Materials Science Forum Vols. 475-479

Abstract: As a part of a study on modeling the microstructural evolution during the welding process, a prediction model of TTT diagram for bainite transformation was studied. This model consisted of a thermodynamic model for the bainite-start(Bs) temperature and a kinetic model for the bainite transformation. A kinetic model was empirically established for low alloy carbon steels, based on Johnson-Mehl-Avrami(JMA) equation. Reaction constants seemed not to have noticeable tendencies for temperatures and were averaged for each alloy. It was, however, found that the mean reaction constant significantly affected the reproducibility for the isothermal kinetics. Therefore, a calibration method to the kinetic parameter was proposed. From calibrations, rate constants were formulated as a function of alloying element and temperature. And TTT diagrams were calculated and compared with experiments.

Abstract: In this paper, the spin configuration and the configurational anisotropy of the clusters were studied by Monte Carlo simulations. An easy magnetization directions and a configurational anisotropy were found for the clusters using the hysteresis and spin configurations in different magnetization processes, and the simulated natural angle and energy distribution. It is considered that the configurational anisotropy is mainly attributed to the non-uniformity in spin configurations.

Abstract: A ferromagnetic monolayer with competing long-range dipolar interaction, short-range interaction and magnetic uniaxial anisotropy is studied using Monte Carlo simulation. Striped domain patterns are found and reveal the microstructure of the magnetization transition at the ground state via the competing between dipolar interaction and uniaxial anisotropy. We present the finite temperature phase diagram and find a temperature-induced magnetization reorientation from out-of-plane to in-plane phase. The influence of the various interactions on the structure of the striped phase and reorientation behavior has been discussed.

Abstract: There are many empirical equations for predicting martensite start temperature (Ms) and the kinetics models of martensitic transformation of plain carbon and low alloy steels. The Ms temperature equations are only dependent upon the chemistry, while the martensite transformation kinetics models are based on the degree of undercooling below Ms temperature. However, the prior austenite grain size (AGS) is also expected to influence both Ms temperature and martensite transformation kinetics as it does in diffusive transformations. In this study, herefore, both Ms temperature and martensite transformation kinetics of a low alloy steel with different austenite grain sizes were investigated using a dilatometer. The new Ms equation and martensite transformation kinetics model including the AGS effect are proposed.

Abstract: Potts model was often used to simulate grain growth without necessary evaluation of the effect of lattice scale and simulation temperature. It is found in this paper that such parameters may affect the simulation results markedly. The results show that simulations at zero temperature or on a small scale lattice (say, the number of sites on one edge of the square lattice L=1000) cannot reach the steady-state period of grain growth, while large-scale simulations (say, L=2000) at a much higher simulation temperature can. The steady-state grain size distribution so obtained may be well described by Weibull function other than log-normal or Rayleigh functions.

Abstract: Isothermal compression tests were conducted at Thermecmaster-Z simulator, and grain size of the prior a phase was measured at a Leica LABOR-LUX12MFS/ST microscope for quantitative metallography. A methodology to establish a constitutive equation with grain size was proposed with the help of the experimental results. Combining FEM and the present constitutive equation at high temperature deformation, grain size of the prior a phase was simulated during the isothermal forging of a TC6 titanium alloy disc. The present results illustrate grain size and distribution of the prior a phase in the forging process of TC6 titanium alloy disc in detail. The maximum difference between the calculated results and the experimental is not more than 15%.

Abstract: A phase field model for step dynamics on vicinal surface is presented. Using this model, time dependent, collective motions of steps were investigated. Through numerical simulations, morphological step instabilities induced by ES barrier were analyzed, and it is shown that this model could interpret various phenomena during step flow growth such as step bunching and meandering.

Abstract: By using the real information of the intended bonding surface and considering the effect of the diffusion distance in a definite time, a new theoretical model for diffusion bonding was proposed. The model, reflecting the characteristic of real bonding process effectively, realizes the visualization and simulation of the bonding process in which many voids disappear dynamically. The results show that the simulation is close to those experimental results from the phenomena of voids closure.

Abstract: We have investigated the ground states of the fcc alloy films in the (001) direction using the Monte-Carlo simulation. The surface field is introduced to describe the surface effects such as surface segregation. The variation of the ground states as a function of the film layer number is determined. The ground states for the films with even layer numbers and odd layer numbers show different variation tendency as the layer number increases. The phase diagrams of the ground states are obtained.

Abstract: A phase field model for alloy solidification was developed to suppress the anomalous interface effects such as enhanced surface diffusion, chemical potential jump and surface stretching by localizing the solute redistribution into a narrow region within the phase-field interface. Application of this model to a free dendritic growth in an undercooled liquid yields quantitatively the same results as previously reported anti-trapping model. By localization of the solute redistribution into a region of single grid spacing the anomalous interfacial effects can be effectively suppressed. This model can be used for quantitative phase field calculation with an enhanced computational efficiency.