Local structure and bonding environment of intermetallic [formula omitted] precipitate phase nucleus in binary Mg-Nd

Abstract

β1 is a key strengthening precipitate phase in Mg-Nd-based alloys, because it is know to improve high-temperature creep resistance. Here, using density functional theory-based first principles calculations, we have examined the structure and local environment of β1 (aβ1~7.4Å) nucleus that forms within hcp-Mg lattice (aMg~3.2Å and cMg~5.2Å). For this purpose, we developed a crystallographically-informed embedded precipitate supercell approach, which allowed us to systematically examine several possible precipitate/matrix configurations, and determine energetically favorable structures. We learn that β1 will initially form within hcp-Mg as smaller structural templates rather than the larger equilibrium structure. In essence, these templates are the “genetic imprint” of the equilibrium structure that contains only a portion of larger β1 crystal structure, while retaining the stoichiometry and nominal symmetry of β1. Such template-nucleus was stabilized by pockets of stiff covalent-bonded environment in hcp-Mg. Phonon density of states and dispersion wave spectra further corroborated such covalent character, and revealed that the predicted template-nucleus structure and its surroundings were stable at temperatures greater than 0 K. Broadly, our DFT results provide crucial insights into intermetallic phase nucleation, whose lattice parameters differ significantly from the host lattice.