Mechanical Properties of Fine-Grained Magnesium Alloys Processed by Severe Plastic Forging

Abstract

Magnesium (Mg) and its alloys have recently attracted growing interest especially in the automobile industries due to their excellent specific properties, as one of the lightest structural materials (Mordike & Ebert, 2001). To date, most of Mg products have been fabricated by casting, in particular, by die-casting because of its high productivity, suitable strength, acceptable quality and dimensional accuracy. The intrinsic poor formability and limited ductility at ambient temperature due to the hexagonal close-packed (hcp) crystal structure and the associated insufficient independent slip systems, however, greatly restricts their practical usage. Then Mg and its alloys are categorized in hard plastic materials and its products of structural light materials are not a lot fabricated by plastic working such as rolling, forging and other forming processes. During warm and hot working, on the other hand, there are several non-basal slip systems operated in addition to the basal slip plane leading to increase of the plastic workability. It is also known that fine-grained structures are frequently developed in Mg alloys after warm and hot working to relatively low strains. The plastic workability of such fine-grained alloys can be much improved accompanied by superplasticity. It has been studied recently (Humphrey & Hatherly, 2004, Valiev & Langdon, 2006) that severe plastic deformation (SPD), such as equal channel angular pressing (ECAP), high pressure torsion (HPT), accumulative roll-bonding, multi-directional forging (MDF), etc., is carried out on many metallic materials, leading to the development of ultrafine-grained (UFGed) structures in a whole volume of the products. The present authors have studied thermomechanical processing for development of UFGed Mg alloys by using MDF under decreasing temperature conditions, and succeeded much in improvement of the plastic workability as well as the mechanical properties at ambient temperature, as described in the following sections. The aim of the present chapter is to review our current studies on (1) the mechanical and microstructural behaviors as well as the grain refinement mechanism operating in Mg alloys during a single-pass compression, (2) outstanding effect of MDF on acceleration of rapid grain refinement under decreasing temperature conditions, and (3) improvement of the mechanical properties of fine-grained Mg alloys processed by MDF. The mechanical properties and characteristics of superplasticity are investigated in tension at various temperatures and strain rates, and the microstructural and textural changes taking place