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On the structure and role of beta '(F) in beta(1) precipitation in Mg-Nd alloys
Journal Contribution - Journal Article
© 2017 Acta Materialia Inc. The crystal structure and atomic coordinates of β F′ 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 β F′ unit cell deviate from the atomic model that has been generally accepted. The newly proposed β F′ structure is similar to that of β 1 which also forms in the alloy, and a simple atomic replacement and structural relaxation can transform the structure of β F′ to that of β 1 . A specific variant of the β F′ phase is often observed to be attached to the end of a β 1 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 β 1 precipitate and may influence β 1 lengthening. DFT computation results suggest that the β F′ phase is more stable than β′, but less stable than β 1 , 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.
Journal: Acta Materialia
Pages: 408 - 426
Number of pages: 19