Papers by Author: Yoshiyuki Saito

Abstract: Numerical simulations of phase separation in Fe-Cr-Mo―Ni or Fe-Cr-MoーTi quaternary alloys similar to ferrite phases in dplex stainless steeswere performed by the Cahn-Hilliard equation. We obtained that the asymptotic behaviour of minor element Ni, Mo, Ti in an Fe-Cr-X quaternary alloy along a trajectory of a peak top of the major element Cr is classified into three groups according to the sign of the second derivative of the chemical free energy with respect to the compositions of Cr and X(X=Mo, Ni or Ti]. It is also predicted that that small addition of Mo upto 10% accelerates phase separation of Cr. It seems that the optimum value of Mo exists to enhance the phase separation of Cr in Fe-Cr-Mo ternary alloys. Simulation result indicates that the phase separation of Cr is most enhanced with addition of 1% Mo. The above mentioned simulation results given by the numerical simulation by the Cahn-Hilliard equation were in good agreement with those obtained by the Monte Carlo simulation Theoretical analyses were performed in order to discuss the simulation results. On the basis of theb simulation results Optimum materials design of the duplex stainless steel has been established.

Abstract: Numerical simulation of phase separation in Fe-Cr-Mo and Fe-Cr-Ni ternary alloys and Fe-Cr-Mo-Ni quaternary alloys were performed with use of the Cahn-Hilliard equation for ternary and quaternary alloys. A new numerical model based on the Gauss-Seidel and Newton Raphson methods was utilized to obtain efficient and accurate solution.

Abstract: Numerical simulations of phase separation in Fe-Cr-Mo and Fe-Cr-Ti ternary alloys and Fe-Cr-Mo-Ti quaternary alloys were performed with use of the Cahn-Hilliard equation for ternary alloys and quaternary alloys. We obtained that the asymptotic behaviours of minor element X(Mo, Ti) in Fe-Cr-X ternary alloys and Fe-Cr-Mo-Ti quaternary alloys along a trajectory of a peak top of the major element Cr was classified into three groups according to the sign of the second derivative of the chemical free energy with respect to the compositions of Cr and X(Mo, Ti). Theoretical analysis for the symptotic behavior Mo in Fe-Cr-Mo ternary has been formed in order to discuss the simulation results. Effect of the other elements, such as Ti on separation behaviours of Cr in Fe-Cr-Ti ternary alloys were also investigated. A simple theory for describing the effect of subsutitutonal element in Fe-Cr-X ternary alloys on the basis of simulations and theoretical analyses was proposed.

Abstract: Grain boundary migration in materials under severe plastic deformation was simulated by the phase field methods. The interface energy and dislocation density on growth kinetics were simulated on systems of 2-dimensional lattice. .In inhomogeneous systems grain size distributions in simulated grain structures were binodal distributions. The classification of the solution of differential equations based on the mean-field Hillert model describing temporal evolution of the scaled grain size distribution function was in good agreement with those given by the Computer simulations. Effect of dislocation on thermodynamic stability was taken into consideration. Dislocation density distribution was calculated by a equation based on the diffusion-reaction equation.. Scaled grain size distribution was known to be affected by the dislocation.

Abstract: Thermodynamic stability of Grain boundary in materials under severe plastic deformation was simulated by the Monte Carlo and the phase field methods. Computer simulations were performed on 3-dimensional textured materials. The Monte Carlo simulation results were qualitatively in good agreement with those by the phase field model. The classification of the solution of differential equations based on the mean-field Hillert model describing temporal evolution of the scaled grain size distribution function was in good agreement with those given by the Computer simulations. The ARB experiments were performed for pure Al and Al alloys-sheets in order to validate the computer simulation results concerning the grain boundary stability of textured materials. With use of the Monte Carlo and the phase field methods. Effect of grain boundary mobilises and interface energy given by the computer simulations.

Abstract: The effects of second-phase particles on the recrystallization kinetics in two-dimensional polycrystalline structures were investigated. Numerical simulations of recrystallization were performed by coupling the unified subgrain growth theory with a phase-field methodology. Simple assumptions based on experimental observations were utilized for preparing initial microstructures. The following results were obtained: (1) The presence of second-phase particles retarded recrystallization speeds. (2) If the mean subgrain size was small enough recrystallized region covered whole system for various values of the particle fraction, f. (3) On the other hand, if the mean subgrain size was not small enough the progress of recrystallization was frozen at some point.

Abstract: The kinetics and topology of grain growth in three dimensions were simulated using a phase-field model with anisotropic grain-boundary mobilities. In order to perform large scale calculations we applied both modifications of algorithms and parallel coding techniques to the Fan and Chen's phase-field algorithm. Kinetics of abnormal grain growth is presented. It is observed that the grains of a minor component which are at the beginning surrounded preferentially by boundaries of high mobility grow faster than the grains of a major component until the texture reverses completely. Additionally, topological results of grain structures, such as grain size distributions and grain face distributions, are discussed