Grain Refinement of Magnesium Alloy by Multiaxial Alternative Forging and Hydrogenation Treatment

Abstract

Owing to their high specific strength, magnesium alloys are promising materials for use in various structural products whose weight is to be minimized. These alloys are drawing attention as materials with reduced environmental load since they save both resources and energy. However, these alloys have low strength and a small number of slip systems and are more difficult to work forming. Therefore, it is necessary to modify them, e.g., improve the mechanical properties and fatigue strength of the alloys, and reduce processing costs, which would help increase their industrial applications. To overcome the above-mentioned drawbacks, the formation of a fine-grained structure would be useful. For magnesium materials, the effect of fine-grained structure on the static mechanical properties, fatigue, and fracture toughness has been investigated to study their practical applicability(Nagata et al,2007). Such investigations are very important for increasing the use of magnesium alloys as industrial materials (Noda et al, 2009). Microstructure control has been achieved through grain refinement by employing several strain work processes such as MA (Mechanical Alloying:Ameyama et al.,1998), ECAP (Equal Channel Angular Pressing:Berbon et al.,1999), ARB (Accumulative Roll-Bonding:Tsuji et al.,1999), and MAF (Multiaxial Alternative Forging:Noda et al.,2005 and Xing et al,2005). These methods were found to enhance strain energy in materials without changing their geometrical shapes. This means that under external deformation, it is necessary to produce in materials, as much as possible, a nonhomogeneous microstructure containing such as micro bands, shear bands, and lamellar boundaries. In the early stages of deformation, even a small increase in the number of dislocation pile-up sources influences the formation of dense dislocation walls and cells. Examples of such sources are static and dynamic recrystallization sources. Magnesium alloys have high hydrogen absorbing capacity, similar titanium alloys, and readily form hydrides under suitable temperature and pressure (Yoshimura et al, 1994). Hydride precipitates of MgH2 obtained in this treatment were useful for developing high-density dislocation pile-up sources. This refinement treatment, which involves the use of hydrogen, titanium alloy, and titanium composite alloy with dispersed TiC particles, resulted in a homogeneous, fine, and equi-axial structure(Machida et al,2004). It appears that the combination of a plastic deformation process and hydrogenation treatment has considerable potential to induce a fine-grained structure in a magnesium alloy (Ishida et al, 2007).