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 β-MgNd precipitates, that grow along 〈110〉 in bulk hcp-Mg and on dislocation lines. The growth kinetics of β 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}/{100} and {111}/{110}- that are commensurate with β/hcp-Mg orientation relationship via first principles calculations. We find that β acquires plate-like morphology to reduce small lattice strain via the formation of energetically favorable {112}/{100} interfaces, and predict that β grows along 〈110〉 on dislocation lines due to the migration of metastable {111}/{110}. 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 β-like precipitates in the hcp-Mg matrix will require well-bonded coherent interface along precipitate broad-faces, while simultaneously destabilizing other interfaces.