Effect of heat treatment on mechanical properties and microstructure evolution of Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy

Jie Zheng, Zhaoming Yan, Jinsheng Ji, Yusha Shi, Heng Zhang, Zhimin Zhang, Yong Xue

School of Materials Science and Engineering, North University of China, Taiyuan 030051, China

Abstract Regarding the as-cast Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy, the effect of heat treatment on its properties at room temperature (RT), as well as the mechanical properties and microstructure evolution of various peak-aging samples at different tensile temperatures were discussed in this article.The results indicated that the optimal heat treatment process of the alloy was: 520 °C × 24 h + 200 °C × 112 h.Under this condition, the yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) at RT were: 238 MPa, 327 MPa and 2.5 %,respectively.As the tensile temperature increases, the strength increases firstl and then decreases, but the ductility increases monotonously.The microstructures evolution of 200 °C peak-aging (200PA) and 250 °C peak-aging (250PA) samples were different with the increasing tensile tenperature.When tensile test processed at 150°C, the dense β' phase and rod-shaped basal γ' phase will be formed in the 200PA sample.However, at 300 °C, the β' phases disappeared.The β' and LPSO phases in the 250PA sample coarsened gradually as the tensile temperature increased, and 14H-LPSO phases were formed during tensile at 300 °C.The 200PA sample reached the highest strength when tensile at 150 °C, which was attributed to the hindrance of the basal dislocation and non-basal dislocation slip by the prismatic β' phases and the newly formed basal γ' precipitates.

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Keywords: Mg-Gd-Y-Zn-Zr alloy; Heat treatment; Elevated temperature mechanical properties; Microstructure evolution.

1.Introduction

With the increasing demand of low energy consumption and warm-gas emission,magnesium(Mg)and Mg alloys have been treated as the most promising material candidates for light-weight components in aerospace and automotive industries due to their low density, high specifi strength and easy recycling [1-3].Compared with traditional Mg-Al-Zn alloys,the additions of rare-earth (RE) elements in Mg-RE alloys can significantl improve the strength at both room and elevated temperatures after T6 heat treatment [4-6].The Gd element has a high solid solubility in Mg (?24 wt.%), but it drops sharply (3.8 wt.%) when the temperature decreases to 200 °C, which will cause a significan aging response [7,8].Zhong et al.[9]reported that the hardness of Mg-12.88Gd(wt.%) alloy after T6 heat treatment increased from 72.8 HV to 98.5 HV at 225 °C isothermal aging for 24 h.Xie et al.[8]suggested that twoβ' phases (βL' andβs') with different structures coexist in Mg-11.5Gd (wt.%) alloy after T6 heat treatment.In addition, adding quantitative Y element to Mg-Gd alloy can significantl improve the tensile strength at room temperature and high temperature [10,11].Gao et al.[12]proved that Y element can significantl improve the mechanical properties in Mg-Gd-Y alloy, which is a signifi cant contribution of solid solution strengthening.Furthermore,adding Zn element to the Mg-Gd-Y-Zr alloy can improve the aging response, but it will also promote the formation of the long period stacking ordered(LPSO)phase during the isothermal aging process.Xu et al.[13]pointed out that the precipitation sequence of Mg-8.2Gd-3.8Y-1.0Zn-Zr (wt.%) alloy isα-Mg (S.S.S.S) →β'' (DO19) →β' (cbco), and the stack-ing fault (SF) and 14H-LPSO phase also precipitated during aging at 200 °C and 225 °C, respectively.

Recently, Mg-Gd-Y-Zn-Zr alloys have received extensive research due to their remarkable aging strengthening response and excellent strength [14-16].The current research of Mg-Gd-Y-Zn-Zr alloys mainly focuses on improving RT performance through deformation[17-19].Ramezani et al.[20]prepared the extruded Mg-8.1Gd-4.3Y-1.6Zn-0.4Zr (wt.%) alloy by multi-directional forging (MDF), and obtained excellent UTS and EL of 581 MPa and 15.9%, respectively.Sun et al.[21,22]obtained high hardness Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr(wt.%) alloy through the combination of high-pressure torsion (HPT) and heat treatment.Because deformation requires energy consumption, some studies are aimed at improving the as-cast Mg-RE alloy; just as importantly, the quality of the as-cast alloy can significantl affect the performance after subsequent deformation.Therefore, the microstructure evolution and RT properties of Mg-6Gd-3Y-1.5Zn-0.6Zr(wt.%) alloy were studied under T6 heat treatment [23].Jin et al.[24]investigated the effect of heat treatment on the microstructure evolution and fracture behavior of the semi-continuous casting Mg-7Gd-5Y-0.6Zn-0.8Zr (wt.%)alloy, and proved that there are two different precipitation sequences during the isothermal aging process.There are few studies on the elevated temperature (ET) properties of cast Mg-Gd-Y-Zn-Zr alloys.Meng et al.[25]studied the effect of tensile strain rate on the hot tensile behavior of solid solution Mg-13Gd-4Y-2Zn-0.5Zr (wt.%) alloy.And the ET properties of the cast Mg-10Gd-3Y-1.2Zn-0.4Zr (wt.%) alloy were investigated under different solution treatment times [26].Mg-Gd-Y-Zn-Zr alloy as a high temperature resistant material is a promising candidate for application in the aerospace and defense industries.Until now, there is no systematic study on the effect of the aging temperature of the cast Mg-Gd-Y-Zn-Zr alloys on the ET performance and the evolution of the mechanical properties and microstructure of peak-aging(PA) samples with the increase of tensile temperature is still unclear.In addition, some researchers have proved that pre-aging treatment before deformation of Mg alloy can significantl affect the microstructure evolution and mechanical properties of the alloy [27-28].Wang et al.[28].proved that pre-aging can significantl increase the strength of deformed alloys.However, there are few researches on the aging treatment of Mg-9.5Gd-4Y-2.2Zn-0.5Zr (wt.%) alloy.Therefore,we hope that this manuscript can support subsequent research.

This article mainly studies the microstructure evolution,RT mechanical properties and ET mechanical properties of the cast Mg-9.5Gd-4Y-2.2Zn-0.5Zr (wt.%) alloy under T6 heat treatment, and proves the optimal heat treatment process of the alloy.And the influenc of tensile temperature on the mechanical properties and microstructure evolution of various peak aging samples has been also discussed.

2.Experimental procedures

Fig.1.DSC curve of the as-cast Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy.

The alloy with the nominal composition Mg-9.5Gd-4Y-2.2Zn-0.5Zr (wt.%) (determined by inductively coupled plasma-atomic emission spectrometry, ICP-AES).The cylindrical ingot ofφ420 mm × 660 mm was prepared from high purity Mg (99.95 wt.%), Zn (99.95 wt.%), Mg-25Gd (wt.%),Mg-25Y(wt.%)and Mg-30Zr(wt.%)master alloys in an electrical resistance furnace.The molten metal was held at 730°C under a mixed gas of CO2and SF6with a volume ratio of 99:1.The phase transformation temperature was tested by differential scanning calorimetry (DSC, TA-DSC250) method at a heating rate of 5°C/min.Fig.1 shows the DSC curve of the cast Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy, the onset and peak temperatures were 522.6 °C and 528.9 °C, respectively, indicating that the dissolution of eutectic phase initiated at 522.6 °C.Therefore, the solution treatment of the as-cast alloy was carried out at 510 °C, 520 °C and 530 °C for different times.Based on the dissolution of the interdendritic phases and the growth of grains, 520 °C × 24 h was selected as the optimal solid solution parameter for this study.After solution treatment, the samples were subjected to aging treatment at 200 °C and 250 °C for 0.5-256 h, and then water cooled to RT (25 °C).All heat treatments are performed under a protective atmosphere of argon.

The microstructures were observed by using the optical microscopy (OM, Zeiss Axio ImagerA2m), scanning electron microscopy(SEM,Hitachi SU5000)with an energy dispersive spectroscopy (EDS) and transmission electron microscopy(TEM, JEOL JEM-F200).Before the OM observation, the samples were grinding with SiC papers and mechanically polished with polishing agent(composed of diamond powder and grinding), then chemically etched in a solution of 1 g picric acid, 2 ml acetic acid, 14 ml alcohol and 2 ml distilled water.The specimens for SEM examination were also prepared by grinding with 800#-7000# SiC papers and then mechanically polishing.The thin foils for TEM observation with thickness of 0.25 mm were punched into discs with 3 mm in diameter and mechanically ground to about 30 μm,and then ion milled to perforation with an acceleration voltage of 4.5 kV and a gun angle of 15° by ion beam thinner (Leica Em Res102).When the mere emergence of a tiny hole in samples, the acceleration voltage and gun angle would be changed to 2.5 KV and 5°.

The hardness test of the specimens was taken on Vickers hardness (HV, UHL-VMHT) testing, which using 200 g load and holding time of 15 s, and taken ten measurements, excluding the maximum and minimum values to calculate the average.And the tensile properties of the samples in the temperature range of 25 °C to 300 °C were tested by using an Instron uniaxial tensile machine (Instron-3382), and the strain rate was 1 × 10-3s-1.Before the tensile testing, samples were mechanically polished by SiC papers and then polished to 1 μm finis with diamond suspension.For each state of the samples, repeated the tensile tests for three times and taken the average value to ensure accuracy.

3.Results

3.1.Microstructure of as-cast state

Fig.2.OM and SEM microstructures of the as-cast Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy: (a) OM micrographs, (b)-(d) SEM images.

Fig.2 shows the OM microstructure and SEM images of the as-cast Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy, respectively.The average grain size of the as-cast alloy was measured as 79.3 μm (deviation of 3.14 μm) using the linear intercept method.The dense interdendritic phases exist at the grain boundaries as shown in Fig.2a and 2b.And Fig.2c and d are high magnificatio SEM images of the interdendritic phase.The EDS results (Table 1) shown that the spherical-shaped particle corresponding to point A was Zr particle, the point B corresponding to the square-shaped particle was RE-rich phase (Fig.2c), which has been confirme in many studies[29,30].Fig.2d is an enlarged view of the red dotted circle in Fig.2c.Point C was the skeletal-shaped white phase in the interdendritic phase, the EDS result shown that this was Mg3(Gd, Y, Zn) phase.The light gray area next to the corresponding point D was Mg12(Gd, Y)Zn (corresponding to the component content of the 18R-LPSO phase), which is consistent with previous research [31].In addition, many fin particles(point E)were found around the skeletal-shaped Mg3(Gd,Y,Zn)phase,similar in shape to point B,and its element content was Mg12.03Gd12.22Y2.82Zn0.94Zr (at.%), this may be another fin RE-rich phase formed during the casting process.

Table 1EDS results corresponding to point A, B, C, D and E in Fig.2c and d.

Table 2Tensile properties of different samples at RT.

3.2.Microstructure evolution with distinctive solution treatment

Fig.3 shows the SEM images and OM micrographs of Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy after different solution treatments.Compared with the as-cast sample, the interdendritic phases were dissolved in theα-Mg matrix during the solution treatment.Almost all the interdendritic phases were dissolved when the solution temperature was 520 °C and 530 °C (Fig.3b and 3c), but when the solution temperature dropped to 510°C,most of the interdendritic phases remained(Fig.3a).In addition, the average grain size of all samples after solution treatment has increased, and the average grain sizes of the 510 °C × 24 h and 520 °C × 24 h samples were 117.2 μm (deviation of 5.23 μm) and 120 μm (deviation of 4.87 μm), respectively.Furthermore, some of grains grew abnormally after solution treatment at 530 °C due to the lack of the second phase hindrance (Fig.3f).Jin et al.[24]confirme that the abnormal growth of grains deteriorates the mechanical properties.Therefore, the preferred solution treatment process for Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy in this study was: 520 °C × 24 h.

3.3.Age hardening behavior

Fig.4 shows the age-hardening curves of Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy at 200 °C and 250 °C, respectively.Compared with the optimal solution treatment sample (78 HV),the hardness of the aging treatment samples was significantl improved.In addition, the hardness increases monotonously with the increase of aging time at the beginning, then fluc tuates up and finall reaches the peak hardness (Fig.4), but various aging temperatures have different aging hardening responses.As the aging temperature increases, the aging time to reach the peak hardness was significantl shortened, but the peak aging hardness was just the opposite.For example,when the aging temperature is 200 °C, the time to reach the peak hardness was 112 h (126 HV,ΔH=HVmax-HVinitial=48 HV), and when the temperature is increased to 250 °C, the peak aging time was shortened to 89 h (115 HV,ΔH = 37 HV).On the other hand, the hardness of all samples slowly increased in the initial stage of the aging treatment, and the 250 °C sample had a higher aging hardening response, which can be attributed to the higher temperature that enables the Gd/Y atoms to diffuse faster to form the strengthening phase[11].Han et al.[23]obtained a peak hardness of 117.69 HV for Mg-6Gd-3Y-1.5Zn-0.6Zr(wt.%)alloy aging at 200°C.Xu et al.[13]obtained 115 HV at 200 °C for 48 h during the aging treatment of Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy.Therefore, it can be known that the experimental alloy in this article has high hardness with the optimal T6 heat treatment process (520 °C × 24 h + 200 °C × 112 h).

Fig.3.SEM images and OM micrographs of solution-treated samples: (a, d) 510 °C × 24 h, (b, e) 520 °C × 24 h, (c, f) 530 °C × 24 h; (a, b, c) SEM images, (d, e, f) OM micrographs.

Fig.4.Age hardening curves of Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy at 200 °C and 250 °C, respectively.

3.4.Room temperature mechanical properties of different states

Fig.5.Tensile properties of different samples at RT.

In order to facilitate understanding, the as-cast sample,solution-treated (520 °C × 24 h) sample, 200 °C peak-aging(520 °C × 24 h + 200 °C × 112 h) sample and 250 °C peak-aging (520 °C × 24 h + 250 °C × 89 h) sample were define as AC sample, ST sample, 200PA sample and 250PA sample, respectively.Fig.5 displays the stress-strain curves of Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy in different states at RT.Table 2 is a summary of the stress-strain curves in Fig.5.It can be seen from Table 2 that the AC sample has the lowest comprehensive mechanical properties, and its YS, UTS and EL were 122 MPa, 202 MPa and 3.4 %, respectively.The mechanical properties of the sample after solution treatment were improved, which is consistent with the results of Yin et al.[32].In addition, the properties of the aging-treated samples were significantl improved, and the YS, UTS and EL of the 200PA sample were 238 MPa, 327 MPa and 2.5%, respectively, which were the preferred properties of all samples.So far, the reason for the significan increase in the strength of the Mg-RE-Zn-Zr alloy after aging treatmentis mainly due to the formation of different strengthening precipitates, such as the prismaticβ''/β' phases or the basal LPSO/γ' phases.Generally, the precipitation behavior of various strengthening phases is related to solute content[31,33-35], aging temperature [36], aging time [9,37], stress fiel distribution [38]and interaction with other precipitated phases [8,39].The various precipitation sequences of Mg-Gd-Y-Zn-Zr alloy at different aging temperatures may lead to different mechanical properties.Therefore, compared with the 200PA sample, the reason for the 250PA sample exhibits weaker strength will be discussed in Section 4.1, and its YS,UTS and EL were 180 MPa,260 MPa and 2.8%,respectively.

3.5.Elevated temperature mechanical properties of 200PA and 250PA samples

And mechanical properties of the 200PA and 250PA samples evolved in the temperature range of 25 °C to 300 °C as shown in Fig.6.It is obvious that the initial strength increases with the increase of the tensile temperature and then decreases, but the ductility increases monotonically.The 200PA sample obtained the highest mechanical properties when the tensile temperature was 150 °C, and its YS and UTS increased from 275 MPa and 352 MPa (100 °C) to 283 MPa and 358 MPa (150 °C), respectively.As the tensile temperature continued to increase, the strength of the 200PA sample began to decrease.When the tensile temperature was 300 °C, its YS and UTS dropped significantl to 155 MPa and 170 MPa, respectively, but the EL reached 12.5 %.In addition, for the 250PA sample, its optimal mechanical properties were obtained at 200 °C, and YS, UTS and EL were 217 MPa, 285 MPa and 7.9 %, respectively.Similar to the 200PA sample, the strength of the 250PA sample decreased significantl when the tensile temperature was 300 °C, and its YS and UTS were only 125 MPa and 155 MPa, but the EL increased significantl , reached 14.3 %.

4.Discussion

Generally, Mg-Gd-Y-Zn-Zr alloy with LPSO phase,solid solution strengthening and aging strengthening can significantl improve mechanical properties, compared to commercial AZ31 alloy.In addition, the strength decreases with the increase of the tensile temperature, while the plasticity increases, which is a general characteristic of traditional metal materials [40-43].Many studies have found that the strength of Mg-RE alloys increases with the increase of the tensile temperature, and then decreases, which is mainly due to the RE precipitation phases with higher thermal stability[44].However, it is not clear about the evolution of ET tensile behavior of properties and microstructures.Next, we will focus on the Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy to discuss the reasons for the difference in the tensile properties of the various samples at RT, and the microstructure evolution and performance of the 200PA and 250PA samples with tensile temperature change.

4.1.Effect of heat treatment on mechanical properties at RT

When tensile at RT, compared with the AC sample, the YS, UTS and EL of the ST sample increased by 15 MPa,45 MPa and 2.4 %, respectively (Table 2).Comparing Fig.2b with Fig.3b, the presence of interdendritic phases can hinder the movement of dislocations and grain boundaries during the tensile process,which may initiate cracks and cause premature fracture of the AC sample, such as fracture before sufficien work-hardening occurs.Additionally, the Gd/Y atomic radius is significantl larger than theα-Mg atomic,and its high solubility and large solute misfit in the solution treatment can be used as an effective solute strengthening for basal slip in Mg alloys.Therefore, the increase of RE solute atoms in theα-Mg matrix produces uniform and significan lattice distortion and the interaction of solute atoms with dislocations prevents dislocation sliding, which caused the ST sample to have better comprehensive performance than the AC sample.Some studies have also found that solution treatment can improve the strength and ductility of Mg-Gd-Y-Zn-Zr alloy, which is consistent with the above [32,45].

During the isothermal aging process after the solution treatment, the solid solubility of Gd and Y elements decreased, which led to the precipitation of various RE phases.Fig.7 reveals the TEM bright-fiel images and corresponding SAED patterns of the 200 °C PA and 250 °C PA samples.The schematic diagram of the microstructure evolution of the Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy during the heat treatment is shown in Fig.9.The denseβ' phase was observed as the main strengthening phase [29], as shown by the red arrow(Fig.7a).However, in addition to theβ' phase in the 250PA sample, the formation of LPSO phase was also found, resulting in the faint streaks in Fig.7f.On the one hand,β'phases arranged in a triangle and perpendicular to theα-Mg basal plane were formed on the prismatic plane of theα-Mg matrix, which can effectively suppress the basal slip during the deformation process [39].On the other hand, the LPSOphase acts as a basal strengthening phase, which can hinder the prismatic and pyramidal dislocations slip.Therefore, dislocations were hindered by the precipitated phases during the RT tensile deformation process, which significantl increases the strength of the 200PA and 250PA samples.Compared with the AC sample, the YS of the 200PA and 250PA samples increased by 95.1 % and 47.5 %, respectively, while the EL decreased slightly.Zhen et al.[46]proved that the YS and UTS of the extruded Mg-11Gd-1Zn (wt.%)alloy after aging at 225 °C increased by 11 MPa and 49 MPa, but the ductility decreased by 3.4 %.The precipitation density ofβ' phase is affected by the RE atom concentration in theα-Mg matrix,so the formation of LPSO phase will reduce the precipitation density ofβ' phase.In addition, the diffusion rate of solute atoms increases as the aging temperature increases, resulting in the formation of coarserβ'phases,which reduces the aging strengthening response [47,48].Therefore, the strengthening effect of the coarser and dispersedβ' phase in the 250PA sample was weaker than that of the 200PA sample by comparing Fig.7b and 7e.

Fig.9.The schematic diagram of microstructure evolution of Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy during heat treatment and tensile process.

Fig.7.TEM bright-fiel images and SAED patterns of different samples:(a, b, c)200PA sample, (d, e, f) 250PA sample; (a, b, d, e) TEM bright-fiel images,(c, f) SAED patterns; (b, e) are the enlarged images of (a, d) respectively.

It is well known that the critical resolved shear stress(CRSS) of the non-basal slip system is hundreds of times that of the basal slip system at RT in the Mg alloys.Nie et al.[49]reported that the YS was mainly related to basal dislocation slip, and it increases with the increase of the CRSS of basal slip.In summary, the prismaticβ' phase as the most important strengthening phase can effectively hinder the basal dislocation slip during the RT tensile process, and signifi cantly increase the YS.Therefore, there were LPSO phases and coarserβ' phases in the 250PA sample, but the 200PA sample with dense and finβ' phases had the most excellent comprehensive mechanical properties.The strengthening effect of theβ' phases was the strongest in the Mg-Gd-Y-Zn-Zr alloy,and the strengthening effect of the prismatic precipitates was always greater than that of the basal precipitates, which has also been confirme by Xu et al.[50], which is consistent with the above research.

4.2.The effect of tensile temperature on mechanical properties and microstructure

As the tensile temperature increases, the strength firs increases and then decreases and the ductility increases monotonously, as shown in Fig.6.The 200PA and 250PA samples reached their highest strength when the tensile temperature was 150°C and 200°C,respectively,but the strength of both dropped sharply when the temperature was 300 °C.The schematic diagram of the microstructure evolution of the Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy with the tensile temperature is shown in Fig.9.Next, we will discuss the microstructure evolution during ET tensile process.

Fig.6.The mechanical properties of different samples in the tensile temperature range of 25 °C to 300 °C.

Fig.8 reveals the TEM bright fiel images of different samples and the corresponding SAED patterns.Except for the presence ofβ' phase, dense rod-shaped basalγ' phases were precipitated when the 200PA sample after tensile at 150 °C.According to the analysis of the precipitation behavior of the Mg-Gd-Y-Zr alloy [51,52], the solute segregation phase on the basal plane was called theγ' phase, which can effectively prevent the non-basal dislocation slip and improve the strength of the alloy.In addition, the prismaticβ' phases and the basal LPSO phases still existed in the 250PA sample after tensile at 200 °C.On the one hand, the non-basal slip system CRSS has high temperature sensitivity, and its CRSS decreases significantl as the tensile temperature increases.Therefore, many non-basal dislocation slips were activated during the ET tensile process, which improved the ductility of the alloy.On the other hand, the prismaticβ' phase and the basal LPSO/γ' phase were formed in the 200PA and 250PA samples, which can prevent the basal and non-basal dislocation slip (shown by the red dashed ellipse in Fig.8a and 8e), thereby significantl improved the strength of the alloy.

Fig.8.TEM bright-fiel images and SAED patterns of different samples:(a, b)200PA-150 °C, (c, d) 200PA-300 °C, (e, f) 250PA-200 °C, (g, h) 250PA-300 °C;(a, c, e, g) TEM bright-fiel images, (b, d, f, h) SAED patterns.

However, the strength of both samples decreased signifi cantly as the tensile temperature continued to increase.The TEM bright-fiel images and SAED patterns of the 200PA and 250PA samples after tensile at 300 °C are shown in Fig.8.It can be known that according to the corresponding SAED pattern, there were onlyγ' precipitates in the 200PA sample (Fig.8c and 8d), while the LPSO andβ' phases in the 250PA sample became coarser (Fig.8g and 8h).Moreover, according to the SAED pattern in Fig.8h, the coarser lamellar precipitation phase was the 14H-LPSO phase in the 250PA sample after tensile at 300 °C.In general, the effect of the precipitated phases in suppressing dislocation slip was weakened, mainly due to the disappearance of theβ' phase in 200PA and the coarsening of theβ' and LPSO phases inthe 250PA sample, resulting in a significan deterioration in strength, but ductility improved.

Fig.10.SEM micrographs of fracture morphology of different samples at various tensile temperatures:(a) AC samples, (b) ST samples, (c, d) 200PA samples, (e, f) 250PA samples; (a, b, c, e) 25 °C (RT), (d, f) 300 °C.

SEM micrographs of the fracture morphology of different samples at different tensile temperatures are revealed in Fig.10.The fracture morphology of the samples in all states was brittle fracture when tensile at RT, which basically composed of cleavage planes,tear ridges and cracks.With the increase of the tensile temperature, many dimples appeared in the fracture morphology, which indicated that the fracture behavior of the samples changed from brittle fracture to brittle-ductile mixed fracture or ductile fracture when tensile at 300 °C.

5.Conclusions

This article studies the effect of heat treatment process on the microstructure and mechanical properties of the as-cast Mg-9.5Gd-4Y-2.2Zn-0.5Zr (wt.%) alloy, mainly discussed the optimal heat treatment parameters of the alloy, the RT mechanical properties of samples in different heat treatment states and the influenc of tensile temperature on the mechanical properties and microstructure evolution of 200PA and 250PA samples.In addition, this article does not prove the aging precipitation sequence of Mg-9.5Gd-4Y-2.2Zn-0.5Zr (wt.%) alloy, and we will discuss related research in other manuscripts.The main conclusions are summarized as follows:

(1) For as-cast Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy, the microstructure is mainly composed ofα-Mg matrix,Mg3(Gd, Y, Zn) phase, LPSO phase, spherical-shaped Zr-rich particle phase and square-shaped RE-rich phase.And the optimal T6 heat treatment process was: 520°C × 24 h + 200 °C × 112 h.

(2) Compared with the AC and ST samples, the strength of the peak-aging samples was significantl improved,mainly due to the precipitation of strengthening phases on the basal and prismatic planes.The prismaticβ'phase suppresses basal dislocation slip more effectively than the basal LPSO phase.Therefore, the 200PA sample with denseβ'phases has the most excellent strength at RT, although the 250PA sample hasβ' and LPSO phases.And the YS, UTS and EL of the 200PA sample were 238 MPa, 327 MPa and 2.5 %, respectively.

(3) The microstructure evolution of 200PA and 250PA samples affected by tensile temperature were different.Except for the denseβ' phase, the 200PA sample formed rod-shaped basalγ' phases when tensile at 150 °C;however, theβ' phase disappeared when tensile at 300 °C.In addition, theβ' and LPSO phases in the 250PA sample gradually coarsened as the tensile temperature increased, and 14H-LPSO phases were formed during tensile at 300 °C.

(4) As the tensile temperature increases, the strength firs increases and then decreases and the ductility increases monotonously.The 200PA sample reached the highest strength when tensile at 150 °C, which was attributed to the hindrance of the basal dislocation and non-basal dislocation slip by the prismaticβ'phase and the newly formed basalγ' precipitate.As the tensile temperature continued to increase, the strengthening effect of theβ' phase was weakened, resulting in the deterioration of the strength of the 200PA sample when tensile at 300 °C.

Declaration of Competing Interest

The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.

Acknowledgements

The present research was supported by the National Natural Science Foundation of China (Grant No.52075501).