Enhanced creep properties of a cast Mg–5Sn alloy subjected to aging-treatment

doi:10.1016/j.msea.2010.04.015

Materials Science and Engineering: A

Volume 527, Issues 18–19, 15 July 2010, Pages 4613-4618

https://doi.org/10.1016/j.msea.2010.04.015 Get rights and content

Abstract

The microstructure and impression creep behavior of the Mg–5Sn alloy are studied in the as-cast and aged conditions. Impression creep tests were performed in the temperature range 423–498 K and under punching stresses in the range 125–400 MPa. Analysis of the data showed that for all loads and temperatures, the creep resistance of the aged alloys was much better than that of as-cast materials, due to the higher volume fraction of Mg₂Sn precipitates distributed in the microstructure. This phase strengthened both matrix and grain boundaries during creep deformation in the investigated system. Based on the steady-state power-law creep relationship, the stress exponents of about 5.2–6.9 were obtained at different temperatures. When the experimental creep rates were normalized to the pipe-diffusion coefficient, stress exponents of 5 and 6.8 were, respectively obtained for the cast and aged conditions, which are in complete agreement with stress exponents determined by the conventional creep testing of the same material reported in the literature. The activation energies in the range 90–98 kJ/mole, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents of about 5 and 7 suggest that the operative creep mechanism is dislocation viscous glide in the as-cast condition, and dislocation climb controlled by dislocation pipe diffusion in the aged material.

Introduction

Mg alloys have great potential for high performance structural applications due to their inherent properties such as acceptable mechanical properties, low density, superior damping capacity and good castability [1], [2]. The most common types of these materials are those based on the Mg–Al system, which have relatively poor mechanical properties at elevated temperatures, mainly due to the low thermal stability of the microstructure containing the net shape Mg₁₇Al₁₂ precipitates along grain boundaries [3], [4], [5], [6]. This makes the Al-containing Mg alloys unsuitable for high temperature creep, by limiting their allowable temperature to less than 125 °C. Therefore, attempts have been made to develop new magnesium alloys having improved structural stability at the elevated temperatures. Among many possibilities, Mg–Sn based alloys are of special interest. These alloys have great potential for creep resistance due to the formation of thermally stable phase Mg₂Sn, which is mainly distributed along grain boundaries in the as-cast condition [7]. According to the Mg–Sn binary phase diagram [8], the solubility of Sn in the α-Mg decreases from 14.85 wt.% at the eutectic transformation temperature of 834 K to less than 0.45 wt.% at 473 K. This means that the Mg₂Sn phase, mainly distributed at the boundaries of the as-cast Mg–Sn alloy, will be redistributed homogeneously in the α-Mg matrix through the solutionizing and aging-treatments, the outcome of which would be enhanced strength properties at both ambient and elevated temperatures.

Accordingly, the creep characteristics of the Mg–Sn alloys have received some attentions. Wei et al. [9] have demonstrated that the compressive creep resistance of the aged Mg–5Sn alloy is much better than that of the as-cast material, mainly due to the dispersive distribution of Mg₂Sn phase in the aging-treated condition. The microstructure, tensile properties and creep behavior of cast Mg–(1–10)%Sn alloys have also been investigated [10]. It has been reported that the indentation creep resistance of these alloys is improved with increasing tin content. Recently, the effects of Sb and Ca additions on the creep of cast Mg–Sn have been investigated [11], [12]. It has been shown that ternary alloys exhibit enhanced creep resistance mainly due to the formation of the thermally stable Mg₃Sb₂ and CaMgSn second phase particles which strengthen both matrix and grain boundaries during creep deformation.

The effect of aging-treatment on the impression creep behavior of the Mg–5Sn alloy has not been studied before, and therefore it is the aim of this study to examine the microstructure and impression creep properties of the Mg–5Sn alloy in the as-cast and aged conditions and compare them with those obtained by the conventional creep tests reported in the literature. Impression creep test, developed by Chu and Li [13], is a unique method for determining creep behavior of different materials. This technique is a modified version of indentation creep method, in which a flat-ended cylindrical punch is pushed into test specimen under a constant load and the impression depth is recorded during dwell time. In contrast to the conventional creep tests which need tensile or compressive specimens, in this method all creep data can be obtained with a small piece of material.

Section snippets

Experimental procedure

The material used in this investigation was Mg–5 wt.% Sn. High purity Mg and Sn were used to prepare the alloy. Melting was carried out in an electrical furnace under the Foseco Magrex 36 covering flux to protect molten magnesium from oxidation. The melt was held at 1023 K for 20 min and mechanically stirred for 2 min before Sn was added to it. Another 20 min was allowed to ensure a homogenous composition and to settle the oxides and other contaminants before pouring the melt into a permanent mold

Results and discussion

Fig. 1 shows the optical microstructure of the investigated alloys in the as-cast and aged conditions. As can be seen, the as-cast condition (Fig. 1a) possesses a coarse dendritic microstructure with a dendritic arm spacing of about 250 μm, while after aging-treatment at 483 K for 21 h (Fig. 1b), the microstructure has changed to a nearly equiaxed grain structure having an average grain size of about 90 μm. It is further revealed that spherical particles are distributed in the grain boundaries as

Summary and conclusions

1.
The structure of Mg–5Sn alloy in the as-cast condition consists of Mg₂Sn intermetallic phase in α-Mg matrix. These intermetallic particles are thermally stable barriers which oppose recovery processes, decreasing the minimum creep rates of the materials. The enhanced creep resistance of the material in the aged condition is related to the dispersive distribution of Mg₂Sn phase in the α-Mg matrix. These thermally stable particles strengthen both grain and grain boundaries and oppose both

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The age-hardening response and compression property of a Mg-7Sn-1Ca-1Ag (wt.%) alloy are improved by extrusion combined with aging treatment (abbreviated as extruded-aged). The corresponding microstructure evolution was investigated and discussed in detail. The results show that the average grain size of alloy was refined obviously from ∼54.6 μm to ∼3.2 μm by extrusion at 320 °C and the refinement of CaMgSn phase is also exhibited. Extrusion process promotes the precipitation of Mg₂Sn and Mg₅₄Ag₁₇ phase and more precipitates of Mg₂Sn phase exists in the extruded-aged alloy. As a result, age-hardening response and compressive strength are significantly enhanced in the extruded-aged Mg-7Sn-1Ca-1Ag alloy. The peak-aged hardness and ultimate compressive strength (UCS) of extruded-aged Mg-7Sn-1Ca-1Ag alloy are 95.2 HV and 482 MPa, respectively, both of which are obviously higher than that of as-aged and as-extruded alloys. This is mainly attributed to the grain refinement, formation of Mg₅₄Ag₁₇ phase and the increased content of finer Mg₂Sn precipitates. Moreover, it was found that an obvious yield behavior occurs in compressive engineering stress versus strain of extruded and extruded-aged alloys, which is mainly attributed to the basal texture caused by extrusion and high value of Schmid factor, leading to the easy formation of {10–12} twinning in the extruded-aged alloy during compression along the extrusion direction.
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Citation Excerpt :
The results reveal that the stress exponent values decrease with increasing temperature from 473 to 563 K for both composites, as presented in Table 2. The observed slight decrease in the stress exponent with increasing temperature is indicative of relative microstructural stability of the tested composites during the high-temperature creep process [27–29]. The present results are close to those found in previous studies on the creep behavior of other aluminum matrix composites [30,31].
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Enhanced creep properties of a cast Mg–5Sn alloy subjected to aging-treatment

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Summary and conclusions

Scripta Mater.

Acta Mater.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

J. Alloys Compd.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

J. Alloys Compd.

Acta Metall.

Acta Metall.

Light Alloys