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 beta'-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 beta' ordering with the hexagons of (beta '' (D0(19)) along the [010](beta') direction. We label this hierarchy of orderings as beta '''. A high degree of degeneracy among the different beta ''' 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 Mg-3 Nd having D0(19) ordering. Rather, the features reported as D0(19) 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) -> beta ''' -> beta(1) (Mg3Nd) -> beta(Mg12Nd) -> beta(e)(Mg-41 Nd-5). (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.