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

Abstract 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: S S S S → GP Zones ( N , V , hexagons ) → β ′ ′ ′ → β 1 ( Mg 3 Nd ) → β ( Mg 12 Nd ) → β e ( Mg 41 Nd 5 ) .