Papers by Keyword: Phase Field Modellig

Abstract: In this presentation, a novel phase field grain growth model combined with a micro-elasticity effect including elastic anisotropy and inhomogeity is presented to demonstrate the effect of micro-elasticity on grain growth and texture evolution. We report on texture evolution and abnormal grain growth induced by external elastic load from the viewpoint of micro-elasticity and first demonstrate that the previous mechanism (macroscopic viewpoint) on the effect of external elastic load on grain growth does not work in strain-controlled system. In contrast to the macro-elastic descriptions, strong localization of strain energy density and inhomogeneous distribution even inside grains are observed. Moreover, elastically soft grains with a higher strain energy density grow at the expense of the elastically hard grains to reduce the total strain energy. It is observed that strong <100>//ND fiber texture was developed in poly-crystalline Cu with initial random texture by biaxial external strain while <111>//ND fiber texture evolved in biaxial external stress condition. Even, grain growth of <100>//ND textured grains is occurred as abnormal grain growth when <100>//ND textured grains are surrounded by <111>//ND fiber textured grains.

Abstract: The present contribution highlights the approach to multi-scale steel design used at the Graduate Institute of Ferrous Technology (GIFT). Multi-scale modeling combining ab-initio methods, molecular dynamics, crystal plasticity modeling etc. enables GIFT researchers to gain a better fundamental understanding of phase and lattice stability, magnetic properties and basic mechanical constants. In addition, these methods allow for the reliable determination of critical material parameters. The opportunities for the development of new steel grade is thereby greatly enhanced and, when these new materials-oriented methods are combined with the more traditional engineering modeling methods, the challenges related to the large scale production of new steel grades can also be addressed.

Abstract: The dendrite grain growth of a succinonitrile based transparent alloy, their fragmentation under an intense thermal shock and the subsequnet morphology evolution during solidification have been simulated using a two-dimensional binary alloy phase field model coupled with heat and solute transfer. The effect of a sudden, rapid change in the thermal environment (thermal shock) was implemented in the model and the resulting effect on the incipient dendritic grain morphology was studied. Thermal shock effectively promoted the fragmentation of the dendritic grains, providing a significant grain multiplication effect to refine the final solidification microstructure.

Abstract: A new algorithm of phase field model is developed to simulate polycrystalline dendritic solidification growth in undercooled melts. The algorithm adopts a single phase field order parameter model incorporated with the anisotropy of solid-liquid interfacial energy and mobility. The model validation is performed by comparing the simulations with the theory analytical results and experimental information for both single and multi-grain dendritic growth, which demonstrates the quantitative capabilities of the proposed algorithm.

Abstract: This study presents the simulation of evolution of Ni4Ti3 variants during stress-assisted aging of NiTi alloys containing nano-scale pores with different sizes, by using phase field approach. The simulation shows that the higher level of applied stress can cause more Ni4Ti3 particles precipitated around pores than that of lower level stress, regardless of pore size; the large pores can “capture” more precipitates while less particles precipitated around the small pores. Moreover, the precipitation of Ni4Ti3 particles exhibits different regional preferences near pores, which means the unixial compressive stress can result in inhomogeneous Ni4Ti3 particle distribution.

Abstract: This paper presents a combined experimental and modelling approach to understand dendrite fragmentation of atomised metal alloy droplets during deposition in spray forming, and to study quantitatively the relationship between this dendrite fragmentation behavior and subsequent microstructural evolution. A Gleeble 3500 physical simulator was used to create controlled thermal shock conditions in solid-liquid mixtures of Ni superalloy IN718 atomised powders, which simulated the environment of droplet deposition during the twin-atomiser spray forming of large diameter IN718 alloy billets at BIAM. The experiments were complemented by phase field modelling studies at Oxford. Experiment and modelling supported the hypothesis that the characteristic equiaxed spray formed microstructure depends critically upon the rapid remelting and thermal shock of fine-scale dendrites in solid particles in the spray to provide a high density of embryonic grains.

Abstract: Phase-field simulation of phase transformation during creep in Type 304 austenitic steel is performed and simultaneous nucleation and growth of both M23C6 carbide and ferromagnetic α phases are reproduced. Nucleation events of these product phases are explicitly introduced through a probabilistic Poisson seeding process based on the classical nucleation theory. Creep dislocation energy near the carbide is integrated into the nucleation driving force for the α phase. We examine the effect of the dislocation density on precipitation of the α phase, and it is found that a small difference in the dislocation density leads to a significant change in precipitation behavior of the α phase.

Abstract: A special feature of Mg solidification is the anisotropy of the hexagonal closed packed lattice, which under directional growth conditions causes a strong crystallographic texture. Although this primary growth texture is in technical processes masked by subsequent solid state processes, its understanding can be helpful for efficient microstructure optimization. The aim of the present work is to study the fundamental orientation selection mechanisms by numerical simulation. For this pur-pose, a phase-field model has been extended to allow for complex 3D anisotropic interfacial ener-gies and interfacial mobilities, calibrated by data from molecular dynamics studies. The model is first applied in 3D to Mg-6%Al, revealing two major stages of texture formation. Directly after nuc-leation, all grains with basal plane parallel to the gradient direction are selected. During further competitive growth, grains with <1120> closely aligned to the temperature gradient commonly pre-vail, but process dependent also other orientations of the basal plane (between <1120> and <1010>) may coexist. The latter phenomenon is investigated in detail in 2D for the ternary alloy AZ31.

Abstract: We present phase-field simulations of isothermal phase transformations in the peritectic system below and above the peritectic temperature TP , and in the monotectic system below the monotectic temperature TM. We focus particularly on the Liquid-Film-Migration (LFM) mechanism, which appears to be the generic process for phase transformations above TP . Below TP , we obtain an assymetric LFM, suggesting the existence of a doublon structure in free space. In the monotectic system, the transformation from a liquid L1 to a solid+liquid L2 mixture proceeds via the migration of a L2 film, which is the analogous of the LFM process. When the metastable state consists of a liquid-liquid mixture, a dendritic-like solidification is obtained.

Abstract: A phase-field simulation is performed to examine the effect of elastic inhomogeneity between the  and ’ phases on coarsening of the ’ phase in Ni-based superalloys. In the calculation of elastic strain energy, the mechanical equilibrium equation in elastically inhomogeneous system is solved by an iterative-perturbation scheme. On the basis of the elastic constants of a practical Ni-based superalloy, a series of simulations is performed in which both elastic anisotropy and shear modulus are varied independently. The variation of elastic anisotropy gives significant effect on both morphology and size distribution function of the ’ particles, whereas the variation of shear modulus gives little effect on them. Furthermore, it is found that the coarsening rate constant of the cubic growth raw changes and increases with increasing the standard deviation of the ’ size distribution.