On the early stages of precipitation in dilute Mg–Nd alloys

We report on a first-principles and experimental study of precipitation in supersaturated solid solutions of Mg-rich Mg–Nd alloys. A cluster expansion Hamiltonian combined with Monte Carlo simulations was used to calculate a metastable HCP temperature-composition phase diagram. Aging studies were performed on dilute Mg–Nd alloys that were then characterized with HAADF-STEM. The early stage decomposition of these alloys is dominated by the thermodynamic stability of different Nd orderings on the HCP crystal structure. The observed progression of Guinier-Preston zones can be rationalized by a successive reduction of the Nd chemical potential. HAADF-STEM studies confirm the appearance of β' -like precipitates, however, the observed Nd ordering does not exclusively consist of zig-zag Nd rows but also contains linear strips of Nd-hexagons at non-periodic intervals. First-principles calculations predict that these orderings are stable compounds that belong to a family of hybrid phases formed by combining the β' ordering with the hexagons of β″ (D019) along the [010]β' direction. We label this hierarchy of orderings as β′′′. A high degree of degeneracy among the different β′′′ phases is predicted. This along with a strong composition dependence of the misfit strain is likely responsible for the observed disorder of Nd-hexagon strips. Our experimental and first-principles evidence does not support the formation of Mg3 Nd having D019 ordering. Rather, the features reported as D019 are isolated hexagonal GP rods that show no long-range periodicity. Our study suggests the following precipitation sequence for binary Mg–Nd alloys: SSSS→GP Zones(N,V,hexagons)→β′′′→β1(Mg3Nd)→β(Mg12Nd)→βe(Mg41Nd5).