The β₁ Triad-Related Configurations in a Mg-RE Alloy

As the building block of a honeycomb precipitate network, the triadic arrangement of β₁ 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 β₁ variants in a Mg-Nd alloy, which are directly related to the characteristic triadic configuration. It is found that the isolated α-Mg crystal in a β₁ triad has a 10.5 deg rotation when it has a perfect equilateral-triangular shape. This rotation angle decreases to ~ 9.4 deg when the isolated α-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 β₁ triadic configuration is revealed for the first time. It originates from the implement of two β₁ 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 α-Mg crystal in the small area close to the approaching ends of these two variants. A low-angle symmetrical tilt boundary around the [0001]_α rotation axis is formed between the rotated and unrotated α-Mg crystals. Most of the tilt boundaries observed in this work have a tilt angle of ~ 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.