Bulk Ultrafine-grained Magnesium Alloys by SPD Processing：Technique, Microstructures and Properties

Abstract

Mg alloys are very attractive as the lightest metallic structural materials and have great potential for applications in automotive, aerospace and electronic industries. However, their usages are still limited due to some undesirable properties of castings (such as strength, ductility, creep and corrosion resistance) and few fabricating methods besides conventional ingot metallurgy processing. Since grain refinement is generally believed to have beneficial effect on properties of Mg alloys, fabrication of bulk ultrafine-grained (UFG, grain size less than 1μm) alloys using the new conceptual metal forming process severe-plasticdeformation (SPD) should attract considerable attention (Azushima et al., 2008). During the last decade, SPD processing has been evolving as a rapidly progressing direction of modern materials science that is aimed at developing materials with new mechanical and functional properties for advanced applications. As well known, all Mg alloys (except for some special Li-containing alloys) exhibit a hexagonal crystal structure, leading to severe limitations in their ductility, strength and creep resistance. This inherent difficulty maybe be reasonably overcame by some special processing of SPD, such as equal-channel angular pressing (ECAP), accumulative rolling bonding (ARB), high pressure torsion (HPT), etc. Enhanced properties have been obtained after these SPD processes in various Mg alloys. For example, ZK60 Mg alloy processed by ECAP (with the grain size to ~0.8μm) is superplastic at a testing temperature of 473K with an optimum ductility of ~1310% when using an initial strain rate of 2.0×10-4s-1 (Figueiredo et al., 2006). Mg-9Li-1Zn alloy had the greater increase in tensile strength of about 41.8MPa and the least decrease in elongation of about 25% at room temperature (Chang et al., 2006). UFG ZE41A alloy after enough ECAP passes obtains higher corrosion resistance besides superior mechanical properties (about 120% higher in yield strength and 75% larger in elongation at room temperature after 32 passes) (Ma et al., 2009). SPD fabrication of bulk UFG Mg alloys is becoming one of the most actively developing areas in the field of advanced structural and functional Mg alloys. SPD-produced UFG materials are fully dense and their large geometric dimensions make them attractive for efficient practical applications. Today, SPD techniques are emerging from their domain of laboratory-scale research into commercial production of various UFG materials (Valiev et al., 2009). In this chapter, we will consider these new trends in SPD processing used to produce bulk UFG Mg alloys and highlight some key results on the development of the