Effect of milling and reinforcement on mechanical properties of nanostructured magnesium composite

Abstract

Recycled Mg chips were used to synthesize nanostructured Mg composite of Mg–5 wt%Al reinforced with x wt% (x = 1, 2 and 5) in-situ formed AlN powder. Mechanical milling was employed to produce the composite powder of crystalline size 30–43 nm. The mechanically milled (MMed) powders were subjected to uniaxial pressing, sintering and hot extrusion processes to produce bulk solid samples. After sintering at 400 °C and hot extrusion at 350 °C, the crystalline size of the composite samples still remains in nanometer range from 52 to 84 nm. The effect of milling and the percentage of reinforced AlN on the mechanical properties such as tensile strength and ductility were discussed with the general explanation of deformation mechanisms involved.

Introduction

The introduction of legislation limiting emission and the need to reduce fuel consumption triggered the renewed interest in Mg especially in the aviation, automotive and consumer electronics industries (Mordike and Ebert, 2001) because Mg has the lowest density (1.74 g/cm³) of all the engineering metals with a density only slightly higher than that of plastics. Furthermore, Mg has excellent machineability, good recyclability and surface finish (Wolf et al., 2004).

However, pure Mg has low elastic modulus, limited strength and creep resistance at elevated temperatures and also poor ductility due to its hexagonal crystal structure. Thus, Mg is usually alloyed with other metals or used as composites (Fritze et al., 1998). It was found that Mg alloy with 3 wt% Al provides the optimal ductility and alloying with 6 wt% Al produces the optimum combination of strength and ductility (Avedesian and Baker, 1999). Enhanced mechanical properties have been reported in Mg composite reinforced with nanoscaled ceramic particles such as SiC (Ferkel and Mordike, 2001), Al₂O₃ (Hassan and Gupta, 2005) and AlN (Maung et al., 2006). The demand for greater performance of the Mg alloys or composites leads to the development of Mg-based metal matrix composite (MMC) with nanocrystalline microstructure reinforced by in-situ ceramics (Tjong and Ma, 2000). Composites not only have the combined properties of their constituents, it can also be tailored to offer improved properties to meet different engineering requirements.

With the increasing demand and usage of Mg, it is of great importance to recycle Mg scraps. Mg chips and scraps have been recycled by consolidation followed by hot extrusion (Hu et al., 2008, Watanabe et al., 2001). It was found that mechanical milling (MM) can provide an alternative cheaper way of recycling the Mg scraps as the nanocrystalline powders can be produced directly from the solid state without going through the melting process which is very expensive and environmentally harmful. MM is one of the most impressive methods for reducing grain size and producing nanocrystalline Mg alloys (Benjamin and Volin, 1974, Suryanarayana, 2001). It is also well documented that the mechanical properties of Mg alloys, especially yield strength and ductility, strongly depend on the grain size due to the large Taylor factor of Mg (Kubota et al., 1999).

In this study, Mg nano-composites with x wt% (x = 1, 2 and 5) of in-situ AlN reinforcement are synthesized at milling durations up to 40 h and their mechanical properties are accessed. The contribution of texture developed during extrusion to the tensile deformation was examined by means of pole figures measurement.