Search

The potential effect of the element Ca on the precipitation behavior was investigated in an Mg-rare earth alloy. A combination of metastable β′′′ and β′ precipitates was observed for the peak aging condition at 200°C. Ca addition was found to have no significant effect on the precipitating phases and evolution sequence. Composition analysis showed that the Ca partitioned to both β′′′ and β′ precipitate phases. First-principles calculations indicated that Ca partitions to the rare-earth sublattice in the precipitate phase. This finding suggests the potential of Ca to partially replace costly rare-earth elements in precipitation-hardened Mg-rare earth alloys. Both experimental investigation and first-principal calculation in a Mg-Nd-Y-Ca alloy revealed the partitioning of non-rare-earth element Ca to Mg-RE precipitates, suggesting the potential of Ca to enhance precipitation and strength. [ABSTRACT FROM AUTHOR]

Materials for high-temperature environments are actively being investigated for deployment in aerospace and nuclear applications. This study uses computational approaches to unravel the crystallography and thermodynamics of a promising class of refractory alloys containing aluminum. Accurate first-principles calculations, cluster expansion models, and statistical mechanics techniques are employed to rigorously analyze precipitation in a prototypical senary Al–Nb–Ta–Ti–V–Zr alloy. Finite-temperature calculations reveal a strong tendency for aluminum to segregate to a single sublattice at elevated temperatures. Precipitate and matrix compositions computed with our ab-initio model are in excellent agreement with previous experimental measurements (Soni et al., 2020). Surprisingly, conventional B2-like orderings are found to be both thermodynamically and mechanically unstable in this alloy system. Complex anti-site defects are essential to forming a stable ordered precipitate. Our calculations reveal that the instability of B2 compounds can be related to a simple electron counting rule across all binary alloys formed by elements in groups 4,5, and 6. The results of this study provide viable routes toward designing high-temperature materials for deployment in extreme environments. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract Alloying Mg with rare-earth elements, such as Nd, Gd, or Y, enables hardening from the precipitation of metastable coherent phases during aging at low temperatures. While the aging potential of binary Mg-Nd alloys is relatively limited due to the nucleation of coarse β 1 and β precipitates at the expense of the strengthening β‴ precipitates, binary Mg-Y alloys exhibit exceptional stability of the strengthening coherent β S ′ phase when aged at 200 °C. Through combination of high-resolution characterization, density functional theory calculations, and finite-element elasticity studies, we demonstrate that the strengthening β S ' phase is thermodynamically stable at low temperatures (as opposed to metastable as in other Mg-RE binaries) and that misfit strains play a key role in not only controlling precipitate structure and morphology but also their unusual evolution into an interconnected network. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

High throughput first-principles calculations were employed to unravel the thermodynamic stability of two sets of hierarchical phases, β P ‴ and β S ‴ , in Mg-RE (rare-earth) alloys. The β P ‴ orderings (with compositions greater than x RE =0.125) are predicted to form in the Mg-{La,Ce,Pr,Nd,Pm,Sm} binary alloys, thus explaining experimental observations of RE atoms arranged as hexagonal rings in these alloys. Mg-{Sc,Y,Tb,Dy,Ho,Er,Tm,Lu} alloys are predicted to form β S ′ ( x RE =0.125) precipitates without hexagonal ring arrangements. The calculated misfit strains are used to infer the qualitative shapes of the precipitates and their relative aspect ratios. High aspect-ratio precipitates may be formed using alloying elements that encourage the formation of β P ‴ . The predictions in this study can be used to inform alloy design of precipitation strengthened magnesium alloys. [ABSTRACT FROM AUTHOR]

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: 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 ) . [ABSTRACT FROM AUTHOR]