简介概要

Phase equilibria of low-Y side in Mg-Zn-Y system at 400℃

来源期刊：Rare Metals2020年第3期

论文作者：Bo-Shu Liu Hong-Xiao Li Yu-Ping Ren Min Jiang Gao-Wu Qin

文章页码：262 - 269

摘要：Phase equilibrium relations of the Mg-Zn-Y system in the low-Y side at 400℃were investigated by alloy-equilibrated method,combined with thermal analysis.The results show that there is a liquid phase which could be in equilibrium with an a-Mg solid solution and an icosahedral quasicrystal I phase in the low-Y side of the Mg-Zn-Y system at 400℃.The liquid phase region originates from the binary Mg-Zn system and extends to 0.4 at%Y in the Mg-Zn-Y system.Besides,the hexagonal structure H phase,fee W phase and LPSO phase（X phase）are in equilibrium with a-Mg.With Y/Zn（atomic ratio,the same as follows） increasing,there exist four three-phase regions consisting of I+liquid+α-Mg,I+H+α-Mg,H+W+α-Mg and W+X+α-Mg,respectively,in the low-Y side of the isothermal section at 400℃.The twophase region a-Mg+I phase exists between I+H+aMg and I+liquid+a-Mg.In this two-phase region,the Y/Zn ratio is in the range of 0.14-0.17;and a three-phase region of a-Mg+I phase+H phase appears when Y/Zn ratio comes up to 0.17-0.27.Not I but W phase is in equilibrium with a-Mg,when Y/Zn ratio> 0.27.The system is in liquid-state phase equilibrium,when Y/Zn ratio <0.14.

稀有金属(英文版) 2020,39(03),262-269

Phase equilibria of low-Y side in Mg-Zn-Y system at 400℃

Bo-Shu Liu Hong-Xiao Li Yu-Ping Ren Min Jiang Gao-Wu Qin

Key Laboratory for Anisotropy and Texture of Materials,Ministry of Education,Northeastern University

作者简介：*Hong-Xiao Li,e-mail:lihx@atm.neu.edu.cn;

收稿日期：14 December 2016

基金：financially supported by the National Natural Science Foundation of China (Nos.51271053 and 5137104 and the National Key Research and Development Program of China (No.2016YFB0701202);

Phase equilibria of low-Y side in Mg-Zn-Y system at 400℃

Bo-Shu Liu Hong-Xiao Li Yu-Ping Ren Min Jiang Gao-Wu Qin

Key Laboratory for Anisotropy and Texture of Materials,Ministry of Education,Northeastern University

Abstract：

Phase equilibrium relations of the Mg-Zn-Y system in the low-Y side at 400℃were investigated by alloy-equilibrated method,combined with thermal analysis.The results show that there is a liquid phase which could be in equilibrium with an a-Mg solid solution and an icosahedral quasicrystal I phase in the low-Y side of the Mg-Zn-Y system at 400℃.The liquid phase region originates from the binary Mg-Zn system and extends to 0.4 at%Y in the Mg-Zn-Y system.Besides,the hexagonal structure H phase,fee W phase and LPSO phase(X phase)are in equilibrium with a-Mg.With Y/Zn(atomic ratio,the same as follows) increasing,there exist four three-phase regions consisting of I+liquid+α-Mg,I+H+α-Mg,H+W+α-Mg and W+X+α-Mg,respectively,in the low-Y side of the isothermal section at 400℃.The twophase region a-Mg+I phase exists between I+H+aMg and I+liquid+a-Mg.In this two-phase region,the Y/Zn ratio is in the range of 0.14-0.17;and a three-phase region of a-Mg+I phase+H phase appears when Y/Zn ratio comes up to 0.17-0.27.Not I but W phase is in equilibrium with a-Mg,when Y/Zn ratio> 0.27.The system is in liquid-state phase equilibrium,when Y/Zn ratio <0.14.

Keyword：

Mg-Zn-Y system; Phase equilibrium; Icosahedral quasicrystal phase; Solid solubility;

Received： 14 December 2016

1 Introduction

Mg-Zn-Y system has received considerable interest due to the discovery of a stable icosahedral quasicrystal phase (I-phase).Further analysis shows that the alloy exhibits many superior mechanical properties when some I-phases distributed in the magnesium matrix [ 1, 2, 3, 4, 5, 6] .The Y/Zn for the formation of two-phase as-cast microstructure consisting ofα-Mg and I-phase is 0.14-0.20 [ 7] .To eliminate the coring segregation and improve mechanical properties,homogenizing annealing processing must be adopted [ 8, 9, 10, 11, 12] .In addition to the I-phase,three ternary compounds,namely H,W and a long-period stacking-ordered (LPSO) phases,were often found in the alloys after heat treatments [ 13, 14, 15, 16, 17] .However,the elongation to failure was limited due to large size cubic W phase particles which form during the high-temperature solutioning [ 3, 16] .Since the second phases are differently depending on the Y/Zn ratio,Ju et al. [ 7] also reported that the main phases in the as-cast alloy wereα-Mg,I phase and W phase when Y/Zn ratio was 0.50-0.67;the main phases wereα-Mg and W phase when Y/Zn ratio was 0.40-0.50.But these results of as-cast alloy are not applicable for guiding heat treatment processes.

Studies have shown that its performance is preferable usually through annealing treatment for 15 or 30 min at400℃ [ 5, 7, 11, 18, 19] .Bae et al. [ 19] investigated the effect of I phase on the deformation behavior under the condition of annealing at 400℃for 0.5 h.

It is vital to establish phase equilibrium at 400℃to pilot alloy design in view of practical heat treatment temperature.Shao et al. [ 20] and Grobner et al. [ 21] optimized thermodynamic parameters and reported isothermal section of Mg-Zn-Y alloy at 400℃using the CALPHAD method,respectively.The two results are not the same in the phase constitutes,phase boundaries and characteristic temperatures.Shao et al. [ 19] calculated it based on previous limited experimental phase diagram data.Subsequently,they changed composition of W phase [ 22] .Grobner et al. [ 21] reproduced isothermal section based on their revised dataset and added some experimental data at temperature higher than 400℃.It is unreliable in case of phase reactions in this temperature range.Indeed,L+W=Mg+I at 448℃was verified in previous research [ 23] .The mistaken mode makes the results doubtful,and they considered transformation temperature to be 521℃ [ 21] .

A major advantage of CALPHAD method lies in the modeling of higher-order systems built on fundamental database structures via optimizing available experimental data [ 20] .However,available experimental data of constituent sub-systems are still limited,especially in equilibrium state at this temperature.All the information is insufficient to guide alloy design [ 24, 25, 26, 27, 28, 29] .

Above all,the phase equilibrium relations of the low-Y side in the Mg-Zn-Y system at 400℃were investigated by alloy-equilibrated microstructure and lattice structure analysis,combined with thermal analysis.It is expected to provide more reliable experimental data for the thermodynamic assessment of Mg-Zn-Y system and for the alloy design of Mg matrix alloy.

2 Experimental

Seven alloys with different Y/Zn atomic ratios,such as Mg₉₆Zn_3.4Y_0.6,Mg_92.6Zn_6.4Y_1.0,Mg₉₀Zn_9.0Y_1.0,Mg_91.4Zn_7.4Y_1.2,Mg₉₄Zn₃Y₃,Mg₈₀Zn₁₇Y₃ and Mg₄₁Zn₄₈Y₁₁ (nominal composition,at%,the same as follows),in the low-Y side were designed in this work.The alloys were prepared by the high purity Mg (99.99%),Zn(99.999%) and Y (99.5%) and melt using medium-frequency induction furnace under an argon atmosphere in graphite crucibles.The weight of each ingot was about50 g.

All the samples taken from the ingots were wrapped in Ta foil and sealed in a quartz tube with the vacuum of1×10^-2 Pa under the protection of high-pure Ar atmosphere,then kept at 400℃for equilibrium treatment for35 days and finally quenched by water.The phase transition temperatures were determined through differential scanning calorimetry (DSC,NETZSCH404F3) at the heating rate of 10 K·min^-1.The microstructure observation was carried out on scanning electron microscope(SEM,JEOL JSM-7001F) with the voltage of 15 kV.The equilibrium phase compositions of the alloys were analyzed on electron probe microanalyzer (EPMA,JXA-8530F) with the voltage of 15 kV and spot size of 1μm.The high-pure Mg,Zn and YP5014 were used as standard samples.The equilibrium phase constituents were determined by X-ray diffractometer (XRD,SmartLab X'Pert PROXRD) with Cu Kαradiation at a voltage of 40 kV and a current of 200 mA.The selected area electron diffraction(SAED) patterns were obtained from the TECNAI-G20transmission electron microscope (TEM) operation at200 kV.

3 Results and discussion

3.1 Stability of icosahedral quasicrystal phase inα-Mg solid solution

The back-scattered electron (BSE) SEM image of the ascast alloy Mg_92.6Zn_6.4Y_1.0 is shown in Fig.1a.The composition of black phase in Fig.1a detected by EPMA contains few Zn and Y.It isα-Mg solid solution.The phase with bright contrast is enriched in Zn or Y,compared withα-Mg with dark contrast.TEM image and the SAED of intermetallics are shown in Fig.1b,c.The characteristic of spots exhibits a fivefold rotational symmetry pattern.B ased on these investigations,one phase of the lamellas isα-Mg solid solution and the other is I phase.It could be deduced that the lamellar microstructure forms through eutectic transformation with the temperature decreasing.However,the symmetry of diffraction spots is distorted,indicating that structural perfectness of quasicrystalline I phase is low [ 30] .

Equilibrium microstructures of the alloy Mg_92.6Zn_6.4Y_1.0 at 400℃are shown in Fig.2a,b.The blocky white phases distribute on the grain boundary of Mg-based solid solution in Fig.2a.EPMA result shows that the composition of white phases is 34.7Mg-55.6Zn-9.7Y,close to the composition of I phase [ 21] .SAED pattern of the ternary compound is shown in Fig.2c.Compared with that of as-cast alloy,it exhibits wellordered fivefold rotational symmetry.This is because the quasicrystalline I phase structure is much more intact after equilibrium treatment.As seen from DSC curve (Fig.1d),no phase transformation occurs from room temperature to448.8℃.With the temperature increasing from room temperature,it is a process of compounds spheroidization.It can be concluded that I phase could keep thermally stable till 448.8℃.So,the alloy Mg_92.6Zn_6.4Y_1.0 is in the two-phase regionα-Mg+I at 400℃.The characteristic diffraction peaks ofα-Mg and I phase are present in XRD analysis (Fig.2d),which also indicates the two-phase constituents of the alloy Mg_92.6Zn_6.4Y_1.0.

Fig.1 a BSE-SEM image,b TEM image,c SAED pattern of fivefold I phase and d DSC pattern of as-cast alloy Mg_92.6Zn_6.4Y_1.0

Fig.2 Equilibrium a SEM image,b TEM image,c SAED pattern of fivefold I phase and d corresponding XRD pattern of Mg_92.6Zn_6.4Y_1.0 alloy at 400℃

3.2 Equilibrium related to icosahedral quasicrystal phase at 400℃

The typical structure of the as-cast Mg₉₀Zn_9.0Y_1.0 alloy(Fig.3a,b) is similar to that of Mg_92.6Zn_6.4Y_1.0 alloy,consisting ofα-Mg matrix and two phases ofα-Mg and icosahedral quasicrystal eutectic network.SAED pattern(Fig.3c) of I-phase shows a twofold rotational symmetry pattern.However,due to lower Y/Zn ratio (Y content is the same in two alloys),the I phase content becomes much more with Zn+Y content increasing.

The equilibrium micro structure of Mg₉₀Zn_9.0Y_1.0 alloy at 400℃is shown in Fig.4a.The blocky white phase and thin lamellas distribute in the black matrix.Investigated by EPMA,the composition of the blocky white phase and the Mg matrix in Fig.4a is 34.4Mg-57.4Zn-8.2Y and96.8Mg-3.2Zn-0Y,respectively.Together with the analysis of TEM (Fig.4b) and XRD patterns (Fig.4c),the white phase is I phase and in equilibrium withα-Mg.It means that these white particles form through spheroidization of the lamellar icosahedral quasicrystal.

The average composition of these eutectic networks is73.6Mg-26.0Zn-0.4Y,with low Y content.According to the binary Mg-Zn phase diagram,it satisfies the scope of the liquid phase composition,which means that the existence of the liquid phase exists in the alloy Mg₉₀Zn_9.0Y_1.0kept at 400℃.First endothermic peak emerges at339.0℃on DSC curve (Fig.4d) of as-cast alloy Mg₉₀Zn_9.0Y_1.0,which might be caused by melting of the sample.Combining the composition of the as-cast,some of the thin lamellas consist ofα-Mg and icosahedral quasicrystal melt at 339.0℃,and then the liquid transforms to the eutectic network during quenching from 400℃.So,Mg₉₀Zn_9.0Y_1.0 alloy is in Mg-I-liquid three-phase equilibrium,i.e.,the three-phase region Mg+I+liquid exists in the isothermal phase diagram of Mg-Zn-Y system at400℃.

The equilibrium micro structure of alloy Mg₈₀Zn₁₇Y₃ at400℃is shown in Fig.5a.The composition of the lightgray phase in Fig.5a is 33.4Mg-56.4Zn-10.2Y,which is identified as I phase,and that of the white phase is21.5Mg-62.2Zn-16.3Y,which is identified as ternary compound H phase.Figure 5b shows SAED pattern of H phase observed in this alloy with zone axes of[1100].The results fit well with the hexagonal structure with parameters of a=0.9132 nm,c=0.9468 nm.The result of XRD(Fig.5c) also shows that intermetallics are not merely I-phase but also H phase.So,the Mg₈₀Zn₁₇Y₃ alloy is in the solid-state equilibrium ofα-Mg+H+I at 400℃.

3.3 Equilibrium related to W phase at 400℃

The equilibrium microstructures of Mg₉₆Zn_3.4Y_0.6 alloy are shown in Fig.6a,b.The blocky white phases distribute on the grain boundary ofα-Mg matrix in Fig.6a.A mass of EPMA results show that the compositions of white phases are classified into two types and they can be distinguished by shape.One average composition of angular phase is23.7Mg-61.8Zn-14.5Y,which is identified as ternary compound H phase.The other composition of spherical phase is 25.8Mg-51.0Zn-23.2Y,which is identified as ternary compound W phase.Figure 6c shows SAED pattern of H phase observed in this alloy with zone axes of[1210].SAED pattern of W phase with zone axes of[110]is shown in Fig.6d.The result fits well with fcc structure with parameters of a=b=c=0.683 nm.The corresponding XRD pattern is shown in Fig.6e.The diffraction peaks of Mg,H phase and W phase are indexed.

The equilibrium microstructure of Mg₉₄Zn₃Y₃ alloy is shown in Fig.7a.A large number of gray phases have a particular lamella structure,which is similar to the LPSO phase,and the composition of which is 88.7Mg-5.2Zn-6.1Y.The composition of bright spherical phase is31.9Mg-44.4Zn-23.7Y.XRD data in Fig.7b also indicate that the alloy is in three-phase equilibrium of Mg+W+X and the three-phase region is verified.

3.4 Isothermal section at 400℃

In the isothermal section at 400℃by Shao et al. [ 20] ,the H phase was absent.But differently,icosahedral quasicrystal phase,hexagonal structure H phase,fcc W phase and LPSO phase are in equilibrium withα-Mg solid solution in the low-Y side.The solubility of Zn atoms inα-Mg solid solution is no more than 3.2%,almost containing no Y.So the supersaturated Zn and Y could strengthen alloy by forming the second phase.

Fig.3 a SEM image,b TEM image and c SAED pattern of twofold I phase of as-cast alloy Mg₉₀Zn_9.0Y_1.0

Fig.4 a Equilibrium SEM image,b SAED pattern of fivefold I phase,c corresponding XRD pattern at 400℃of alloy Mg₉₀Zn_9.0Y_1.0 and d DSC curve of as-cast alloy

Fig.5 a Equilibrium SEM image,b SAED pattern of H phase with zone axes of

and c corresponding XRD pattern (c) of Mg₈₀Zn₁₇Y₃ at400℃

The partial isothermal section of phase diagram of lowY side in Mg-Zn-Y system at 400℃was constructed according to the above experimental data (Table 1) and is shown in Fig.8.

On the basis of these phase compositions,the boundaries of each phase region are confirmed.With the Y/Zn ratio increasing,the phase regionsα-Mg+liquid+I,α-Mg+I+H,α-Mg+H+W andα-Mg+W+X arranged.Different from the work in Ref. [ 20] ,there does not exist the three-phase regionα-Mg+W+I.

The vertical section at 80 at%Mg in the Mg-Zn-Y system was calculated by Grobner et al. [ 21] .At 400℃,the phase regions liquid,liquid+α-Mg+Z,α-Mg+Z+I,α-Mg+H+I,α-Mg+H+W andα-Mg+W+X line up with the increase in Y,the same as their isothermal section.In their work,the liquid phase could be in equilibrium withα-Mg solid solution and Z phase rather than I-phase at 400℃.The two-phase regionα-Mg+I phase exists between I+H+α-Mg and I+Z+α-Mg.However,from the experimental results of Mg₉₀Zn_9.0Y_1.0alloy,the composition of the liquid phase in the Mg+I+liquid three-phase equilibrium of Mg₉₀Zn_9.0Y_1.0 alloy at 400℃is 73.6Mg-26.0Zn-0.4Y,and the content of Y atoms in it is about 0.4%.The Z phase could not equilibrate withα-Mg solid solution in the low-Y side.

Fig.6 Equilibrium a SEM image and b TEM image,c SAED pattern of H phase with zone axes of[1210],d SAED pattern of W phase with zone axes of

and e corresponding XRD pattern at 400℃of Mg₉₆Zn_3.4Y_0.6

Fig.7 a Equilibrium SEM image and b corresponding XRD pattern of Mg₉₄Zn₃Y₃at 400℃

下载原图

Table 1 Composition of equilibrium phase at 400℃(at%)

Fig.8 Isothermal section of low-Y side in Mg-Zn-Y ternary system at 400℃

The existence of two-phase regionα-Mg+I provides more possibilities for the design of quasicrystal-reinforced alloys,and alloys with Y/Zn ratio of 0.14-0.17 are in that region.Besides,the system is in liquid-state phase equilibrium when Y/Zn ratio<0.14.As for Mg-Zn binary alloy,the liquid phase appears over the eutectic temperature (～340℃).However,the small amount of Y could not increase the eutectic temperature of the Mg-Zn-Y alloy surmounting 400℃.It is vital to control Y/Zn ratio to avoid liquid phase appearing in the alloy design.

The alloy is in three-phase regionα-Mg+I phase+H phase when Y/Zn ratio is 0.17-0.27.When Y/Zn ratio>0.27,W phase appears and equilibrates withα-Mg.It can be concluded that Y/Zn ratio between 0.14 and 0.17 is a dominant parameter to avoid the formation of W and H phases,thus optimizing mechanical properties.Additionally,in all these previous works,the scope of liquid phase and the solubility of Zn atoms inα-Mg solid solution are not explicit.Therefore,the experimental data of Mg-Zn-Y system at 400℃provide a fundamental and complementary work to calculate phase diagrams and design Mg matrix alloy using intermetallics as reinforcement.

4 Conclusion

There exists liquid phase which could be in equilibrium withα-Mg solid solution and icosahedral quasicrystal I-phase in the low-Y side of the Mg-Zn-Y system at400℃.The liquid phase region originates from the binary Mg-Zn system and extends to 0.4 at%Y in the Mg-ZnY system.Icosahedral quasicrystal I phase,hexagonal structure H phase,fcc W phase and LPSO phase (X phase) are in equilibrium withα-

Mg.With Y/Zn ratio increasing,there exist four three-phase regions consisting of I+liquid+α-Mg,I+H+α-Mg,H+W+α-Mg and W+X+α-Mg in the low-Y side of the isothermal section at 400℃.The two-phase regionα-Mg+I-phase exists between I+H+α-Mg and I+liquid+α-Mg.In this two-phase region,the Y/Zn ratio is in the range of 0.14-0.17;and a three-phase region ofα-Mg+I phase+H phase appears when Y/Zn ratio comes up to0.17-0.27,when Y/Zn ratio is 0.17-0.27.Not I but W phase is in equilibrium withα-Mg,when Y/Zn ratio>0.27.The system is in liquid-state phase equilibrium,when Y/Zn ratio<0.14.

Acknowledgements This study was financially supported by the National Natural Science Foundation of China (Nos.51271053 and5137104 and the National Key Research and Development Program of China (No.2016YFB0701202).