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Alloy solid solution strengthening of Mg alloys: Valence effect



Ab initio density functional theory (DFT) calculations were carried out to investigate alloy solid solution strengthening of hexagonal close packed (HCP) Mg. The binary solid solution Mg–X base alloys (X = Gd, Nd, Ce, Bi, In, and Sn) were examined to identify a possible valence effect on the solution strengthening in terms of electronic density of states (DOS) and electron localized function (ELF). An addition of rare earth (RE) elements Gd, Nd, and Ce in Mg results in a hybridization between the localized f electrons of RE elements and the p electrons of Mg. The hybridization is primarily responsible for the formation of covalent bonds between RE elements and Mg. This electronic structure was then validated by the calculated ELF, illustrating a distribution of localized electrons between Mg and RE atoms. This peculiar chemical bonding characteristic successfully explains the experimentally observed solid solution strengthening induced by Gd, Nd, and Ce additions. An addition of sp-elements of Bi, In, and Sn in Mg, however, leads to the formation of typical ionic bonds, which are not capable of inducing solid solution strengthening in Mg alloys. A discussion was made on a possible relationship between the chemical bonds and solid solution strengthening in Mg alloys.