Effect of heat treatment on LPSO morphology and mechanical properties of Mg–Zn–Y–Gd alloys

Hongxin Liao,Jonghyun Kim,?,Taekyung Lee,Jiangfeng Song,Jian Peng,Bin Jiang,Fusheng Pan,?

a College of Materials Science & Engineering,Chongqing University,400044,China

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

cSchool of Mechanical Engineering,Pusan National University,Busan 46241,Korea

Received 20 August 2019;accepted 30 June 2020 Available online 15 August 2020

Abstract The mechanical properties and microstructure were investigated under different Zn content and heat treatment conditions in a Mg–Zn–Y–Gd cast alloy.A part of the long period stacking order(LPSO)phases transformed to W-Mg3Zn3RE2 phases with an increase in Zn content from 0.9 at.% to 1.8 at.%,and the ultimate tensile strength(UTS)increased from 229MPa to 248MPa.With solution treatment at 480°C,the content of the LPSO phase and strength sharply decreased in the Mg-1.8Zn-0.8Y-0.8Gd alloy,whereas this change was not significantly observed in the Mg–0.9Zn–0.8Y–0.8Gd alloy.After solution treatment,the elongation significantly improved and the UTS sharply decreased in both alloys.The lamellar and filminess LPSO phases were observed with aging treatment at 200°C.Moreover,the strengthening efficiency of lamellar and filminess LPSO phases was lower than that of the block LPSO phases.Therefore,the UTS of the T6 state was lower than that of the as-cast alloy.

Keywords:Mg alloy;LPSO;Age behavior;Mechanical properties;Heat treatment;Strengthening efficiency.

1.Introduction

Because magnesium alloys have many advantages,such as high specific strength,low density,and high damping capacity,they are widely used in electronic products and automotive industries[1-2].However,their poor mechanical properties limit the applications of as-cast Mg alloys[3].Recently,the addition of rare earth(RE)elements to magnesium alloys has been widely reported to improve the microstructure and enhance mechanical properties of these alloys[4-6].

Mg–Zn–RE alloys have attracted considerable attention as an important metallic material because of their good damping and excellent mechanical properties[7].This alloys has different types of reinforced phases,such as Mg30Zn60RE10(I)phase,Mg3Zn3RE2(W)phase,and Mg12ZnRE(X)phase with an LPSO structure,that can be obtained under different heat treatment condition and by different elementary compositions[8-9].In addition,the good mechanical properties of the Mg–Zn–RE alloys can be attributed to the presence of the second phase in the LPSO structure in these alloys[10].Kawamura et al.[11].reported that Mg97Y2Zn1(at.%)prepared by rapidly solidified powder metallurgy has excellent mechanical properties,such as yield strength and elongation of 610MPa and 5% at room temperature,respectively.Fei et al.[12]revealed that the substitution of half of Gd for Y imparts better mechanical properties than the nonsubstituted alloy.Furthermore,the Mg–Zn–Y–Gd alloy has good mechanical properties due to strengthening by the LPSO phase[13].

Table 1Chemical compositions of experimental alloys(at.%).

As is well known,the microstructural evolutions in Mg–Zn–Y–Gd alloys during heat treatment are complicated;some microstructures are formed depending on not only alloy compositions[14]but also solution treatment processes[15].Under different heat treatment condition,the alloy has different morphologies and contents of LPSO phase[16].Thus,a heat treatment condition has significant effects on the mechanical properties of the Mg–Zn–Y–Gd alloy.Lu et al.[17]reported that the Mg–Zn–Y alloy with a block shape LPSO phase possesses higher strength than that with a lamellar LPSO phase.However,the effects of morphology and LPSO phase content have not been investigated in detail.

The content of Zn has a significant effect on the microstructure and mechanical properties during the heat treatment process of Mg–2.0Gd–1.2Y–0.2Zr[18].However,in most studies,the content of Zn is less than that of the RE;therefore,the effect of high Zn content on the cast alloys has not been elucidated thus far.In this study,the Mg–Zn–Y–Gd alloy with different Zn contents has been shown to exhibit different microstructures and age-hardening behavior.Moreover,different contents and morphologies of the LPSO phase were observed after heat treatment.The effect of Zn content on the microstructure,aging behavior,and mechanical properties of the Mg–Zn–Y–Gd alloy has been investigated.

2.Experimental procedure

The alloy ingots were prepared in an electric resistance furnace with a mild steel crucible under an atmosphere of mixed protective gases including 99% CO2and 1% SF6.The raw materials were commercially available pure Mg,pure Zn,and Mg-30%Gd(wt.%)and Mg-30%Y(wt.%)master alloys.The melt was held at 720°C for 20 min and then poured into a permanent mold preheated at 300°C.The actual chemical composition of the alloys was determined by X-ray fluorescence spectrometry,and its details are listed in Table 1.To study the effect of Zn content,we designed two alloys with different Zn contents.The samples cut from the ingots were solution-treated at 480°C for 24 and 48h,followed by water quenching.The T4 alloy samples were subjected to aging treatment at 200°C for 12–120h.

The Vickers hardness test was carried out at a load of 5kg for 15s.The tensile samples with a length of 25mm and a center diameter of 5mm were tested using a CMT5105 material testing machine at a crosshead speed of 1.5mm/min at room temperature and 200°C.The tensile yield strength(TYS),UTS,and elongation to fracture were averaged over at least three individual tests.The microstructures of the ascast samples were observed by field-emission scanning electron microscopy(FE-SEM;JEOL JSM-7800F)and an HKL

Fig.1.XRD patterns of alloys A and B.

Chanel 5 electron backscatter diffraction(EBSD)system.The second phases were identified by X-ray diffraction(XRD)analysis using a Rigaku D/MAX-2500PC system.Transmission electron microscopy(TEM)was performed using an FEI Tecnai G2 F20 instrument operating at 200kV.All samples for the microstructural observation and property tests were taken from the center of the alloy ingots.

3.Results

3.1.Microstructure

Fig.1 shows the XRD patterns of the as-cast alloys with different Zn contents.Three main phases were found,namely,α-Mg(matrix),theW-phase(Mg3Zn3RE2),and theX-phase(Mg12Zn1RE1)with an LPSO phase.Alloy B showed onlyα-Mg and LPSO phases.The peak of the LPSO phase in alloy B was higher than that in alloy A.This indicated that the content of the LPSO phase in alloy B was more than that in alloy A.With the increase in Zn content,theW-Mg3Zn3RE2phase was observed in alloy A.This phenomenon is in agreement with previous reports[19].

Fig.2 shows the SEM micrographs of the as-cast alloys.A net structure was observed in alloys A and B.According to the XRD result,the dark gray and light gray sections were the LPSO(in alloy B,as marked in Fig.2(b))andW-Mg3Zn3RE2phases are shown in alloy A(as marked in Fig.2(a))respectively.Moreover,in alloy A,theW-Mg3Zn3RE2and LPSO phases showed alternate distribution in the matrix.Compare to alloy B,the block LPSO phase in alloy A showed considerably more regularity and orderliness.

The microstructures of the alloys after heat treatment were observed by SEM.SEM images of solution-treated alloys are displayed in Fig.3.According to the calculation results,determined using the software X,the volume fraction of 4.9%and 8.2% LPSO phases dissolved intoα-Mg in alloy A with solution treatment times of 24 and 48h at 480°C,respectively.Compared to alloy A,alloy B showed a decrease of only 1.8%and 2.2% LPSO phases during 24 and 48h solution treatment time at 480°C,respectively.In alloy A,the content of theW-Mg3Zn3RE2phase did not significantly change during the solution treatment.This indicates that theW-Mg3Zn3RE2phase cannot dissolve intoα-Mg at 480°C.A large number of the LPSO phases did not dissolve inα-Mg;thus,some particles of theW-Mg3Zn3RE2phase existed alone atα-Mg,rather than near the LPSO phase.Due to solution treatment,the net structure of some LPSO phases was broken and was sporadically distributed in alloy B.

Fig.2.SEM micrographs of as-cast alloys:(a)alloy A and(b)alloy B.

Fig.3.SEM micrographs of T4 alloy specimens:solution treatment time for alloy A:(a)24h,and(b)48h and that for alloy B(c)24h and(d)48h.

Fig.4 shows the SEM micrographs of the T6 alloys.The highly magnified images showed some lamellar LPSO phases in both alloys A and B(marked by capital B).Also,as shown in the figure,the lamellar LPSO phases in alloy B were considerably more than those in alloy A.The block LPSO phases(marked by capital A)were present near the lamellar phases in alloy B;however,the lamellar phases just separate from theα-Mg matrix and have no contact with the block phases in alloy A.TheW-Mg3Zn3RE2phase(marked by capital C)also existed in alloy A alone,i.e.,the heat treatment did not affect this alloy.Wu et al.[16]reported that the block LPSO phases dissolve inα-Mg at 500°C for 12h in a Mg–10Gd–1Zn–0.5Zr(wt.%)alloy,followed by separation of the lamellar phases at 420°C for 1h.

Fig.5 shows the inverse pole figure(IPF)maps of different state alloys under different heat treatment conditions.Color of grains represents grain orientation;from the figure it can be observed that the orientation of all alloys is random,no significant texture exists,and the grains were coarse equiaxial crystals.The grain size is listed in Table 2.The grain sizes of as-cast alloy A and B were 70.0 and 58.9μm,respectively.With the increase in the heat treatment period,the grain sizes of alloy A and B showed an obvious increase.The grain sizes of alloys A and B increased to 82.8 and 87.6μm after T4 treatment,respectively.With T6 treatment,the grain sizes of alloys A and B changed to 92.9 and 95.7μm,respectively.

Fig.4.SEM micrographs of T6 alloy specimens.Low magnification images of(a)alloy A and(c)alloy B.High magnification images of(b)alloy A and(d)alloy B.

Fig.5.IPF maps of the different state alloys:alloy A(a)as-cast,(b)48h solution treatment,(c)T6(age-peak),alloy B,(d)as-cast,(e)48h solution treatment,(f)T6.

3.2.Age-hardening behavior

Fig.6 shows the age-hardening curves of alloy A and B subjected to 48h solution treatment at 200°C.In the solutiontreated stated,the hardness of alloy A was only 67.4 HV.After aging at 200°C,the hardness increased with increase in solution treatment time before 60h.At 60h,the agepeak occurs in alloy A and the highest hardness of 75.7 HV was achieved.Alloy B had initial hardness(68.2 HV)similar to that of alloy B.However,peak aging occurred at just 12h for alloy B.Furthermore,the increase in the agepeak of alloy B was caused by with the increase in Zn content.Honma et al.[18]also reported that the increase in Zn content delays the age-peak time of Mg–2.0Gd–1.2Y–0.2Zr alloys.

Table 2Grain size and volume content of different state alloys.

Fig.6.Age-hardening curves of alloys A and B at 200°C.

Table 2 shows the result of calculated volume content with different states calculated using at least three photos.The content ofW-Mg3Zn3RE2phases did not significantly change with heat treatment.Also,the LPSO phase contents of alloys A and B were 21.7% and 23.6%,respectively.With longer solution treatment time at 480°C,the content of LPSO phases decreased.Compared to alloy B,the transformation in alloy A was more significant with the increasing solution treatment time.The contents of LPSO phases increased with age treatment and were 20.1% and 22.9% for alloys A and B,respectively.The content of LPSO phases in T6 state was lower than that in as-cast state,due to the presence of lamellar LPSO phases in the as-cast alloys.

Fig.7 shows the bright-field images,high-resolution images,and corresponding selected-area electron diffraction(SAED)patterns of alloy A with the aging precipitates generated during aging treatment.The aging precipitates with lamellar morphology andW-Mg3Zn3RE2particles were observed,as shown in Fig.7(a)and(b),respectively.The presence of aging precipitates of Mg–Zn–Y–Gd alloys with aging treatment has been confirmed[19].Furthermore,theW-Mg3Zn3RE2particles were found to exist at the grain boundary and grain interiors.The filmy LPSO phase(marked by arrow in Fig.7(c)),with a thickness of～1nm,was observed in the high-resolution image.Also,from this figure,the LPSO phase at the bottom right corner was a part of the lamellar LPSO phase.The corresponding SAED pattern indicated that the lamellar LPSO phase exhibited the features of the 14H-type LPSO phase[20].

Fig.7.TEM images of alloy A with T6 heat treatment:(a)and(b)bright field images,(c)high-resolution image,(d)corresponding SAED pattern.

Fig.8.TEM images of alloy B with T6 heat treatment:(a)bright field images,(b)high-resolution image.

Fig.8 shows the bright-field and high-resolution images of alloy B with T6 treatment.As shown in Fig.8(a),the black lamellar phase was the LPSO phase.Also,the LPSO phase exhibited much more orderliness and was thinner than that of alloy A.In addition,the filminess LPSO phase,marked by arrow,was observed in the high-resolution image.The lamellar LPSO phase in alloy B was thinner than that in alloy A,the thickness was～33nm.

3.3.Mechanical properties

Tensile stress–strain curves of the alloys under different heat treatment conditions at room temperature are shown in Fig.9.The as-cast alloy A exhibited the highest yield strength(YS)of 134.9MPa,the highest UTS of 247.8MPa,and the lowest elongation of 2.9%.The YS,UTS,and elongation of the as-cast alloy B were 123.2MPa,228.2MPa,and 3.2%,respectively.The UTS increased from 228.2 to 247.8MPa as the Zn content increased from 0.9 to 1.8wt%.On the other hand,alloy A with the T4 state showed the lowest YS of 90.8MPa,the lowest UTS of 180.7MPa,and the highest elongation of 8.6%.Thus,the mechanical properties of the T6 state were between lower than the as-cast alloys but higher than the T4 state in both alloys A and B.Given the differences in the as-cast and T4 and T6 states,the alloy with high Zn content has lower strength at the T4 and T6 states.

Fig.9.Tensile stress-strain curves of the alloys under different heat treatment conditions at room temperature:(a)alloy A,(b)alloy B.

Table 3Mechanical properties of different state alloys.

The mechanical properties of the different state alloys at room temperature and 200°C are listed in Table.3.The strength of alloy B reduced significantly at 200°C;however,alloy A exhibited superior strength.Also,the reduction in mechanical properties of alloy B from room temperature to 200°C was obvious.The YS,UTS,and elongation at 200°C of alloy B with T4 and T6 states were 67.5MPa,147.5MPa,and 9.0% and 78.9MPa,158.2MPa,and 4.8%,respectively.This indicates that alloy A still has a high strength because of its excellent thermal stability at 200°C.

4.Discussion

As shown in Table 2,the volume fraction of LPSO andWMg3Zn3RE2phases were 23.9% in alloy A,which were similar to the volume fraction of LPSO phases in alloy B(23.6%).This indicated that a part of the LPSO phase transforms to theW-Mg3Zn3RE2phases as the Zn content increased from 0.9 at.% to 1.8 at.%.As is well-known,the value of Zn/RE is an important parameter to evaluate the type of the second phase in Mg-Zn-RE alloys.When this value is close to 0.5,the second phase is the LPSO phase,as shown for the Mg97Zn1Y2alloy[21].As this value increases to more than 1,the LPSO phase transforms to theW-Mg3Zn3RE2phases and theI-Mg30Zn60RE10phase[22].Moreover,the grain size does not significantly change with this value;because the structure composed of combined LPSO phases andW-Mg3Zn3RE2phases,the mechanical properties of alloy A are improved.TheW-Mg3Zn3RE2phase,a common phase found in Mg–Zn–RE alloys,is generally not realized to be an excellent strengthening phase[23].However,the combination ofWand LPSO phases exhibits better phase strength efficiency than that by the presence of only LPSO phases[19].Because of the higher melting point ofW-Mg3Zn3RE2phases than that of LPSO phases,alloy A has considerably higher thermostability than that of alloy B,thereby enabling it to exhibit superior strength at 200°C.

With solution treatment at 480°C,the content of the LPSO phase significantly decreased from 21.7% to 13.5% in alloy A.However,this change was not significantly observed in alloy B,with a decrease from 23.6% to 21.5% for 48h solution treatment at 480°C.This indicates that the increase in Zn content has a positive impact on the solution behavior at 480°C.The strength of alloy A with T4 decreased because of the absence of the LPSO phase.Lesser the number of LPSO phases,lower is the alloy strength.Moreover,the LPSO phase is the main strengthening phase rather thanWMg3Zn3RE2phase[9].Therefore,the strength of alloy B was higher than that of alloy A at the T4 state.Also,the grain size increased with the solution treatment,which decreased the alloy strength according to the Hall–Petch equation.

Fig.10.Schematic illustration of dislocation pass kink band of different LPSO phases:(a)filminess,(b)lamellar,(c)block.

The aging times of alloy A and B were 60h and 12h,respectively.As a great portion of the LPSO phase dissolved in theα-Mg phase in alloy A at the T4 state,the latter needed more time to separate the LPSO phase than alloy B did to get to the highest hardness.There are not only many lamellar LPSO phases,but also a filminess LPSO phase observed at the T6 state,as shown in Fig.4,Fig.7,and Fig.8.Wang et al.[24]reported that the lamellar LPSO phases exist with heat treatment,and the alloy with lamellar LPSO phases and block LPSO phases have lower strength than that of the alloy with just block LPSO phases.As is well known,the strength of alloys will increase if the dislocation slip is blocked.Gao et al.[25]found that the LPSO phases begin to exhibit kink deformation due to compression along the basal plane of the LPSO phase,and the angle of the kink boundaries is increased with further compressive deformation.Fig.9 shows a schematic illustration of dislocation pass kink band of different types of LPSO phases.When dislocation slips to the filminess LPSO phase,it is easy to pass because the filminess LPSO phase is easy to kink by compression.The filminess LPSO phase cannot effectively prevent the dislocation slip.The thicknesses of lamellar and block LPSO phase are much higher than that of the filminess LPSO phase,which is difficult to kink by compression.As seen in Fig.10,the dislocation is hard to pass the lamellar and block LPSO phases.Therefore,the block LPSO phases have the highest strengthening efficiency of alloys,the second highest provided by the lamellar LPSO phases,and filminess LPSO phase affording the lowest strengthening efficiency.The grain size increases with age treatment,but the increase in the LPSO phase content is the main reason that grain size increases with age treatment in the T6 compared to T4 state.The precipitation strengthening causes a major effect on the alloy,rather than the effect of grain size.Because the lamellar and filminess LPSO phases cause lower strengthening efficiency than that of block LPSO phases,the strength of T6 alloys is lower than that of as-cast alloys.

5.Conclusions

1.A part of LPSO phases transformed toW-Mg3Zn3RE2phases because of the increase in Zn content from 0.9 at.%to 1.8 at.%.The combination of LPSO andW-Mg3Zn3RE2phases caused stronger strengthening efficiency than that by just the LPSO phase both at room temperature and 200°C.

2.With increase in Zn content,the LPSO phases in alloy A were much more easy to transform toα-Mg and the content of LPSO phases sharply decreased in the T4 state of alloy A.Thus,the maximum aging time in alloy A was much longer than that in alloy B.

3.Because different types of LPSO phases have different resistance to stress,every type of LPSO phases has different strengthening efficiency.The highest strengthening efficiency was offered in the following order:block LPSO phases>lamellar LPSO phases>filminess LPSO phases.

Declaration of Competing Interest

None.