(V) INVITED: Structure and Local Environment of Intermetallic Precipitate Phase Nucleus

A physics-based model was developed for the formation of intermetallic phase nucleus, whose structure, symmetry and lattice parameter differ substantially from the parent phase. Towards that end, we have examined the nucleus structure and local environment of an intermetallic β1 strengthening precipitate phase within hcp-Mg of creep-resistant Mg-Nd alloys. First principles computations were conducted, which allowed the calculation of an energy-structure parameter landscape, and determination of the minimum energy nucleus structure. DFT analysis yielded two critical insights. First, the nucleus contains only the template or imprint of the equilibrium intermetallic structure, while nominally maintaining the symmetry and stoichiometry of the equilibrium phase. Such template-mediated nucleation allows the intermetallic phase to accommodate itself within the parent phase of different symmetry. Second, the template-nucleus is stabilized by being immersed within pockets of covalent-bonded environment in the metallic parent matrix.