No abstract

Grain refining is one of the most important issues in the applications of Mg alloys, which directly determines mechanical properties and deformability. Therefore the understanding of grain refining mechanism during solidification will be benefit to develop new grain refiners. Herein refining role was elucidated by the first principles calculations based on adsorption behavior of a Mg atom on the closest-packed planes of grain refiners (Zr (001), Al2Y(311) and Al4C3(102)). Taking into account different sites, the site with the maximum adsorption energy value generally corresponded to the most possible location. The adsorption energy results show that the possible refining turn follows Al4C3(102)>Zr (001)>Al2Y(311). Meanwhile, the structural optimization confirmed that the Mg atom connected with two C atoms on the top of zig-zag plane of Al4C3(102), three Zr atoms at the hcp position on Zr (001), and two Y atoms and one Al atom at the bottom of zig-zag plane of Al2Y(311). The density of states revealed that the variation of d-orbital electrons of Mg atom became apparent during adsorption process. The values of Mulliken charges were 0.898 e in Al4C3(102), 0.410 e in Zr (001) and 0.245 e in Al2Y(311), respectively. This tendency agrees well with the previous experimental results. It indicates the adsorption energy on the closest-packed planes can be regarded as a prerequisite to select new grain refiners for Mg alloys in future.

Mg-0.5Mn-1Nd (wt.%) alloy has been prepared by zone purifying method followed by backward extrusion (BE) technique. The microstructures, recrystallization behavior from 150 to 400 °C and mechanical properties of Mg-0.5Mn-1Nd alloy were investigated. The alloy exhibited bimodal microstructures after BE deformation. The equiaxed grains changed to more homogeneous after annealing treatment in range of 150 to 250 °C. The similar trend was confirmed after thermal annealing treatment at different annealing temperatures. The abnormal grains growth was clearly observed when the temperature was higher than 350 °C. Compared with BE-Mg-0.5Mn-1Nd alloy, both yield strength and ultimate tensile strength were reduced after annealing treatment, whereas the elongation was increased correspondingly. These results were mostly related to the fine grain size and homogeneous secondary precipitate.