A Review on Developments in Magnesium Alloys

Abstract

In order to facilitate the understanding of the current research efforts and directions, this article first introduces the anomalous/problematic features of magnesium (Mg) and presents the recent approach of stacking fault energy (SFE)-based alloying element selection to lessen or eliminate this problem. Stacking fault energy computationsvia ab initiotechniques necessitate an understanding of the free electron density distribution around atoms in a solid solution. Therefore, the assessment of the role of atoms by also considering the possibility of short range order (SRO) formation rather than a random solid solution has been revisited. Two possible types of SRO have been indicated. The relevant electronic interactions between the host Mg and the alloying element atoms are more clearly incorporated in a generally less known model by Miedema based on atomic-level thermodynamics rather than in Hume-Rothery rules. This more successful approach has also been addressed here. An evaluation founded on these premises, introducing the relatively more recent Mg alloy systems, has been given in terms of their achievements toward healing the problematic features of Mg alloys. The spectrum of alloy systems discussed ranges from doping of Mg to dilute alloy systems and to some rich alloy systems that offer remarkable properties. Among the first category, an unorthodox addition, doping with oxygen, and its implications, has been presented. The dilute alloy systems and their compositional design based on SRO and SFE together with their potentials have been reviewed. Among the rich alloy compositions, the most interesting precipitate systems, that is, the ones involving order and intermetallic formations, long-period stacking order phases, and quasi-crystals, have been discussed. Among all the alloying elements, one that deserves particular attention, calcium, with its implications such as being economical, offering environmentally friendly Mg metallurgy, and remedial effects on the shortcomings of engineering properties, and a closely related issue of calcium oxide (CaO) addition have been scrutinized. This article also makes an attempt to point out the future directions throughout the text, whenever possible.