The effect of grain size on the tensile and creep properties of Mg–2.6Nd–0.35Zn–xZr alloys at 250 °C

Abstract

A systematic study has been made of the effect of grain size on the tensile and creep properties of Mg–2.6Nd–0.35Zn–xZr (in wt%) alloys (ML10 or ZM6) at 250 °C, the maximum long-service temperature for these alloys. Grain sizes ranging from 102 to 920 μm were produced in the T6 state after grain refinement with Zr. The sand cast Mg–2.6Nd–0.35Zn alloy solidified as largely columnar grains (900±430 μm). An addition of 1.8% Zr reduced the grain size to 84 μm but most grains were still irregular rather than equiaxed. The grain size increased linearly with 1/Q (Q: growth restriction factor) calculated from the soluble Zr content only. Significant grain hexagonalization occurred during solid solution treatment at 530 °C (720 min). Both the yield strength and ultimate tensile strength at 250 °C followed the Hall–Petch relationship while the ductility increased linearly with decreasing grain size. The steady-state creep rate at 250 °C increased significantly with decreasing grain size but followed the general power law at each stress level evaluated (50–90 MPa). The effect of grain size on creep is related to the applied stress where the grain size effect exponent p showed a linear dependency on applied stress. To satisfy the requirements for both the tensile and creep properties at 250 °C, the T6 grain size of these alloys should be limited to about 174 μm. However, when the T6 grain size is <100 μm, it may result in a significant increase in the steady-state creep rate.

Introduction

MgNdZnZr based alloys in the composition ranges of (2.0–2.8)% Nd, (0.2–0.7)% Zn and (0.4–1.0)% Zr (in wt%) are commercially important creep resistant Mg alloys [1]. Compared with the more creep resistant yttrium-containing (the WE series) magnesium alloys, they are more cost affordable for applications up to 250 °C. The alloy system was first developed in Russia in the early 1960s for aerospace applications, known as ML10 [2], [3]. It was subsequently introduced into China and classified as ZM6, where ZM refers to cast Mg alloys and the digit 6 represents one of the 10 series sand cast Mg alloys in the category of aeronautical materials [1]. Over the last decade researchers have continued to develop magnesium-rare earth (RE) based sand cast creep-resistant Mg alloys for potential automotive and aerospace applications [4], [5], [6]. Our assessment is that the MgNdZn system will continue to be a prime candidate Mg system for long-term applications up to 250 °C. A recent detailed CALPHAD assessment of the MgNdZn system [7] has provided a useful thermodynamic basis for its further improvement.

The main role of each alloying element in the ML10 or ZM6 alloys can be summarized below. Nd is the prime strengthening element which forms fine Mg₁₂(Zn,Nd) precipitates with Mg and Zn in the T6 microstructure [2], [3], [8]. The small presence of Zn (0.2%–0.7%) promotes precipitation of basal precipitate plates in the alloy in addition to prismatic precipitate plates [8]. Zr imparts to these alloys structural uniformity and consistency in performance through potent grain refinement [9].

The room temperature and elevated temperature (up to 300 °C) properties, including creep properties, of the ML10 or ZM6 alloys have been well documented in Ref. [1] based on detailed Russian and Chinese work. However, there is little systematic information about the effect of the grain size on the tensile and creep properties of these alloys at elevated temperatures. Commercial practice generally aims to produce a fine as-cast grain structure through a significant addition of Zr [10]. Bettles et al. [11], [12] have recently studied the influence of grain size (46–536 μm) on the tensile, compressive, creep and bolt load retention (BLR) properties of a similar Mg alloy (AM-SC1) in the T6 state up to 177 °C (similar to ML10 or ZM6, AM-SC1 typically contains 1.7% Nd, 1.0% other RE elements, 0.5% Zn, 1.0% Zr and balance Mg [12]). The yield strength of the alloy showed a clear Hall–Petch relationship at 177 °C with grain size in the range of 235–536 μm and the steady-state creep rate peaked at the grain size of 235 μm [12]. However, grain refinement below 200 μm did not show any significant effect on the properties of the AM-SC1 alloy at either 150 °C or 177 °C [12]. These results indicate that the influence of the grain size on the mechanical behavior of Mg–RE based alloys is not straightforward.

The purposes of this study are threefold: to assess the effect of grain size on the tensile and creep properties of Mg–2.65Nd–0.30Zn–xZr alloys at 250 °C; to identify the maximum T6 grain size, rather than the as-cast grain size, that can still meet the design performance criteria of the alloy at 250 °C; and to evaluate the effect of the solid solution treatment on grain growth and morphology. The motivation for the last two purposes is that commercial practice has long focused on achieving a fine equiaxed as-cast grain structure using excessive Zr. It is therefore of practical importance to clarify both issues.