microstructural analysis was conducted using a Leica DM/LM optical microscope(OM).The OM samples were firs polished to 2000 grit,followed by polishing with a 0.5 mm diamond paste.The samples were then chemically etched for 30～40 s with a mixed solution that consisted of 4 g picric acid((NO2)3C6H2OH),2mL nitric acid(HNO3),25 mL acetic acid(CH3COOH),50 mL anhydrous ethanol(CH3CH2OH),60 mL ethylene glycol(CH2OH)2and 25 mL water(H2O).

Electron Back Scattered Diffraction(EBSD)analysis was conducted using the EDAX-OIMTMsoftware in a Zeiss Ultra 55TMFEG-SEM scanning electron microscope to measure the microstructure and texture evolution.The EBSD samples were firs grinded with 4000 SiC paper,then mechanically polished with a 0.05μm silica suspension and fina electro-chemically polishing for 30 s at 33V using the AC2TMcommercial electrolyte.The measure area was～30000μm with a step of 0.3μm for each sample.

Transmission electron microscope(TEM)analysis was conducted with Electron Microscope JEM-2100.The TEM samples were cut along the tensile plane of the test specimens using a cutting machine.These sections were ground to a thickness of 50～60μm using SiC paper,with 3 mm disks punched from the foils.The foils were electropolished using a twin-jet polisher to perforation using a solution of 3 vol%perchloric acid in ethanol.The samples were then cleaned by ion milling for 1 hour,with liquid nitrogen applied during the milling.

3.Results and discussion

3.1.Dynamic tensile properties

The dynamic tensile true stress-true strain curves of extruded EW75 alloy at ambient temperature with different strain rates are shown in Fig.3(a).The dynamic tensile true stress-true strain curves are all concave-down shape which are like those observed at low strain rates[33-34].Such a shape may indicate that the dominated deformation mechanism is slip in tension[35].The dynamic deformation mechanisms based on the microstructural evolution will be discussed in the following section.

In dynamic tests,it is difficul to measure the true yield stress,since at the early stage of deformation the strain rate increases gradually[21].σ0.005is used in this study as approximation to the dynamic yield property.Variations of the true stress atε=0.005(σ0.005)and dynamic maximum f ow stress(σmax)of EW75 alloy are shown in Fig.3(b).The extruded EW75 alloy obviously exhibits a conventional positive strain rate sensitivity in dynamic tension.σmaxincreases with increasing the strain rate resulting in an increase in energy absorption[36].As shown in Fig.3(b),σ0.005is nearly unchanged when the strain rate varies.At the strain rate of 3010 s?1,the extruded EW75 alloy has the relative highest dynamic ultimate tensile strength(DUTS)with a value of 271 MPa.

Both the strain hardening rate and strain rate sensitivity were analyzed as a function of true strain to further understand the dynamic tensile properties of extruded EW75 alloy,as shown in Fig.4.The strain hardening rate(dσ/dε)is define as the change rate of true stress vs true strain[37].It is calculated by the numerical differentiation of the true stress-true strain data in Fig.3(a).The strain hardening rate(dσ/dε)of extruded EW75 alloy at dynamic tension is shown in Fig.4(a).It is indicated that the strain hardening rate is rate insensitive in the strain rate range from 1406 to 3010 s?1.All strain hardening rate curves decrease rapidly from the very beginning of deformation withε＜0.01 due to the occurrence of dynamic adiabatic softening.Subsequently they reduce gradually withε＞0.01 due to plastic deformation and dislocation multiplication.

The strain rate sensitivity(β)can be determined as follows[37-38]:

Whereσis the f ow stress,˙εis the strain rate.βvalues were calculated at different true strains with each 2.5% increment in dynamic tension up to onset of plastic instability[38].The strain rate sensitivities of extruded EW75 alloy are shown in Fig.4(b).They increase with increasing the plastic strain,but decrease with increasing the strain rate.It was reported that the strain rate sensitivity for pure Mg and Mg-Al alloy increased with increasing deformation temperature[39].The dynamic adiabatic softening becomes stronger when strain rate is higher,resulting in a decrease in the strain rate sensitivity.

Fig.3.(a)Dynamic tensile true stress-true strain curves of extruded EW75 alloy along ED at different strain rates;(b)variations of the true stress atε=0.005(σ0.005)and maximum f ow stress(σmax)with the strain rate for extruded EW75 alloy.

Fig.4.(a)Strain hardening rate and(b)strain rate sensitivity of extruded EW75 alloy along ED at ambient temperature and different strain rates.

3.2.Microstructure and texture evolution

Microstructural evolution analysis can help to investigate the dynamic tensile deformation mechanisms for the extruded EW75 alloy.The initial microstructure along ED is shown in Fig.5.It is composed of two kinds of grains.The large grains elongate along ED and the small equiaxed grains distribute among the former,with average sizes of 10～15μm and 1～2 μm,respectively.

It is well known that slipping and twinning are the predominant deformation mechanisms for magnesium and its alloys.The dominant slip systems are{0001}＜11-20＞basal slip system,{10-10}＜11-20＞prismatic slip system and{11-22}＜11-23＞pyramidal slip system[40-41].The basal and prismatic slip systems have the＜a＞type Burger's vector.They are not able to accommodate general plastic deformations along c-axis.The pyramidal slip system has Burger'svector containing both＜c＞and＜a＞components,so it can accommodate plastic deformations along c-axis.The activation of a specifi slip system is dependent on deformation temperature and crystallographic texture[26].Three types of deformation twinning in magnesium alloys are{10-12}tensile,{10-11}compression and{10-11}-{10-12}secondary twinning[41].Tensile twinning supplies extension along c-axis,whereas compression and double twinning supply contraction along c-axis.To analyze the dynamic tensile deformation mechanism,EBSD technique was used to investigate the microstructure and texture.Fig.6 shows the inverse pole figur(IPF)maps and pole figure of extruded EW75 alloy before and after dynamic tension.The true strains after dynamic tension at the strain rate of 1406 s?1,2187 s?1and 3010 s?1are 4.3%,4.5% and 4.7%,respectively,as shown in Fig.3(a).The grain colors correspond to their orientations.Unlike quasi-static deformation,dynamic deformation heat is hard to be released in a very short time like an adiabatic condition.After certain plastic deformation,deformation localization occurs on the microscopic level.Thus,the grain size in dynamic tensile tested specimens has changed compared to that in the initial specimens as observed.

Fig.5.Initial microstructure of extruded EW75 magnesium alloy along ED.

Fig.6.IPF maps,inverse pole f gure,{0001}pole f gure and{10-10}pole f gure of extruded EW75 alloy:(a)initial microstructures;(b)after dynamic tension at the strain rate of 1406 s?1;(c)after dynamic tension at the strain rate of 2187 s?1;(d)after dynamic tension at the strain rate of 3010 s?1.

{0001}pole figure show that all EW75 alloys have fibe texture components,as shown in Fig.6.The extruded EW75 alloy has a weak initial texture and the c-axis of texture mainly gathers at 60°～90° away from ED direction.The extrusion texture of EW75 alloy is very different from that of AZ31 alloy.It can be mainly attributed to the addition of Y,Gd and Nd,which changes the balance of the various deformation mechanisms and dynamic recrystallization behavior[42-45].For extruded EW75 alloys,the angle between c-axis of basal pole peak and ED direction(e.g.,loading direction)is approx.60 degree.A main texture component is＜7-2-57＞//ED texture in extruded EW75 alloy.Dynamic tension causes the c-axis of texture rotate towards the direction perpendicular to the ED.The extruded EW75 alloy has a favorable orientation for the activation of basal slip when stretched along the ED.This will responsible for the formation of c-axis⊥ED texture[46-48].Both inverse pole figure and{10-10}pole figure indicate that a strong＜10-10＞//ED texture is formed after dynamic tension,as shown in Fig.6(b)-(d).Formation of＜10-10＞//ED texture during tension along the ED is usually attributed to the activity of prismatic slip[35,43,49].It indicates that both basal slip and prismatic slip have large contribution for dynamic tension.Moreover,dynamic tension can also enhance the fibe texture intensity.

A very small number of twins are also observed in some grains for all samples,as shown in Fig.6.To identify the type and distribution of deformation twins in the extruded EW75 alloy along ED,the EBSD orientation maps(OM)before and after dynamic tension are shown Fig.7.The{10-12}twinboundary(86°±5°＜1-210＞),{10-11}twin boundary(56°±5°＜1-210＞)and{10-11}-{10-12}secondary twin boundary(38°±5°＜1-210＞are highlighted by blue,red and yellow lines,respectively.The max deviation is for identifying twin boundaries is±5.Fig.7 shows all samples contains small number of{10ˉ12}tensile twins,and thus a distinct peak at a misorientation angle of about 86° can be found in all samples.Based on EBSD data,the fraction of{10-12}twin boundaries(fTTB)is also calculated and also shown in Fig.7.It indicates that dynamic tension has little influenc on the amount of{10-12}twins.The amount of{10-12}twins is still very low for all samples.This is different from previous research on dynamic deformation of AZ31 alloys.Previous investigations on AZ31 have revealed that the{10-12}tensile twins were developed at the early stage of deformation[23,25-26].Additionally,Dudamell et al.[50]reported that increasing the strain rate(～103s?1)enhanced the activation of{10ˉ12}tensile twinning dramatically.

Fig.7.EBSD orientation maps and corresponding misorientation distribution profile of extruded EW75 alloy:(a)initial microstructures;(b)after dynamic tension at the strain rate of 1406 s?1;(c)after dynamic tension at the strain rate of 2187 s?1;(d)after dynamic tension at the strain rate of 3010 s?1.The fraction of tensile twin boundaries(fTTB)is the ratio of the length of tensile twin boundaries to the length of high-angle grain boundaries(＞15°).

Fig.8.KAM maps of extruded EW75 alloy:(a)initial microstructure;(b)after dynamic tension at the strain rate of 1406 s?1;(c)after dynamic tension at the strain rate of 2187 s?1;(d)after dynamic tension at the strain rate of 3010 s?1.

Schmid factor is one parameter to be considered when the deformation modes are predicted.Besides,the critical resolved shear stress(CRSS)should also be concerned.It is generally accepted that at ambient temperature CRSSbasal＜CRSStensiletwinning＜CRSSprismatic＜CRSSpyramidal＜CRSScompressiontwinning＜CRSSsecondarytwinning[41,51].Normally,basal slip and tensile twinning are the most easily activated mechanisms during deformation.This can explain why only{10-12}twins are easily activated.However,present results indicate that the contribution of tensile twinning to plastic deformation of extruded EW75 alloy is not so significant Firstly,the weak texture in extruded EW75 alloyis conducive to the activation of the base slip,as shown in Fig.6a.Secondarily,it has been reported that the addition of rare earth elements will greatly increase the active stress of{10-12}twinning[42,44,49,52].Thirdly,the CRSS for prismatic and pyramidal(non-basal)slips decrease with the increase of temperature[13].The dynamic deformation process is assumed to be adiabatic,with most of the strain energy(about 90%)transformed into heat.The deformation induced temperature rise(ΔT)is due to both rapid mechanical deformation and lack of time to dissipate heat through the material[37].These are the reasons for the low amount of{10-12}twins in deformed EW75 alloys.Thus,dislocation slip will be the main deformation mechanism to accommodate plastic strains at such high strain rates for extruded EW75 alloy.

3.3.Analysis of dislocation substructure

To evaluate the density of dislocation,Kernel average misorientation(KAM)is calculated by the average misorientation.The distributions of KAM value and average KAM values are illustrated in Fig.8.As a rule,Kernel average misorientation is high(＞1°)in the deformed grains due to high density of dislocation.In contrast,Kernel average misorientation is low(＜1°)in the recrystallized grains[53].In Fig.8,the blue color shows the lower dislocation density areas and the green color shows the higher dislocation density areas.It is clear that before dynamic tension,most grains have a high density of dislocations as shown in Fig.8(a).

With increasing the strain rate,low dislocation density(the blue color)can be clearly observed throughout the dynamic tensile microstructures as shown from Fig.8(b)to Fig.8(d).It results in that the average KAM value is reduced by dynamic tension.The evolution of dislocation density indicates the occurrence of dynamic recovery during dynamic tension.The lower dislocation density is the consequence of dynamic recovery caused by adiabatic process during high strain rate tensile deformation.The dynamic recovery is related with the thermal effect via dynamic deformation.The temperature rise(ΔT)can be determined as follows[54]:

Whereρand Cp are the density and the specifi heat capacity at constant pressure of EW75 alloy which are 1820 kg/m3and 9830 J/(kg·K),respectively.ηis the proportion of work converted to heat which is 0.9.TheΔT results after dynamic tension at the strain rate of 1406 s?1,2187 s?1and 3010 s?1are 37 K,42 K and 53 K,respectively.AlthoughΔT results indicate that they are relatively small to affect the mechanical properties,they are very significan in some regions that undergo a strong degree of deformation with the localization process[55].Therefore,the activity of non-basal slip systems and dynamic recovery may be enhanced by dynamic tension.The occurrence of recovery can also explain that the strain hardening rate reduce rapidly due to the dynamic adiabatic softening in Fig.4(a).

TEM analysis was conducted on the extruded EW75 magnesium alloy specimens after dynamic tension at high strain rates to gain further insight into the deformation mechanism.Bright fiel TEM micrographs for the specimens after the dynamic tension along ED at the strain rate of 3010 s?1are shown in Fig.9.High-density dislocations were produced and tangled with each other after dynamic tension as shown in Fig.9(a).Although the dislocation density is difficul to be accurately measured because of serious dislocation-dislocation entanglement,the dislocation density of Mg and its alloys after high strain rate deformation was estimated to be around 1014m?2[37].Pan et al.[56]suggest that the high-density dislocations are non-basal dislocations which results in the positive strain rate sensitivity of ultimate tensile strength.In the present work,no compressive twins were observed.Thus,the very high strain hardening rates in Fig.4 are as a result of non-basal dislocation activation and the formation of dislocation tangles by dislocation-dislocation interactions.

A typical dislocation cell formed due to dislocationdislocation interactions is shown in Fig.9(b).Normally,the dislocation cell structure is difficul to form and to be observed at low strain rate[45].However,high-density dislocations could be produced at high strain rate(＞103s?1).The enhanced dislocation-dislocation interactions result in the formation of the dislocation cell as an evidence of dynamic recovery.It is also observed that there are pile-ups of dislocations around a second phase and the second phase acted as a barrier to dislocation motion in Fig.9(c).

The selected area diffraction pattern(SAED)of the second phase shown in Fig.9(d)is a typical fin particle identifie as the Mg5RE phase[57].The TEM results indicate that the interactions of dislocation-dislocation and dislocation-second phase lead to that both the fl w stress and strain hardening increase with increasing strain rate.

4.Conclusions

In this work,the dynamic tensile properties and microstructural evolution of extruded EW75 alloy at high strain rates were investigated.The main conclusions are summarized as follows:

1.Extruded EW75 alloy exhibits a conventional positive strain rate sensitivity in dynamic tension.At the strain rate of 3010 s?1,it has the relative highest dynamic ultimate tensile strength under the present testing strain rates,which is 271 MPa.

2.Dynamic tension enhances the fibe texture intensity in the extruded EW75 alloy.After dynamic tension,a strong＜10-10＞//ED texture is formed.

3.Only{10-12}tensile twins are mainly observed in the extruded EW75 alloy during dynamic tension process.However,dynamic tension cannot enhance the activity of{10-12}twinning.Both basal slip and non-basal slip are required to accommodate the dynamic tensile deformation at high strain rates.4.The interactions of dislocation-dislocation and dislocationsecond phase result in the increase in the fl w stress and strain hardening.

Acknowledgements

The authors would like to thank Professor Kui Zhang,Beijing General Research Institute for Nonferrous Metal,for providing EW75 magnesium alloy for this work and acknowledge the funding from the Natural Science Foundation of the Jiangsu Higher Education Institutions of China(Grant No.17KJD430006),Scientifi and Technological Innovation Team Foundation of Wuxi Institute of Technology(No.30593118001)and Scientifi Research Project of Wuxi Institute of Technology(No.ZK201901).The help of EBSD experiment provided by Yukyung Shin from Helmholtz-Zentrum Geesthacht is gratefully acknowledged.