On the structure and role of βF′ in β1 precipitation in Mg-Nd alloys

The crystal structure and atomic coordinates of βF′ in aged samples of a Mg-3wt.%Nd alloy are examined using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and the first-principles density functional theory (DFT). It is found that the lattice parameters and the atomic positions in the orthorhombic βF′ unit cell deviate from the atomic model that has been generally accepted. The newly proposed βF′ structure is similar to that of β1 which also forms in the alloy, and a simple atomic replacement and structural relaxation can transform the structure of βF′ to that of β1. A specific variant of the βF′ phase is often observed to be attached to the end of a β1 precipitate after ageing for longer time. The stress field analysis and interaction energy calculations indicate that this attachment can reduce the stress field around the end facet of the β1 precipitate and may influence β1 lengthening. DFT computation results suggest that the βF′ phase is more stable than β′, but less stable than β1, in the Mg-Nd system, and this finding is extendable to other Mg-RE (where RE represents rare earth elements) systems such as Mg-La, Mg-Ce, Mg-Pr, Mg-Pm, Mg-Sm and Mg-Eu alloys.

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