Interfacial structures and energetics of the strengthening precipitate phase in creep-resistant Mg-Nd-based alloys

The extraordinary creep-resistance of Mg-Nd-based alloys can be correlated to the formation of nanoscale-platelets of beta(1)-Mg3Nd precipitates, that grow along < 11 (2) over bar0 >(Mg) in bulk hcp-Mg and on dislocation lines. The growth kinetics of beta(1) is sluggish even at high temperatures, and presumably occurs via vacancy migration. However, the rationale for the high-temperature stability of precipitate-matrix interfaces and observed growth direction is unknown, and may likely be related to the interfacial structure and excess energy. Therefore, we study two interfaces-{112}(beta 1)/{(1) over bar 100}(Mg) and {111}(beta 1)/{11 (2) over bar0}(Mg)-that are commensurate with beta(1)/hcp-Mg orientation relationship via first principles calculations. We find that beta(1) acquires plate-like morphology to reduce small lattice strain via the formation of energetically favorable {112}(beta 1)/{(1) over bar 100}(Mg) interfaces, and predict that beta(1) grows along < 11<(2)over bar0 >(Mg) on dislocation lines due to the migration of metastable {111}(beta 1)/{11<(2)over bar>0}(Mg). Furthermore, electronic charge distribution of the two interfaces studied here indicated that interfacial-energy of coherent precipitates is sensitive to the population of distorted lattice sites, and their spatial extent in the vicinity of interfaces. Our results have implications for alloy design as they suggest that formation of beta(1)-like precipitates in the hcp-Mg matrix will require well-bonded coherent interface along precipitate broad-faces, while simultaneously destabilizing other interfaces.