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 ′′ (D0 19 ) 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 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:Mg 41 Nd 5 ) (Emmanuelle A. Marquis), avdv@engineering.ucsb.edu (Anton Van der Ven) tion sequence during aging of Mg-Nd alloys, however, remains controversial, having been the topic of a number of experimental[22-28] and computational[29-33] studies. A recent review reports the following aging sequence[2]:During typical aging experiments, supersaturated solid solutions (SSSS) containing small amounts of Nd (of the order of 0.5 at. %) are solution treated around 800K and are then aged around 500K. Guinier-Preston zones (GP zones) containing Nd atoms arranged as rods along the HCP caxis form first in the early stages of precipitation [22,24]. These rods exhibit local orderings that resemble N's, V's, Λ's and hexagons when viewed along the ⟨0001⟩ direction (figs. 1a to 1c). At early times they appear as isolated columns (figs. 1a to 1c) but later self-organize into long chains along the ⟨1010⟩ direction (figs. 1d and 1e). All orderings proposed in past studies are illustrated as 2dimensional projections along the ⟨0001⟩ axis in fig. 1. The red circles in fig. 1 represent columns of Nd atoms along the c-axis of the HCP crystal. Two patterns have been proposed for the chains consisting of V's and Λ's,

The Center for Predictive Integrated Structural Materials Science (PRISMS Center) is creating a unique framework for accelerated predictive materials science and rapid insertion of the latest scientific knowledge into next-generation ICME tools. There are three key elements of this framework. The first is a suite of high-performance, open-source integrated multi-scale computational tools for predicting microstructural evolution and mechanical behavior of structural metals. Specific modules include statistical mechanics, phase field, crystal plasticity simulation and real-space DFT codes. The second is the Materials Commons, a collaboration platform and information repository for the materials community. The third element of the PRISMS framework is a set of integrated scientific ''Use Cases'' in which these computational methods are linked with experiments to demonstrate the ability for improving our predictive understanding of magnesium alloys, in particular, the influence of microstructure on monotonic and cyclic mechanical behavior. This paper reviews progress toward these goals and future plans.

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