­­­ Influence of heat treatment on microstructure and mechanical properties of Mg-10Gd-3Y-1.2Zn-0.4Zr alloy

LI De-jiang(李德江)¹, ZENG Xiao-qin(曾小勤)^{1, 2}, DONG Jie(董 杰)¹, ZHAI Chun-quan(翟春泉)¹

1. National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University,

Shanghai 200240, China;

2. State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University,

Shanghai 200240, China

Received 12 June 2008; accepted 5 September 2008

Abstract: Microstructure evolution and mechanical properties of the cast Mg-10Gd-3Y-1.2Zn-0.4Zr (mass fraction, %) alloy during annealing at 798 K for different time were investigated. In the as-cast state, the microstructure consists of α-Mg, Mg₅(Gd, Y, Zn) eutectic compounds and stacking faults(SF) of basal plane distributed from grain boundary to inner grain. During heat treatment at 798 K, the SF and parts of eutectic compounds dissolve into the matrix gradually, simultaneously, a new straight lamellar phase with 14H type of long period stacking ordered(LPSO) structure comes into being. Microstructure observation indicates that the lamellar phase is transformed from the un-dissolved eutectic compounds and shows a coarsening and disappearing process with annealing time prolonging. Three alloy specimens with typical microstructure after being annealed for 6, 36 and 72 h were selected to age at 498 K to peak hardness. Room and elevated temperature tensile tests show that the LPSO structured phase contributes to the change of high temperature tensile properties.

Key words: Mg-Gd-Y-Zn-Zr alloy; microstructure; LPSO structured phase; mechanical properties

1 Introduction

Magnesium alloys containing rear earth metals are attractive as high performance structural materials used in aerospace and automotive industries[1]. KAWAMURA developed the Mg97Y2Zn1 (molar fraction, %) alloy firstly by warm extrusion of rapid solidified powders at 573 K, which exhibits the tensile yield strength of about 610 MPa and elongation of 5% at room temperature. The excellent properties are considered to be due to the ultra-fine Mg grains of 100-200 nm in diameter with novel long period stacking ordered(LPSO) structure and homogeneously dispersed Mg₂₄Y₅ particles less than 10 nm in diameter[2-3]. In recent years, Mg-RE-Zn alloy systems have attracted much attention and many researches are focused on the novel LPSO structure. It has been reported that different types of LPSO structures including 18R, 24R, 6H, 10H, 14H etc can easily form during solidification or by thermo- mechanical treatment in many Mg-RE-Zn series such as Mg-Gd-Zn, Mg-Y-Zn, Mg-Er-Zn, Mg-Ho-Zn and Mg-Dy-Zn[4-14]. It is believed that mechanical properties at room or elevated temperatures of these alloys will be improved by the formation of LPSO structure[6-8]. However, the formation and strengthening mechanisms of LPSO phase were scarcely discussed.

In previous study[9], the formation process of stable LPSO structured phase in Mg-10Gd-3Y-1.2Zn-0.4Zr (GWZK) alloy during heat treatment at 773 K was clearly discussed. The purpose of the present work is to study the LPSO structure transformation process and its effect on mechanical properties during heat treatment at a rather high temperature of 798 K for the GWZK alloy and then try to elucidate the strengthening mechanisms of this type of phase.

2 Experimental

The Mg-10Gd-3Y-1.2Zn-0.4Zr (mass fraction, %)(GWZK) alloy was prepared by melting in an electrical resistance furnace with steel crucible under protecting gas consisting of SF₆ (1%, volume fraction) and CO₂ (99%, volume fraction) in order to prevent burning of the melts. Specimens were cut from the ingot by electric spark linear cutting and heat treated at 798 K for different time and then aged at 498 K for 16 h in an oil bath followed by quench in cold water.

Microstructures of the specimens were analyzed with optical microscope(OM) and FEI SIRION 200 scanning electron microscope(SEM) equipped with an Oxford energy dispersive X-ray spectrometer(EDS).

Characterization of phases was performed in a JEOL-2010 transmission electron microscope, operated at 200 kV. Thin foils for the TEM observations were prepared by twin jet electro-polishing in a solution of 25% HNO₃ and 75% methanol cooled down to 253 K, and then low energy beam ion thinning was carried out. Tensile tests were performed on a Zwick-20kN material testing machine at the rate of 1 mm/min.

3 Results and discussion

3.1 Microstructure of as-cast GWZK alloy

SEM and TEM morphologies of the as-cast GWZK alloy are shown in Fig.1. As can be seen, there are four phases in the as-cast alloy designated as α-Mg, Mg₅(Gd, Y, Zn) eutectic compounds, cuboid-shaped phase and needle-like phase. The compounds and cuboid-shaped phase are both of FCC crystal structure; however, the crystal lattice constants are different with each other. The needle-liked phase is just basal plane stacking faults(SF) of Mg as shown in Fig.1(b). The more detailed discussion of the microstructure is presented in Ref.[9].

3.2 Microstructure of GWZK alloy during heat treat- ment

Optical microstructures of 798 K heat treated alloy for different time are presented in Fig.2. After heat treatment for 6 h, as shown in Fig.2(a), the SF and parts of the eutectic compounds dissolve into the Mg matrix. The residual compounds display in a scattered block-shape and become broader than before. Besides that, a new fine lamellar-shaped phase forms through growing from grain boundaries to inner grain.

As the heat treatment time prolongs to 18 h, as shown in Fig.2(b), the compounds continue to dissolve into the matrix, on the other hand, the fine lamellar- shaped phase displays a coagulating process. In the center of the grain, some irregular-shaped particles concentrate. Investigation of EDS confirms they are RE-rich particles, but the composition varies from each other. With increasing heat treatment time to 36 h, microstructure of the GWZK alloy consists of fine and coarse block-shaped lamellar phase in company with some coarse block-shaped phase. Fig.3 shows the TEM image and electron diffraction pattern of lamella shaped phase in a 36 h heat treated specimen. Electron beam direction is approximately parallel to  From the obtained diffraction pattern, the lamellar-shaped phase is identified as 14H type of LPSO structure. Periodic small diffraction spots are observed at the interval of 1/14 of distance between direct spot and (0002)_Mg reflection. In TEM investigation results, any eutectic compounds have not been found and it is sure that the block-shaped phases at grain boundaries are just coagulated 14H LPSO structured phase. When the heat treatment time increases to 72 h, the GWZK alloy shows a dissolving process of the LPSO phase. As seen in Fig.2(f), there is already no obvious LPSO phase in the grains except small amounts of contrast streaks after heat treatment. It is worth noting that some block-shaped phases come into being during the whole heat treatment process and they can be also identified as RE-rich phases with FCC crystal structure by EDS and electron diffraction technique[9]. Combined with microstructure observation and other investigation[10], it can be regarded that the LPSO structured phase is transformed from the eutectic compounds and gradually dissolves into the matrix with heat treatment time increasing.

Fig.1 SEM image (a) and TEM image and SAED pattern of needle-like phase in inner grains (b) of as-cast GWZK alloy

Fig.2 Optical microstructures of heat-treated alloy specimens at 798 K for different time: (a) 6 h; (b) 18 h; (c) 36 h; (d) 48 h; (e) 60 h;  (f) 72 h

Fig.3 TEM image (a) and electron diffraction pattern (b) of lamellar shaped phase in 36 h heat treated specimen

3.3 Mechanical properties

Room temperature tensile properties of some typical heat-treated GWZK alloys are shown in Fig.4. For the as-heat treated alloy, the tensile yield strength(TYS) and ultimate tensile(UTS) strength are decreased with increasing heat treating time while the elongation shows a reverse trend.

Fig.4 Room temperature tensile properties of GWZK alloy:  (a, d) 798 K, 6 h; (b, e) 798 K, 36 h; (c, f) 798 K, 72 h; (a, b, c) Without peak aging; (d, e, f) With peak aging at 498 K for 16 h

For heat treated and peak aged alloy, the test results are just on the contrary with those of the non-aged alloy specimens. The reason should be as follows. For the non-aged alloy, the fact that the TYS decreases with increasing heat treatment time is due to the dissolving of the eutectic compounds and grain coarsening during heat treatment. While for the peak aged alloy, because amounts of RE atoms dissolve into the matrix, as shown in Fig.2, more metal-stable phases that are the main strengthening particles to hinder dislocation movement during deformation will precipitate after peak aging. It is obvious that the TYS should be improved.

The elevated temperature tensile properties of heat treated and peak aged alloy specimens are presented in Table 1. The ultimate tensile strength(UTS) at 523 K of the 598 K, 72 h treated alloy specimen is 301 MPa, 34 MPa higher than that of 36 h treated specimen. However, it is 27 MPa lower than that tested at a comparatively high temperature of 573 K. This means that the high temperature tensile strength for the 36 h treated alloy is superior to that of the 72 h treated although there are more metal-stable phases in the 72 h heat-treated alloy specimen.

Although that at high temperatures (commonly higher than 523 K) the prismatic plane dislocation sliding of Mg alloys can be easily operated, the meta-stable phase precipitated perpendicular to basal plane can only effectively hinder basal plane dislocation moving, as reported in Ref.[15]. So as to further improve

Table 1 Elevated temperature tensile properties of GWZK alloy

the strength at such high temperatures, another type of strengthening phase is needed which should be arranged parallel to basal plane. The LPSO structured phase formed during heat treatment can be just regarded as a type of plane defect parallel to basal plane of Mg alloys, as shown in Fig.3. It is observed in Fig.2 that the amount of LPSO phases in 36 h heat-treated alloy is much more than that of 72 h heat-treated alloy. Then the higher UTS of the 36 h treated alloy must be reasonable. Controlling the microstructure by heat treatment is meaningful for further improving the high temperature mechanical properties of Mg-RE-Zn series alloys and is worthy of deep investigation.

4 Conclusions

1) The microstructure of as-cast Mg-10Gd-3Y- 1.2Zn-0.4Zr alloy consists of α-Mg, Mg₅(Gd, Y, Zn) eutectic compounds, cuboid-shaped phase and basal plane stacking faults of Mg.

2) During heat treatment at 798 K, the basal plane stacking faults and parts of Mg₅(Gd, Y, Zn ) eutectic compounds dissolve into Mg matrix. The residual compounds show a transformation to lamella-shaped LPSO structured phase and gradually dissolving process with time prolonging.

3) The elevated temperature tensile properties of heat-treated and peak-aged Mg-10Gd-3Y-1.2Zn-0.4Zr alloy can be improved by LPSO structured phase.

Acknowledgement

The authors would like to appreciate the help of the SEM and TEM observations received from the Instrumental Analysis Center, Shanghai Jiao Tong University, China.

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(Edited by YANG Bing)

Foundation item: Project(2007CB613701) supported by the National Basic Research Program of China; Project(08XD14020) supported by Shanghai Subject Chief Scientist Program, China

Corresponding author: ZENG Xiao-qin; Tel: +86-21-54742619; Fax: +86-21-34202794; E-mail: xqzeng@sjtu.edu.cn