Shinji Sakane

Boron (B) is an effective element for improving the hardenability of steel, and therefore comprehensive understanding of its diffusion behavior is desirable for controlling the microstructure under heat treatment. This study evaluates the volume and grain-boundary diffusion coefficients of B (DV and DGB) in bcc (α) and fcc (γ) iron (Fe) by profiling the long-range penetration distribution by means of secondary ion mass spectrometry. DV is in the range of 10-21 –10-16 m2 s-1 at 550–700 ºC in α-Fe and 10-22 –10-17 m2 s-1 at 400–550 ºC in γ-Fe. DV is close to the self-diffusion coefficient of Fe, but far smaller than the previously reported DV. It is highly likely that vacancy related mechanisms operate rather than interstitial mechanism for B, which can occupy at both octahedral interstitial and substitutional sites. DGB was obtained to be in the range of 10-23 – 10-20 m2 s-1 at 200–350 ºC by anlyzing the distribution in the nanocrystalline films. DGB is several orders of magnitude smaller than the GB self-diffusion of Fe, suggesting that the diffusion of B is structurally constrained in the GB of Fe.

Understanding the motion and growth behaviors of equiaxed dendrites during solidification is important for predicting macrosegregation. In this study, we develop a phase-field lattice Boltzmann (PF-LB) simulation method for the settling and growth of an equiaxed dendrite during the nonisothermal solidification of a binary alloy. The PF-LB computations are accelerated by employing parallel computation using multiple graphic processing units (GPUs) and the octree block-structured adaptive mesh refinement method, which incorporates multiple mesh and time increment methods. By using the developed method, we can simulate the three-dimensional long-distance settling dendrite while considering the effects of latent heat release and natural convection. From the simulation results, we confirm that the natural convection due to the high solute concentration around a dendrite reduces the settling velocity. In addition, we observe that the temperature increase owing to latent heat release slows dendrite growth, which in turn slightly slows the settling velocity. From these results, we confirm that the effects of latent heat release and natural convection are not negligible in the quantitative evaluation of settling dendrites.