Influence of thermomechanical processing on microstructure,mechanical and strain hardening properties of single-phase Mg-4Li-0.5Ca alloy for structural application

N.Srirmn,S.Kumrn,Sthiy Nrynn N

aSchool of Mechanical Engineering,SASTRA Deemed to be University Thirumalaisamudram,Thanjavur,Tamilnadu 613401,India

b Department of Metallurgical and Materials Engineering,National Institute of Technology Tiruchirappalli,Tamil Nadu 620015,India

Received 2 January 2020;received in revised form 22 July 2020;accepted 7 August 2020 Available online 5 October 2020

Abstract The single-phase Mg-4Li-0.5Ca alloy was rolled at three different temperatures(250,300 and 350°C)and followed by annealing at 200°C for 10min.To evaluate the mechanical properties,the tensile test was conducted at a constant strain rate of 10?3 s?1.Factors influencing the tensile strength and strain hardening properties were assessed by microscopy,XRD and EBSD analysis.Besides,Kocks–Mecking plots(K-M)were used to determine the different stages of strain hardening exhibited by the variously processed Mg-4Li-0.5Ca alloy test specimens.The ultimate tensile strength has decreased as hot-rolling temperature increases with increased ductility.The strain hardening properties such as hardening capacity(Hc),strain hardening exponent(n)are increased significantly with an increase in hot rolling temperature and subsequent annealing.

Keywords:Mg-4Li-0.5Ca;Hot rolling;Twins;Mg2Ca;Hardening capacity.

1.Introduction

Globally,researchers are giving increasing attention towards Mg and its alloys owing to their light-weight,high stiffness,recyclability,weldability and easy castability[1–3].Due to these unique properties of magnesium and its alloys are used in automobile,aerospace and electronic industries[4,5]However,the poor formability of Mg due to its limitedslip systems at room temperature makes it of limited use in structural applications[6,7].To overcome this limitation,and extend the potential for structural applications,suitable alloying and thermo-mechanical processing are needed.Adding lithium to Mg increases formability by reducing critical shear resolved stress of non-basal slip planes and promotes cross slip[8,9].The addition of lithium reduces the density of the magnesium[10].Due to these essential characteristics,Mg-Li alloys are used in ultra-lightweight communication systems and aircraft structures[11].Despite these advantages,Mg-Li alloys exhibited poor strength[12,13],the addition of lithium greater than 4 wt% exhibited less corrosion resistance due to high reactive characteristics of Li than Mg in the aqueous medium[14].The corrosion resistance ofα-phase alloy(Mg-4Li)was better than that ofβ-phase alloy(Mg-12Li)[15].Addition of calcium increased the strength of Mg-Li alloy by refining the grain size and forming Mg2Ca compound[16].Calcium also improved the oxidation resistance of magnesium,making it widely used in structural applications[17].

Strain hardening behaviour is considered to be the most important parameter in various structural applications because it is closely related to ductility and strength properties[18].It is reported that strength and strain hardening capacity of the materials are greatly influenced by grain size,twins,and temperature[19,20].Due to limited-slip system activation at 200°C,the magnesium alloys can be deformed only above this temperature[21,22].However,the literature on the impact of thermo-mechanical processing on tensile and strain hardening properties of Mg-4Li-0.5Ca alloy is limited.Therefore,in this study,efforts have been made to investigate the effect of hot rolling at different temperatures(250,300 and 350°C),annealing at 200 °C for 10min on tensile properties and strain hardening behaviour of Mg-4Li-0.5Ca alloy.

Fig.1.SEM micrographs of(a)as-received,(b)R250 and(c)EDS analysis of as-received Mg-4Li-0.5Ca alloy.

2.Experimental procedure

Mg-4Li-0.5Ca(wt%)billet was rolled at different temperatures(200,300 & 350°C)with a 75% reduction from its initial cross-section and annealed at 200°C for 10min.The following abbreviations are used for varying conditions of Mg-4Li-0.5Ca alloys:(1)as-received-Mg-4Li-0.5Ca,(2)R250-hot-rolled at 250°C,(3)R300-hot-rolled at 300°C,(4)R350 hot-rolled at 350°C,(5)AR250-hot-rolled at 250°C and annealed at 200°C,(6)AR300–hot-rolled at 300°C and annealed at 200°C and(7)AR350-hot rolled at 350°C and annealed at 200°C.Microstructures and phase identification were characterized by an optical microscope,scanning electron microscope(SEM)equipped with Energy dispersive spectroscopy(Model-Hitachi,3000 series)and X-ray diffraction studies.Twin density variations of thermomechanically processed alloys are evaluated by Electron Backscattered Diffraction(EBSD)with step size of 0.5μm and 70°title angle.The Colloidal silica suspension and diamond paste are used to polish the samples for EBSD analysis.To correlate the microstructures with mechanical properties,the tensile test was conducted at a strain rate of 10?3s?1at room temperature using the universal testing machine(Model:Tinus Olsen 100R,which having high elongation extensiometer,Make:U.K)according to ASTM E8.To determine the strain hardening behaviour,hardening capacity(Hc)and strain hardening exponent(n)were calculated.Hardening capacity was calculated by using Eq.(1)[23,24].

To quantify the strain hardening response of Mg-4Li-0.5Ca alloy,the strain hardening exponent(n)was evaluated by the Eq.(2)

Where n is the strain hardening exponent and K is the strength coefficient of the material.The factors influencing strain hardening behaviours such as activation of slip systems and variation in accumulation of dislocations are evaluated through EBSD.The various stages of strain hardening rate exhibited by the hot-rolled and subsequently annealed Mg-4Li-0.5Ca alloys are illustrated through Kocks-Mecking(K-M)plot.

Fig.2.Optical micrographs of Mg-4Li-0.5Ca alloy in different processing conditions(a)R250,(b)AR250,(c)R300,(d)AR300,(e)R350 and(f)AR350.

3.Results and discussion

3.1.Characterization of Mg-4Li-0.5Ca alloy

The backscattered SEM micrograph of the as-received alloy shows single-phase Mg alloy and coarser grains of 84±3μm with an eutectic(Mg+Mg2Ca)segregates along the grain boundaries(Fig.1a)and confirmed by EDS analysis(Fig.1c).Mg-4Li-0.5Ca alloy rolled at 250°C for 75%reduction exhibits relatively fine grains(21±2μm)with deformation twins and uniformly dispersed Mg2Ca compound throughout the matrix,which is shown in Fig.1b.Further,a slight increase in the grain size and reduction in twin density are observed with increasing rolling temperature(250 to 350°C)which is shown in Fig.2.The decrease in twin density is due to the activation of non-basal planes by lowering the critical resolved shear stress when rolled above 250°C[25].It is evident from the XRD patterns shows(Fig.3)that basal planes and non-basal planes get activated upon hot rolling.Also,the reduction in twin density may be due to the transformation of twinned grains into new recrystallized grains which is shown in Fig.2.As twins have much-stored deformation energy than the matrix,this leads to favourable sites for dynamic recrystallization nucleation[26].Also,the annealing of the hot-rolled alloys shows a significant decrease in the twin density with a slight increase in the grain size.The effect of reduction in twin density after short annealing in hot-rolled Mg-4Li-0.5Ca alloy is evident through OIM-EBSD image of R350 and AR350 alloy,as shown in Fig.4.

Fig.3.XRD patterns Mg-4Li-0.5 Ca alloy processed in different conditions.

Fig.4.OIM image of(a)R350,(b)AR350 and(c)Tension twins volume fraction of R350 and AR350 Mg-4Li-0.5 Ca alloy.

3.2.Tensile and strain-hardening characteristics of Mg-4Li-0.5Ca alloy

Fig.5 shows the true stress-strain curves of Mg-4Li-0.5Ca alloy under different processing conditions.The as-received alloy shows an ultimate strength(UTS)of 80±7MPa with an elongation of 4±1%.After hot rolling(R250,R300 &R350),Mg-4Li-0.5Ca alloys exhibited a significant increase in strength and elongation than the as-received alloy.The improvement in strength is due to grain refinement and dispersion of Mg2Ca compound in the matrix.The enhancement in elongation is attributed to activation of more slip planes with an increase in the hot rolling temperature.

Fig.5.True stress-strain curves of as-received and thermo-mechanically processed Mg-4Li-0.5Ca alloy.

Amongst the different temperature hot rolled Mg-4Li-05Ca alloy,R250 alloy exhibits higher UTS of about 221±3MPa with lower elongation of 5±1%.The grain refinement during rolling at 250°C is the contributor for increasing strength.Also,the presence of twins inside the grains has high deformation energy,which hinders the dislocation motions and leads to dislocation pile-up,consequently further increased strength of the alloy[27,28].While increasing rolling temperature from 300 to 350°C,the strength gets reduced with increase in elongation.Also,annealing of hot-rolled alloys(AR250,AR300,and AR350)shows a slight decrease in strength with a remarkable increase in ductility(nearly 20–30%)than its corresponding hot-rolled alloy.The notable increase in elongation is due to an increase in grain size,reduction of twin density and activation of more slip systems.

Fig.6.EBSD based slip trace analysis of AR350 alloy(a)basal plane slip traces of(0001),(b)and(c)denotes 1st and order 2nd order pyramidal slip plane traces(011)and(112).

Fig.7.Strain hardening behaviour(K-M Plot)of as-received and thermomechanically processed Mg-4Li-0.5Ca alloy.

The hardening capacity(Hc)values and strain hardening exponent(n)of the as-received,hot-rolled and annealed Mg-4Li-0.5Ca alloys are given in Table 1.The hot rolled and subsequently annealed alloys show a significant increase in Hcvalues(100–400%)than the as-received alloy.Since the as-received Mg-4Li-0.5Ca alloy has limited slip systems,it results in poor deformability,which leads to poor hardening capacity.Besides,Hcvalue increases with increasing rolling temperature and subsequent annealing due to activation of more slip systems.Apart from the slip systems,Hcis also dependant upon material properties such as yield strength and grain size of the material as per the Hall-Petch relation.Hence,Hcincreased upon increasing rolling temperature and subsequent annealing of Mg-4Li-0.5Ca alloy.

The strain hardening exponent(n)results showed a similar trend as observed in hardening capacity(Hc).The AR350 alloy exhibited higher strain hardening exponent(n)than the asreceived and other thermo-mechanically processed Mg-4Li-0.5Ca alloys.Thus,enough slip mode is available for the accommodation of more plastic strains upon an increase in rolling temperature and subsequent annealing,which results in the increasing of strain hardening exponent.To quantify the activation of slip systems,slip trace analysis is carried out for AR350 alloy using EBSD(Fig.6a,b and c).From the slip trace analysis,it is evident that basal(0001),primary and secondary pyramidal(011)and(112)planes are activated.

Table 1Grain size and tensile properties of as-received and thermo-mechanically processed Mg-4Li-0.5Ca alloy.

Further,to analyse the strain hardening behaviour,the strain hardening rate(dσ/d€)vs true plastic strain curve is plotted(Fig.7).Amongst the as-received and thermomechanically processed Mg-4Li-05Ca alloys,except R250,all the alloys revealed both stage 3 and 4.Stage 3 indicates instantaneous storage and annihilation of dislocations and stage 4 represents the dislocation storage process which reached saturation at critical stress/strain.The as-received alloy showed a lower strain hardening rate than the thermo-mechanically processed Mg-4Li-05Ca alloys,which reveals poor strain hardening behaviour of the alloy.

Initially,R250 exhibited higher strain hardening rate,but it dropped drastically and fractured before 0.05 true plastic strain relative to other hot rolled and annealed alloys which confirm the plastic instability of the R250 alloy.The fine grain structure and the high twin density in R250 would act as a barrier for dislocation movement resulting in higher dislocation strain field interactions leading to higher initial strain hardening rates.The fine grains and more twins decrease the mean free path for dislocation movement reducing its plastic deformation,which contributes to the sudden failure of the alloy.AR250,R300,AR300 R350 and AR350 alloys showed lower initial strain hardening rate and higher strain hardening rate at the later stages.The lower initial hardening rate is due to the effect of coarse grains and lower twin density.Also,the presence of coarser grains provides more space for the accumulation of more dislocations at later stages which improved the strain hardening behaviour[29].

The K-M plot(Fig.7)reveals the AR350 alloy shows the highest strain hardening rate of 675MPa at 0.052 true plastic strain which indicates AR350 alloy’s strong load-bearing potential than the other hot rolled and annealed Mg-4Li-0.5Ca alloys.

Further to probe the strain hardening behaviour of hot rolled and annealed Mg-4Li-05Ca alloys,the dislocation density accumulation during tensile test is taken through Kernel Average Misorientation mapping(KAM)of fractured R250 and AR350 alloy tensile samples(Fig.8a and b).The red colour code region corresponds to the highest dislocation stored area.AR350 alloy shows more dense red colour region which indicates higher strain energy storage area i.e.more dislocation density accumulation compared to R250 alloy.It indicates superior strain hardening behaviour of AR350 alloy.

Fig.8.KAM map of(a)R250 and(b)AR350 alloy after tensile test.

4.Conclusions

(1)The microstructure of as-received Mg-4Li-0.5Ca alloy composed of single phaseα-Mg and an eutectic(Mg+Mg2Ca)segregates along the grain boundaries.After hot rolling,Mg-4Li-0.5Ca alloy showed fine grains with the fragmented Mg2Ca compound dispersed throughout the matrix.

(2)The twin density reduced with an increase in hot rolling temperature from 250 to 350°C.Further subsequent annealing of hot-rolled alloys showed a 40–50% reduction in twin density with a slight increase in the grain size.

(3)The tensile strength of Mg-4Li-0.5Ca alloy increased significantly after hot rolling.Besides,tensile strength is decreased with a significant increase in ductility with an increase in hot rolling temperature and subsequent annealing.

(4)The strain hardening characteristics are increased significantly after different temperature hot rolling and subsequent annealing.Amongst the variously processed Mg-Li-Ca alloy,AR350 exhibited relatively superior strain-hardening characteristics like higher hardening capacity(Hc)and strain hardening exponent(n)nearly 2–3 fold times than the other hot-rolled Mg-4Li-0.5Ca alloys.This is due to the combined effect of coarser grain structure,reduction in twin density and activation of more slip planes.

(5)AR350 alloy showed higher strain hardening rate at later stages amongst all hot rolled and annealed Mg-4Li-05Ca alloys due to accumulation of more plastic strains(dislocation density)as evident in KAM mapping.Therefore,AR350 can be a potential candidate suitable for structural application amongst the variously processed Mg-4Li-05Ca alloy.