Characterization of phases in Mg-10Y-5Gd-2Zn-0.5Zr alloy processed by heat treatment

WANG Qu-dong(王渠东)¹, GAO Yan(高 岩)^{1, 2}, YIN Dong-di(尹冬弟)¹, CHEN Chang-jiang(陈长江)¹

1. National Engineering Research Center of Light Alloys Net Forming,

Shanghai Jiao Tong University, Shanghai 200240, China;

2. Grikin Advanced Materials Co., Ltd, Beijing General Research Institute of None-ferrous Metals,

Beijing 100220, China

Received 23 October 2009; accepted 9 August 2010

Abstract: Characterizations of phases in Mg-10%Y-5%Gd-2%Zn-0.5%Zr (WGZ1052) alloy during heat treatments were investigated by OM, XRD, SEM and EDS. The mechanisms of microstructure evolution were discussed. The results show that, after high temperature heat treatments, the Mg₁₂ZnY phases still exist. During solution-treatment at 535 °C, the amount of the long-period stacking order structures decreases. At 545 °C for 20 h and 24 h, there are still remnant Mg₁₂ZnY compounds in the Mg matrix, the shape of which does not change and the amount does not decrease obviously.

Key words: Mg-Y-Gd-Zn alloy; long-period stacking order; heat treatment

1 Introduction

Mg-base alloys with rare-earth elements are becoming more and more attractive due to their high strength and creep resistance[1-2]. Mg-Gd-Y-Zr alloys were developed with higher specific strength at both room and elevated temperatures and higher creep resistance than WE54 and QE22[3-5].

On the other hand, the addition of Zn or Cu can further improve the strengthening response of Mg-RE alloys. Mg₉₇Zn₁Y₂ (molar fraction, %) was developed using rapidly solidified powder metallurgy in 2001[6]. It is a novel alloy with superior mechanical properties including tensile yield strength of 610 MPa and elongation of 5%. In 2003, AMIYA[7] and MATSUURA et al[8] subsequently reported that the Y and Zn elements were enriched in the long-period region and the periodicity changed from 6H-type to 14H-type with increasing cooling rate in melt spinning Mg₉₇Zn₁Y₂. In 2004, ITOI et al[9] reported that a long-period stacking order (LPSO) structure with 18R-type was observed in Mg₉₇Zn₁Y₂ alloy prepared by Cu-mould casting and induction melting methods. It is very interesting that such small additives like 1%-2% (molar fraction) Zn and Y into pure Mg dramatically change its microstructure and mechanical properties.

All these methods need highly non-equilibrium synthesis in a thermo-dynamically unstable state. So, industrial applications of the new materials are often limited. It was reported that Mg-10%Y-5%Gd-2%Zn- 0.5%Zr (mass fraction) alloy with novel long-period ordered structure was processed via a conventional casting method[10]. The cast alloy showed excellent high temperature strength and creep resistance at 200-300 °C[11], indicating it was a very promising casting creep-resistant Mg alloy for high temperature applications[12-17]. Furthermore, this alloy has successfully been used as manufacturing engine piston by CHEN et al[18]. However, very little is known about the characterizations of phases in the alloy. A better understanding is very important from both the scientific and engineering points of view. In this work, the evolutions of microstructure and LPSO structure were investigated and the formation mechanism was discussed.

2 Experimental

Alloys with actual composition of Mg-10.3Y-5.6Gd-2.0Zn-0.5Zr (mass fraction, %) (WGZ1052) were prepared from high purity (≥99.9%) Mg, Zn and Mg-25Y (mass fraction, %), Mg-25Gd (mass fraction, %) and Mg-30Zr (mass fraction, %) master alloys by induction melting in a mild steel crucible at approximate 750 °C under a mixed atmosphere of CO₂ and SF₆ with the volume ratio of 100:1. Then, the alloy was cast into a steel mould and pre-heated to 200 °C. As-cast specimens of WGZ1052 alloy were solution treated at 535-545 °C for 16-24 h and then quenched into cold water. The constituent phases were identified by X-ray diffraction (XRD) with Cu K_α radiation. The microstructures of specimens were analyzed by optical microscope (OM) and scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS).

3 Results and discussion

Optical microstructure of the conventional casting WGZ1052 alloy is shown in Fig.1. The phase composition of the alloy obtained by XRD analysis is shown in Fig.2. Fig.3 and Table 1 show SEM images of WGZ1052 alloy and its corresponding EDX analysis results, respectively. It is found that WGZ1052 alloy mainly consists of four phases, matrix of primary α-Mg solid solution supersaturated with Gd, Y and Zn (Point A in Fig.3); network-shaped eutectic compound      Mg₂₄(GdYZn)₅ (Point B in Fig.3); network-shaped compound Mg₁₂YZn (Point C in Fig.3) and fine-lamellar phases (Point D Fig.3). The magnified images of fine-lamellar phases are shown in Fig.3(b). Most Mg₂₄(GdYZn)₅ compounds are distributed at the grain boundaries. The fine-lamellar phase uniformly distributes from eutectic compounds to the inner of α-Mg grains. These phases have specific orientation in the matrix grains and only one orientation in one grain (Fig.3(b)), which suggests that the fine-lamellar phases have specific orientation relationship with the α-Mg matrix crystals.

The temperature of solution heat treatment was determined on the basis of Mg-Gd and Mg-Y phase diagrams, and DSC curves[4-5, 11]. The microstructure evolutions at different solution temperature and time are shown in Fig.4. It is found that at 535 °C, the Mg₂₄(GdYZn)₅ net-work of second phase is completely dissolved after heated for 16 h (Fig.4(a)), but the black network-shaped compound Mg₁₂YZn still survives in the matrix.

There are still remnant Mg₁₂YZn compounds in the matrix at 545 °C for 20 h and 24 h, the shape of which does not change and the amount does not decrease obviously. It can be concluded that the Mg₁₂YZn is a high temperature stable phase and does not dissolve into the magnesium matrix at high temperatures.

Fig.1 Optical microstructure of as-cast WGZ1052 alloy

Fig.2 XRD pattern of as-cast WGZ1052 alloy

Fig.3 SEM images of WGZ1052 alloy

Fig.4 Microstructures of WGZ1052 alloys with different treatment: (a) 535 °C, 16 h; (b) 535 °C, 20 h; (c) 545 °C, 20 h; (d) 545 °C, 24 h

Table 1 EDX analysis results of as-cast WGZ1052 alloy in Fig.3 (molar fraction, %)

The XRD pattern of the WGZ1052 alloy solution heat treated at 535 °C for 16 h is shown in Fig.5, which indicates that T4-WGZ1052 alloy mainly consists of α-Mg solid solution and Mg₁₂YZn compounds.

Fig.6 shows the SEM image of the solution treated sample and corresponding EDX analysis results, which indicates that all the network-shaped eutectic compound Mg₂₄(GdYZn)₅ are dissolved in the matrix (Point A). It can be seen that the contents of Zn and Y elements at point B are high. The EDS analysis shows that the average molar ratio of Zn to Y is equal to 1:1, which indicates the stoichiometry of the compound is Mg₁₂YZn.

At elevated temperature, the grain-boundary softens and grain-boundary sliding becomes another possible deformation mechanism[19]. The denser Mg₁₂YZn phases distributing along the grain boundary which have a good thermal stability and greater strength than the grains of solid-solutionized HCP-Mg[20], can pin the grain-boundary sliding and strengthen the boundaries effectively. So, this Mg₁₂YZn phase may play an important role in strengthening the present WGZ1052 alloy because it acts as an effective second hard phase dispersed in the matrix. But the exact role of Mg₁₂YZn phase in strengthening has not been clarified yet. Now, the high temperature strength and creep properties of the WGZ1052 alloy are being studied, and the systematic results will report the relation of intensity, Mg₁₂YZn formation and the interactive effect of dislocation structure and Mg₁₂YZn phase.

Fig.5 XRD pattern of WGZ1052 specimen solution treated at 535 °C for 16 h

Fig.6 SEM images (a, b) and EDS patterns (c, d) of solution treated WGZ1052 alloy

4 Conclusions

1)  The conventionally casting WGZ1052 alloy mainly consists of matrix of primary α-Mg solid solution supersaturated with Gd, Y and Zn, network-shaped eutectic compound Mg₂₄(GdYZn)₅, network-shaped compound Mg₁₂YZn and fine-lamellar phases.

2)  For 16 h solution treatment at 535 °C, the net-work second phase Mg₂₄(GdYZn)₅ completely dissolves, but the black network-shaped compound Mg₁₂YZn still survives in the matrix.

3)  There are still remnant Mg₁₂YZn compounds in the matrix at 545 °C for 20 h and 24 h, the shape of which do not change and the amount does not decrease obviously. It can be concluded that the Mg₁₂YZn is a high temperature stable phase and does not dissolve into the magnesium matrix at high temperatures.

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(Edited by FANG Jing-hua)   

Foundation item: Project(2009AA033501) supported by the National High-tech Research and Development Program of China; Project(2006BAE04B01-2) supported by the National Key Technology R&D Program of China; Project(51074106) supported by the National Natural Science Foundation of China

Corresponding author: WANG Qu-dong; Tel: +86-21-54742715; Fax: +86-21-34202794; E-mail: wangqudong@sjtu.edu.cn

DOI: 10.1016/S1003-6326(09)60420-9