The beta(1) Triad-Related Configurations in a Mg-RE Alloy

YM Zhu and H Zhang and SW Xu and JF Nie, METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, 51, 1887-1896 (2020).

DOI: 10.1007/s11661-020-05651-1

As the building block of a honeycomb precipitate network, the triadic arrangement of beta(1) strengthening precipitates is technologically important for the potential development of highly tuneable nanostructure in rare-earth (RE) containing Mg alloys. In this work, we provide systematic experimental observations of those impinged beta(1) variants in a Mg-Nd alloy, which are directly related to the characteristic triadic configuration. It is found that the isolated alpha-Mg crystal in a beta(1) triad has a 10.5 deg rotation when it has a perfect equilateral-triangular shape. This rotation angle decreases to similar to 9.4 deg when the isolated alpha-Mg crystal exhibits a non-equilateral shape. In this case, one or two of its lateral interfaces include a step with a height of 0.56 nm to reduce the lattice mismatch. Furthermore, the pre-stage prior to the formation of beta(1) triadic configuration is revealed for the first time. It originates from the implement of two beta(1) variants with the same sense of shear. The presence of such two variants, in the absence of the third variant, leads to a rotation of the alpha-Mg crystal in the small area close to the approaching ends of these two variants. A low-angle symmetrical tilt boundary around the 0001(alpha) rotation axis is formed between the rotated and unrotated alpha-Mg crystals. Most of the tilt boundaries observed in this work have a tilt angle of similar to 4.7 deg. The atomic structure of such a tilt boundary is constructed using a crystallographic model and validated by molecular dynamics simulation. The unique distribution of Nd-rich solute clusters along the tilt boundary is qualitatively discussed based on crystallographic analysis.

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