A phase-field study of elastic stress effects on phase separation in ternary alloys

Most of the commercially important alloys are multicomponent, producing multiphase microstructures as a result of processing. When the coexisting phases are elastically coherent, the elastic interactions between these phases play a major role in the development of microstructures. To elucidate the key effects of elastic stress on microstructural evolution when more than two misfitting phases are present in the microstructure, we have developed a microelastic phase-field model in two dimensions to study phase separation in ternary alloy system. Numerical solutions of a set of coupled Cahn-Hilliard equations for the composition fields govern the spatiotemporal evolution of the three-phase microstructure. The model incorporates coherency strain interactions between the phases using Khachaturyan's microelasticity theory. We systematically vary the misfit strains (magnitude and sign) between the phases along with the bulk alloy composition to study their effects on the morphological development of the phases and the resulting phase separation kinetics. We also vary the ratio of interfacial energies between the phases to understand the interplay between elastic and interfacial energies on morphological evolution. The sign and degree of misfit affect strain partitioning between the phases during spinodal decomposition, thereby affecting their compositional history and morphology. Moreover, strain partitioning affects solute partitioning and alters the kinetics of coarsening of the phases. The phases associated with higher misfit strain appear coarser and exhibit wider size distribution compared to those having lower misfit. When the interfacial energies satisfy complete wetting condition, phase separation leads to development of stable core-shell morphology depending on the misfit between the core (wetted) and the shell (wetting) phases

Enhancing elevated temperature strength of copper containing aluminium alloys by forming L12 Al3Zr precipitates and nucleating θ″ precipitates on them

Strengthening by precipitation of second phase is the guiding principle for the development of a host of high strength structural alloys, in particular, aluminium alloys for transportation sector. Higher efficiency and lower emission demands use of alloys at higher operating temperatures (200 °C-250 °C) and stresses, especially in applications for engine parts. Unfortunately, most of the precipitation hardened aluminium alloys that are currently available can withstand maximum temperatures ranging from 150-200 °C. This limit is set by the onset of the rapid coarsening of the precipitates and consequent loss of mechanical properties. In this communication, we present a new approach in designing an Al-based alloy through solid state precipitation route that provides a synergistic coupling of two different types of precipitates that has enabled us to develop coarsening resistant high-temperature alloys that are stable in the temperature range of 250-300 °C with strength in excess of 260 MPa at 250 °C

A phase-field study of elastic stress effects on phase separation in ternary alloys

Most of the commercially important alloys are multicomponent, producing multiphase microstructures as a result of processing. When the coexisting phases are elastically coherent, the elastic interactions between these phases play a major role in the development of microstructures. In order to elucidate the key effects of elastic stresses on microstructural evolution when more than two misfitting phases are present in the microstructure, we have developed a microelastic phase-field model in two dimensions to investigate microstructural evolution during phase separation in a ternary alloy system with coherent elastic misfit. Numerical solutions of a set of coupled Cahn-Hilliard equations for the composition fields govern the spatiotemporal evolution of the three-phase microstructure. The model includes elastic driving forces arising due to coherency strain interactions between the phases. We systematically vary the misfit strains (magnitude and sign) between the phases along with the bulk alloy composition to study their effects on the morphological development of the phases and the resulting phase separation kinetics. We also vary the ratio of interfacial energies between the phases to understand the interplay between elastic and interfacial energies on morphological evolution. The sign and degree of misfit affect strain partitioning between the phases during spinodal decomposition, thereby affecting their compositional history and morphology. Moreover, strain partitioning affects solute partitioning and alters the kinetics of coarsening of the phases. The phases associated with higher misfit strain appear coarser and exhibit wider size distribution compared to those having lower misfit. When the interfacial energies satisfy complete wetting condition, phase separation leads to development of stable core-shell morphology depending on the misfit between the core (wetted) and the shell (wetting) phases

Phase-Field Modelling of Evolution of Compact Ordered Precipitates in Ternary Alloy Systems

Several technologically important alloys like Al-Li-Zr, Al-Li-Sc, Al-Sc-Zr, Al-Li-Sc-Zr, modified Inconel etc., exhibit compact precipitates in their microstructure. We present a phase-field model in two dimensions to study the morphological evolution of composite precipitates in ternary alloys. The model employs a modified regular solution description of the bulk free energy of the disordered matrix phase and ordered precipitates. Elastic strain energy of the three-phase system is described using Khachaturyan's microelasticity theory. The temporal evolution of the spatially dependent field variables is determined by numerically solving coupled Cahn-Hilliard and Allen-Cahn equations for composition and order parameter fields, respectively. We systematically vary the misfit strains, alloy chemistry and mobilities of the diffusing species to study their effect on the development of compact precipitates. Compact core-shell morphology destabilizes when the precipitate phases have misfit strains of opposite signs with the matrix phase although the relative interfacial energies between the phases satisfy Cahn's spontaneous wetting condition. Thus, the stability of monodisperse core-shell microstructures is determined by the interplay between the relative interfacial energies and elastic interactions between the phases. Further, our simulations show that low solute mobility within the core leads to sluggish coarsening of the compact particles

A phase field model combined with genetic algorithm for polycrystalline hafnium zirconium oxide ferroelectrics

Ferroelectric hafnium zirconium oxide (HZO) thin films show significant promise for applications in ferroelectric random-access memory, ferroelectric field-effect transistors, and ferroelectric tunneling junctions. However, there are shortcomings in understanding ferroelectric switching, which is crucial in the operation of these devices. Here a computational model based on phase field method is developed to simulate the switching behavior of polycrystalline HZO thin films. Furthermore, we introduce a novel approach to optimize the effective Landau coefficients describing the free energy of HZO by combining the phase field model with a genetic algorithm. We validate the model by accurately simulating switching curves for HZO thin films with different ferroelectric phase fractions. The simulated domain dynamics during switching also shows amazing similarity to the available experimental observations. The present work also provides fundamental insights into enhancing the ferroelectricity in HZO thin films by controlling grain morphology and crystalline texture. It can potentially be extended to improve the ferroelectric properties of other hafnia based thin films.Comment: Supplementary information is available with the main manuscrip

Enhancing elevated temperature strength of copper containing aluminum alloys by forming L1₂ Al₃Zr precipitates and nucleating θ" precipitates on them

This article presents a new approach in designing an Al-based alloy through solid state precipitation route