Qin Zho,Yujun Wu,b,?,Wei Rong,Ke Wng,Lingyng Yun,Xingwen Heng,Liming Peng,b

a National Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composite,Shanghai Jiao Tong University,School of Materials Science and Engineering,Shanghai Jiao Tong University,Shanghai,200240 China

b School Innovation Institute for Materials,Shanghai,200444,China

Abstract Microstructural evolution of Mg–15Gd–1Zn–0.4Zr(GZ151K,wt%)alloys,cast under 0MPa(gravity cast)and 6MPa(squeeze cast),were comparatively studied.It is found that the grain size of squeeze cast GZ151K alloy with applied stress 6MPa is much smaller than that of the gravity cast counterpart.Moreover,the squeezing pressure hinders the transition from β’precipitates toβ1 precipitates during subsequent aging process,leading to reduced volume fraction ofβ1 precipitates in the squeeze cast alloy.Thus,the relatively lower volume fraction of β1 precipitates in the squeeze cast GZ151K results in higher hardness increment and stronger precipitation hardening effect.

Keywords:Mg–Gd–Zn–Zr alloy;Squeeze casting;Microstructure;Precipitation hardening.

1.Introduction

Environment-friendly magnesium(Mg)alloys with highspecifi strength,high elastic modulus,and good machine processing property show increasing applications in automobile,electrical,aeronautics,and space industries[1].However,the poor thermal resistance and low-yield strength(YS)of Mg alloys remain major obstacles for their wider use[2].Recent research reports suggest that Mg alloys containing rare earth(RE)elements provide a promising solution to solve those above problems[3].In variety Mg–RE alloy systems,Mg–Gd alloy systems are quite promising,due to their excellent room temperature strengths and elevated creep resistance.Besides,the adding Zn to Mg–Gd alloys has been reported to promote the formation of long period stacking ordered(LPSO)structures[4].It has been acknowledged that the LPSO phases/structures can not only strengthen but also toughen Mg alloys[5].Therefore,strong and ductile Mg–Gd–Zn alloys are anticipated via optimizing the compositions of Mg–Gd–Zn alloys and processes to make the most of precipitation hardening and LPSO structures strengthening.

Mg–Gd–Zn–(Zr)alloys were basically fabricated by gravity casting.Under this process,it is difficul to control the defects(shrinkage)effectively in castings.And this conventional casting process is low efficient In this regard,it is essential to seek other casting options.Squeeze casting is a relatively new casting technology[6].Yue and Chadwick[7]reported that squeeze casting as a casting process,in which molten metal was solidifie under the direct action of a pressure,was more effective to prevent the appearance of both gas porosity and shrinkage porosity as opposed to all other casting processes.In the process of solidification the application of mechanical pressure can be summarized as:(1)eliminate the gap between the casting and the mold wall to increase the thermal diffusion coefficient(2)reduce or eliminate shrinkage during solidification(3)change the solvus and liquidus lines in the equilibrium phase diagram[8].

Applied pressure is one of the most important processing parameters in squeeze casting.Goh et al.[9]reported that squeeze casting pressure was critical to affect the microstructures,properties,and solidificatio behavior of AZ91–Ca alloys.Regarding Mg–RE alloys,Wang’s work[10]showed that increasing the applied pressure led to microstructural refinemen of Mg–10Gd–3Y–0.5Zr(wt%)alloy,with accompanying enhancement of tensile yield strength,ultimate tensile strength,and elongation to failure.

Although,some researchers investigated the influenc of applied pressure on Mg alloy[11–15],the effect of squeeze pressure on microstructures of Mg–Gd–Zn-Zr alloys was not reported previously.The aim of this paper is to comparatively investigate the microstructural changes due to the squeeze pressure in a typical high-strength Mg–15Gd–1Zn–0.4Zr(GZ151K,wt%)alloy.

2.Experimental procedures

The GZ151K alloy was produced from high-purity Mg(99.9wt%),high-purity Zn(99.9%,wt%),Mg–87Gd(wt%),and Mg–30Zr(wt%)master alloys using an electrical resistance furnace under the gas protection of CO2and SF6mixture.After the pure Mg was melt,Mg–87Gd and Mg–30Zr master alloys and pure Zn were added into to the molten Mg.The melt was held at 750°C for 20 min following by mechanical stirring for 5 min.After that,the melt was transferred for squeeze casting.The processes of squeeze casting were divided into the following steps:(1)check the die and increase the mold temperature followed by spray paint;(2)clamp the die;(3)manually pour the melt and seal the die;(4)start pressuring and keep the punch to let the melt to solidify under pressure;(5)open the die and ejection.

On the basis of our previous work,the solution treatment time was optimized.During the solution treatment,the samples were under the protection of sulfur dioxide,after which the samples were quenched into ～90°C water.The parameters used for solution treatments are as follows:(1)485°C×12h;(2)520°C×12h;(3)520°C×36h;(4)530°C×12h.Aging treatments were carried out in oil baths at 175°C,200°C,and 225°C.

The actual chemical composition of GZ151K was measured as Mg–15.21Gd–1.14Zn–0.32Zr(wt%)using an inductively coupled plasma atomic emission spectroscopy(ICPAES)analyzer.Phase analysis was conducted by X-ray diffractometer(XRD,copper target Rigaku D/max 2550V)with a scanning speed of 1°/min.Phase transformation temperature was confirme by differential scanning calorimetry(DSC,a Netzsch STA449 F3 machine)with a heating rate of 5°C rose from 25°C to 560°C.The microstructure observation was performed using an optical microscope(OM,Zeiss Axio Observer A1 optical microscope)and a scanning electron microscope(SEM,FEI Nova NanoSEM 230)equipped with a backscatter electron(BSE)detector and an energy dispersive X-ray spectrometer(EDS).The crystal structure of the second phase in the alloy was analyzed by selected area electron diffraction(SAED)on a JEOL 2100 transmission electron microscope(TEM)at 200kV.The grain sizes of the samples were measured from at least ten pieces of optical micrographs(OM),according to ASTM E112-12.The age-hardening testing was carried out by a HVS-30P Vickers hardness tester under a load of 49N and a duration time of 49s.

Fig.1.X-ray diffraction(XRD)patterns of the as-cast GZ151K-G and GZ151K-S samples.Y-axis offset of 20,000 was applied on the curve of GZ151K-S.

3.Results and discussion

3.1.Effect of squeezing cast pressure on the as-cast microstructures

XRD patterns of as-cast GZ151K samples,fabricated by squeeze casting(with applied pressure of 6MPa)and gravity casting,are shown in Fig.1,respectively.For clarity of following descriptions,the samples prepared by squeeze casting and gravity casting are designated GZ151K-Sand GZ151K-G,respectively.The XRD patterns indicate that both GZ151KS and GZ151K-G samples are mainly composed ofα-Mg matrix,(Mg,Zn)3Gd compound and LPSO structure.The squeeze pressure does not change the phases constituents of as-cast samples.

Fig.2 shows OM and SEM micrographs of the as-cast GZ151K-S and GZ151K-Gd samples.All the phases identifie from XRD patterns in Fig.1 exist in OM and SEM micrographs of GZ151K samples with and without squeeze pressure.The eutectic compounds are distributed along the grain boundaries.Besides,lamellar structures were observed inside grains.The grain sizes of the as-cast GZ151K-G and GZ151K-Swere measured as 41μm and 28μm,respectively.Compared with the GZ151K-G sample,the grain size of the GZ151K-S sample is significantl reduced.The main effect of pressure on microstructural refinemen is ascribed to the higher heat transfer coefficien between melt and mold surface[14].When the pressure applied on the melt,the punch came closer to the melt.Therefore,the interface distance between the casting and die was greatly reduced.The heat transferred across the interface more easily.As a result,the increase of heat transfer coefficien leads to a refine microstructure.

Fig.2.(a,b,c,d)Optical and(e,f)BSE-SEM micrographs of(a,c,e)GZ151K-G alloy and(b,d,f)GZ151K-S alloy.

Microstructures revealed in Fig.2 also shows that the eutectic structure is randomly distributed in GZ151K-G sample when no pressure was acted on the casting.When squeeze pressure was applied on the casting during solidification the eutectic structure became much thinner and distributed more uniformly.Moreover,with the application of squeeze pressure,the average volume fraction of the eutectic phase increased from 6.8%in the GZ151K-G sample to 8.6%in the GZ151K-S sample with the application of squeeze pressure.

The composition and structure of the eutectic phase in the GZ151K-S sample were further analyzed by SAED and EDS spectra shown in Fig.3.By measuring the distance between the diffraction spots in SAED pattern,the lattice constant of a of the eutectic compound was measured to be 0.72nm,which is a face-centered cubic structure.Through TEM–EDS analysis,the compound was identifie to be(Mg,Zn)3Gd,which is the same phase in the GZ151K-G sample without pressure.

The DSC curves of GZ151K-G and GZ151K-S samples are showed in Fig.4.Two endothermic peaks appeared at about 521.4 °C and 520.9 °C,respectively.The temperature of the endothermic peak in the DSC curve of the GZ151K-S sample is almost identical to that of the GZ151K-G sample.Therefore,the squeeze pressure does not obviously change the eutectic temperature of the GZ151K alloy.

3.2.Effect of squeeze pressure on solution-treated microstructures

Microstructures of GZ151K-G samples after solution treatment at 485°C×12h,520°C×12h,520°C×36h,and 530°C×12h are shown in Fig.5.With increasing solution treatment temperature,from 480°C to 520°C,more secondary eutectic phase particles around the grain boundaries were dissolved into theα-Mg matrix.When the temperature was increased to 530°C,over-heating was observed in the GZ151K sample,as shown in Fig.5(d).As shown in Fig.5b and c,with time increasing from 12h to 36h,the overall volume of second-phase particles was further reduced.Hence,optimized solution treatment parameter was adopted as 520°C×36h.

Fig.3.(a,b)Transmission electron micrographs and(c)energy dispersive X-ray spectra recorded from a GZ151K-Ssample containing intermetallic compound:(a)bright-fiel micrograph,(b)[001]zone axis SAED pattern recorded from A particle.

Fig.4.DSC curves of the squeeze-cast GZ151K-G and GZ151K-S alloys in the as-cast state.

As shown in Fig.6,the microstructures of the solutiontreated GZ151K-G and GZ151K-S samples are both composed of α-Mg matrix,LPSO structures,Zn–Zr compounds and retained second-phase particles.The grains in the solution-treated alloys(as shown in Table 1)were coarser than those in the as-cast alloys.After the solution treatment,the volume fraction of secondary eutectic phase along the grain boundaries was reduced[16].

Fig.7 shows the TEM micrographs and corresponding SAED pattern of the lamellar structure in the solution-treated GZ151K-S sample.It is found that a series of diffraction spots with equal intervals and arranged in rows existed in theα-Mg matrix diffraction spots.Closer examination of the SAED pattern found that there were 13 diffraction spots appearing in between(0000)αto(0002)αpositions.Based on the above features of the SAED pattern,the lamellar structure was identifie to be 14H LPSO.The lattice constant of LPSO structure was reported to be a=0.325nm and c=3.722nm.Meanwhile,the LPSO structure has a coherent relationship with the α-Mg matrix,i.e.,(0002)2H?Mg‖(0014)14H-LPSOand[20]2H?Mg‖and[20]14H-LPSO[17,18].

3.3.Age-hardening response and microstructures in the peak-aged state

Fig.5.OM microstructures of GZ151K-G alloy solution-treated at(a)485°C×12h;(b)520°C×12h;(c)520°C×36h;(d)530°C×12h.

Fig.6.(a,b)OM micrographs,(c,d)BSE-SEM micrographs of(a,c)GZ151K-G sample and(b,d)GZ151K-S sample under T4 heat-treated for 36h at 520°C.

Table 1 Average grain sizes and area ratios of eutectic phase of the GZ151K alloy under the as-cast and solution-treated condition.

Fig.7.(a,b)Bright-fiel TEM micrographs of T4-treated squeeze-cast GZ151K-S alloy and(c,d)SAED patterns of the lamellar 2H-Mg and the 14H-LPSO structure.The beam was parallel to the[20]direction.(a)Low magnifcaion;(b)High magnificaion

Fig.8.Age-hardening curves of the solution-treated(a)GZ151K-G sample and(b)GZ151K-S sample.

Table 2 Key features of the ageing-hardening curves of the GZ151K alloy at 175°C,200°C,225°C.

Fig.9.Bright-fiel TEM micrographs and corresponding SAED patterns of the peak-aged(a,b)GZ151K-G alloy and(c,d)GZ151K-S alloy.The beams were parallel to the[0001]αdirection in(a,c)and the[20]αdirection in(b,d).

Fig.8 illustrates the age-hardening curves of GZ151K-G and GZ151K-S samples at 175 °C,200 °C,and 225 °C,respectively.Table 2 provides key features of the age-hardening curves.The results show that,with the time prolonging,Vickers hardness value of the GZ151K-G sample under different aging temperatures(175 °C,200 °C,and 225°C)increased from 84 HV,to the peak hardness at peak-ageing time and then gradually decreased with further prolong-aging time.The peak hardness of the GZ151K-G sample at 175 °C,200 °C,and 225°C were 132 HV,126 HV,and 128 HV,respectively,and the time to reach the peak hardness were 516h,64h,and 8h,respectively.The age-hardening tendency of the GZ151K-Ssample is similar to that of the GZ151K-Gsample.The peak hardness of the GZ151K-S sample were 134 HV,131 HV,and 124 HV,respectively,and the time to reach the peak hardness were 516h,32h,and 8h,respectively.With increasing aging temperature from 175 °C to 225 °C,hardness increment of both GZ151K-G and GZ151K-S samples decreased(GZ151K-G:from 48 HV,42 HV to 44 HV;GZ151KS:from 49 HV,46 HV to 39 HV)and the time to reach the peak hardness was reduced(GZ151K-G:from 516h,64h,to 8h;GZ151K-S:from 516h,32h,to 8h).Take both aging efficien y and precipitation hardening effect into consideration,the optimized aging treatment parameters for GZ151K-G and GZ151K-S samples were 200°C for 64h and 32h,respectively.As shown in Table 2,it was found that an increase in squeeze pressure led to an increment in the peak hardness.The reason responsible for the different peak hardness of the GZ151K-G and GZ151K-S samples is the applied pressure affecting the precipitation.

To observe the nano-scale precipitates formed at relatively low-ageing temperature,microstructures of peak-aged GZ151K-Sand GZ151K-G samples were examined by TEM.Fig.9 shows the TEM images and corresponding SAED patterns of the peak-aged GZ151K-S and GZ151K-G samples,with electron beam parallel to[001]α and[100]α directions.The prismaticβ’precipitate and the basedγ’precipitates coexist in both alloys after aging.These precipitates uniformly distributed within theα-Mg matrix.It means that the application of pressure did not affect the phase makeup of metastable precipitates.The diffraction spots of theβ’precipitates were located at the 1/4,2/4,3/4 positions between(0000)αand(1100)α.The β’phase is fully coherent with the α-Mg matrix and has an orthorhombic lattice,with a=2aα?Mg=0.65nm,b=8d(1010)Mg=2.22nm,c=cα?Mg=0.52nm [19,20].Based on proposed atomic model in the literature,theβ’phase has a composition of Mg7Gd[19].When the electron beam is parallel to the[001]αdirection,the β1precipitates are observed between neighboring β’precipitates.Theβ1phase has a fcc structure with a=0.73nm and a composition of Mg3Gd.According to the reported research[19,21],the decomposition ofα-Mg supersaturated solid solution in Mg–Gd(-Y)–Zn(-Zr)alloys was generally accepted as:SSSS(hcp)→β”(D019)→β’(cbco)→β1(fcc)→β(fcc).Theβ’is the main precipitates in the peak-aged Mg–Gd(-Y)–Zn(-Zr).The formation and growth of theβ1andβphases will deteriorate the performance of the alloy during over-aging.The β1phase is incoherence with the matrix,and it nucleates in the necking ofβ’phase.He et al.[22]reported thatβ1precipitates are generated from β’precipitates,which decreases the hardness of the Mg–Gd–Y–Zr alloy.Comparing Fig.9(a)and(c),GZ151K-S samples contained lessβ1precipitates than the GZ151K-G counterpart.Thus,the squeeze pressure was supposed to hinders the transition from β’precipitates to β1precipitates,which led to the reduction of volume fraction ofβ1and thus resulted in an increase in peak-aged hardness.

4.Conclusions

In this work,the effects of applied pressure on the squeeze casting GZ151K alloy under the as-cast,solution-treated,and peak-aged conditions were investigated.The main finding are summarized as follows:

(1)Under the applied pressure,the grain size of the as-cast GZ151K-S alloy is significantl refine from 41μm to 28μm.

(2)During solution heat treatment,the secondary eutectic phase is dissolved into theα-Mg matrix and the volume ofβphase is reduced.

(3)In the aging heat treatment,the peak value of hardness among the age-hardening curves of squeeze casting GZ151K alloy increases with the applied pressure,which results in an improved performance.

Acknowledgment

This work is supported by the National Key Research and Development Program of China(No.2016YFB0701201),National Natural Science Foundation of China(No.51771113,51671128,51605288),the United Fund of National Department of Education and Equipment Development(No.6141A02033213)and the 111 Project(Grant No.B16032).