­­­ Compressive creep behavior of Mg-Sn binary alloy

WEI Shang-hai(魏尚海), CHEN Yun-gui(陈云贵)?, TANG Yong-bai(唐永柏),

LIU Ming(刘 明), XIAO Su-fen(肖素芬), ZHANG Xiao-ping(章晓萍), ZHAO Yuan-hua(赵源华)

School of Materials Science and Engineering, Sichuan University, Chengdu 610065, China

Received 12 June 2008; accepted 5 September 2008

Abstract: Mg-Sn based alloy is one of the potential alloys for application at elevated temperature. The compressive creep behavior of ageing-treated Mg-xSn (x=3%, 5%) alloys was investigated at the temperatures of 423 and 473 K and the stresses from 25 MPa to 35 MPa. When the tin content varies, the ageing-treated Mg-xSn alloys show quite different creep resistance, which are mainly attributed to the size and distribution of Mg₂Sn phases in the ageing-treated Mg-xSn alloys. The calculated value of stress exponent, n=6.3, suggests that the compressive creep behavior of the ageing-treated Mg-5%Sn alloy is controlled by dislocation creep at the temperature of 473 K and the stresses from 25 MPa to 35 MPa.

Key words: Mg alloy; aging treatment; creep deformation; stress exponent

1 Introduction

Magnesium-based alloys, as the lightest structural metal materials, have great potential for high-performance automotive applications[1]. Most commercial use of magnesium alloys is based on the well-developed Mg-Al system at present. However, Mg-Al alloy system particularly with high Al content shows poor creep resistance at elevated temperatures, since β-Mg₁₇Al₁₂ phase in Mg-Al alloys is not a good strengthening phase at high temperature. Therefore, it is of great need to develop other magnesium-based alloy systems. Recently, Mg-Sn based alloys have aroused many concerns[2-8]. Mg-Sn based alloys show some positive characteristics in terms of potential creep resistance because of the formation of stable phase Mg₂Sn, which typically distributes along the grain boundaries in as-cast Mg-Sn alloy[9]. LIU et al[10-12] analyzed the indentation creep behaviors of the as-cast Mg-Sn and Mg-Sn-Di alloy at the temperatures of 423 and 448 K, and found that the indentation creep process of Mg-Sn and Mg-Sn-Di alloys is controlled by dislocation climb. LIM et al[13] reported that the secondary solidification phase in Mg-MM alloy changes dramatically with the addition of Sn, and the small rod-shaped phase forms in the Mg-rich Mg-MM-Sn alloys. According to the Mg-Sn binary phase diagram, the solubility of Sn in the α-Mg solid solution drops sharply from 14.85% (mass fraction) at the eutectic transformation temperature of 834 K to 0.45% at 473 K. This means that the Mg₂Sn phases mainly distributed at the boundaries of as-cast Mg-Sn alloy will be redistributed homogeneously in the α-Mg matrix through the solid solution and ageing-treatment, which is predicted to have an important influence on the creep behavior of Mg-Sn alloy. WEI et al[14] compared the compressive creep behavior of as-cast and aging-treated Mg-5%Sn alloys, and revealed that Mg-5%Sn alloys show better creep resistance after aging-treatment. However, the influence of different content tin on the creep process is not understood clearly. The main objective of this work is to investigate the compressive creep behavior of ageing-treated Mg-xSn (x=3%, 5%, mass fraction) alloys at the temperatures of 423 and 473 K and the stress from 25 MPa to 35 MPa. The size, morphology and distribution of Mg₂Sn phases have great influences on the hardness and compressive creep behavior of the ageing-treated Mg-xSn alloys.

Pure magnesium (99.95%, mass fraction) and pure tin (99.98%) were melted in a graphite crucible with the protection of RJ-2 fusing agent. The melt was stirred to ensure homogeneity. Finally it was cast into an iron mould which was preheated up to 523 K. The pouring temperature was fixed at 983 K. The cavity dimensions of the iron mould were 20 mm×120 mm×140 mm.

The as-cast ingots were solid solution treated at the temperature of 753 K or 793 K (Table 1), quenched into water, and then subsequently aged at 513 K. The Vickers hardness was measured by HD-1000TM digital microhardness tester with the load of 0.25 N. The metallographic specimens were sliced from the same part of each ingot. They were polished and etched with a solution of 20% nitric acid+ethyl alcohol. The overall phase structure was analyzed by X-ray diffraction (D/ Max2rA) with Cu K_α radiation in a step scanning 2θ from 20? to 80? at a scanning rate of 0.01 (?)/s.

Table 1 Solid solution treatment of Mg-xSn alloys

Thin foil specimens for transmission electron microscopy(TEM) were prepared as follows. Block specimens were cut into 0.5-1.0 mm thick slices using a cutting machine. After mechanically grinding to 0.08- 0.1 mm thin foil, several discs of 3 mm in diameter were punched from the thin foil. The discs were finally ion thinned for the observation of TEM. TEM analysis was conducted on JEM-2010UHR operated at 20-200 kV.

The compressive creep tests were carried out on a self-made tester, which consisted of a constant load system, a temperature controller and an experiment data collector. The creep specimens were cut by electric spark machining from the middle part of the ingots. The specimen size of creep test was 10 mm×10 mm×15 mm. Oil was used as protecting medium during the creep test. The compressive creep tests were performed at the applied stresses from 25 MPa to 35 MPa and the temperatures of 423 K and 473 K.

Fig.1 shows the microhardness curves of Mg-xSn (x=3%, 5%) alloys obtained during isothermal ageing at 513 K. The hardness of Mg-3%Sn alloy increases slowly with the increase of aging time, and reaches a value of about HV_0.25 58 after 40 h. When Sn content is increased to 5%, the hardness value of Mg-Sn alloy exhibits larger age hardening response during isothermal ageing. The maximum hardness value of age-treated Mg-5%Sn alloy is HV_0.25 66.

The compressive creep behaviors of peak-aged samples of Mg-3%Sn and Mg-5%Sn alloys at 423 K, 35 MPa and 473 K, 35 MPa are provided in Figs.2(a) and (b),

Fig.1 Age hardening response of Mg-xSn (x=3%, 5%) alloys at 513 K

Fig.2 Constant-load creep curves of peak-aged samples of Mg-3Sn and Mg-5Sn alloys: (a) 423 K, 35 MPa; (b) 473 K, 35 MPa

respectively. Each of these creep curves includes a primary or transient creep stage and a near steady-state creep stage. Comparison of these curves indicates that the Mg-5%Sn alloy shows the highest creep resistance. The minimum creep rates of the age-treated Mg-3%Sn and Mg-5%Sn alloys, obtained at 423 K, 473 K and 35 MPa, are listed in Table 2.

Table 2 Secondary creep rate of Mg-xSn alloys at temperature of 423 K, 473 K and stress of 35 MPa

The creep behavior of magnesium alloy is affected by many factors, of which the microstructure is the most important[15]. The peak-aged microstructures of Mg-3%Sn and Mg-5%Sn alloys are shown in Fig.3. The TEM micrographs reflect the microstructure of samples at the beginning of creep tests. By comparing the peak- aged samples of Mg-3%Sn with Mg-5%Sn alloy, the size and morphology of Mg₂Sn phases have significant difference. In Fig.3(a), the content of precipitate Mg₂Sn phase is a little, precipitates shape is mainly plate-like, and their average size is about 150 nm. However, when

Fig.3 TEM images of peak-aged samples of Mg-3%Sn (a) and Mg-5%Sn (b) alloys

the Sn content is increased to 5%, the Mg₂Sn particles show two shapes, plate-like and rod-like, in Fig.3(b). The size of plate-like Mg₂Sn is 100 nm, and the length of rod-like Mg₂Sn is about 1-2 mm. SASAKI et al[2] reported that most of the precipitates are on the {0001} plane. The rod-like precipitates exist along the <> direction. And for the peak-aged samples of Mg-9.9%Sn alloy at 453 K, coarse precipitates of Mg₂Sn with a length of 1-2 mm are dispersed in the magnesium matrix α-phase, in most regions. NIE[16] reported that the rod precipitates on the pyramidal planes and prismatic planes are more effective for strengthening than that on the basal plane. However, the orientation relationship of Mg₂Sn phases with matrix has not been studied in this work. Further study is in progress to clarify how the orientation relationship of Mg₂Sn phases with matrix influences the compressive properties of Mg-Sn alloys. Thus, the morphology of Mg₂Sn particles is evolved from plate-like to rod-like when the content of Sn increases. The changes in the hardness and the minimum creep rates of the peak-aged Mg-xSn alloys (Fig.1 and Fig.2) are attributed to the size and morphology of the precipitates phase of Mg₂Sn.

Fig.4 shows the variation of minimum creep rate as a function of stress from 25 MPa to 35 MPa at the temperature of 473 K for the age-treated Mg-5%Sn alloys. The stress exponent obtained from the plot is 6.3 for the peak-aged sample of Mg-5%Sn alloy. The stress exponent n value of approximate 2 is related to grain boundary sliding as the prominent creep mechanism, while n=4-6 is generally reported for dislocation creep [1]. The stress exponent n value suggests that the compressive creep behavior of the peak-aged samples of Mg-5%Sn alloys is controlled by dislocation creep at the temperature of 473 K and stress from 25 MPa to 35 MPa.

Fig.4 Minimum creep rates as function of applied stress for ageing-treated Mg-5%Sn alloy at temperature of 473 K and stress from 25 MPa to 35 MPa

1) The compressive creep behavior of peak-aged Mg-xSn (x=3%, 5%) alloys at the temperatures of 423 K, 473 K and the stress of 35 MPa shows that the ageing- treated Mg-5%Sn alloy demonstrates quite better creep resistance than Mg-3%Sn alloys.

2) In the peak-aged sample of Mg-3%Sn alloy, the Mg₂Sn phases mainly consist of plate-like grains with average size of about 150 nm. When Sn is added to 5%, the precipitates of Mg₂Sn display in two shapes: the plate-like and the rod-like. The size of plate-like Mg₂Sn is 100 nm and the length of rod-like Mg₂Sn particles is about 1-2 mm. The difference of compressive creep behavior is mainly attributed to the size, morphology and distribution of Mg₂Sn phases in the peak-aged Mg-xSn alloys.

3) The calculated value of stress exponent, n=6.3, suggests that the compressive creep behavior of the ageing-treated Mg-5%Sn alloys is mainly controlled by dislocation creep at the temperature of 473 K and the stresses from 25 MPa to 35 MPa.

Acknowledgements

This research was supported by a grant from the Science & Technology Bureau of Sichuan Province of China. The authors would like to thank Prof. LIU Ming for the very helpful assistance and also thank the Analysis and Testing Center of Sichuan University for providing the necessary testing equipments.

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(Edited by YUAN Sai-qian)

Corresponding author: CHEN Yun-gui, Tel: +86-28-85405670; E-mail: ygchen60@yahoo.com.cn