Akinori Yamanaka

Peta-scale phase-field simulation for dendritic solidification on the TSUBAME 2.0 supercomputer

The mechanical properties of metal materials largely depend on their intrinsic internal microstructures. To develop engineering materials with the expected properties, predicting patterns in solidified metals would be indispensable. The phase-field simulation is the most powerful method known to simulate the micro-scale dendritic growth during solidification in a binary alloy. To evaluate the realistic description of solidification, however, phase-field simulation requires computing a large number of complex nonlinear terms over a fine-grained grid. Due to such heavy computational demand, previous work on simulating three-dimensional solidification with phase-field methods was successful only in describing simple shapes. Our new simulation techniques achieved scales unprecedentedly large, sufficient for handling complex dendritic structures required in material science. Our simulations on the GPU-rich TSUBAME 2.0 super- computer at the Tokyo Institute of Technology have demonstrated good weak scaling and achieved 1.017 PFlops in single precision for our largest configuration, using 4,000 CPUs along with 16,000 CPU cores.

Rapid fabrication of an ordered nano-dot array by the combination of nano-plastic forming and annealing methods

In this paper, a new fabrication method for an ordered nano-dot array is developed. Combination of coating, nano-plastic forming and annealing processes is studied to produce uniformly sized and ordered gold nano-dot array on a quartz glass substrate. The experimental results reveal that patterning of a groove grid on the gold-coated substrate with NPF is effective to obtain the ordered gold nano-dot array. In the proposed fabrication process, the size of the gold nano-dot can be controlled by adjusting the groove grid size. A minimum gold nano-dot array fabricated on a quartz-glass substrate was 93 nm in diameter and 100 nm in pitch. Furthermore, the mechanism of nano-dot array generation by the presented process is investigated. Using a theoretical model it is revealed that the proposed method is capable of fabrication of smaller nano-dots than 10 nm by controlling process conditions adequately.

Fabrication of three-dimensional ordered nanodot array structures by a thermal dewetting method

A new fabrication method for three-dimensional nanodot arrays with low cost and high throughput is developed in this paper. In this process, firstly a 2D nanodot array is fabricated by combination of top-down and bottom-up approaches. A nanoplastic forming technique is utilized as the top-down approach to fabricate a groove grid pattern on an Au layer deposited on a substrate, and self-organization by thermal dewetting is employed as the bottom-up approach. On the first-layer nanodot array, SiO(2) is deposited as a spacer layer. Au is then deposited on the spacer layer and thermal dewetting is conducted to fabricate a second-layer nanodot array. The effective parameters influencing dot formation on the second layer, including Au layer thickness and SiO(2) layer thickness, are studied. It is demonstrated that a 3D nanodot array of good vertical alignment is obtained by repeating the SiO(2) deposition, Au deposition and thermal dewetting. The mechanism of the dot agglomeration process is studied based on geometrical models. The effects of the spacer layer thickness and Au layer thickness on the morphology and alignment of the second-layer dots are discussed.

Phase-Field Modeling for Dynamic Recrystallization

Hot working is a process in which metallic materials are worked at the elevated temperatures above the recrystallization temperature. During the hot working of the low-to-medium stacking fault energy metals, the dynamic recrystallization (DRX) occurs. The mechanical properties of the DRX materials during the hot working are largely affected by the nucleation and growth of the dynamic recrystallized grains. In this article, the application of a phase-field method, which has emerged as a powerful numerical tool to simulate the material microstructure evolutions, to the simulations of the deformation and microstructure during the DRX is reviewed. First, the multi-phase-field dynamic recrystallization (MPF-DRX) model, which can simulate the mechanical behaviors of a computational domain based on the DRX microstructure evolutions simulated by the multi-phase-field (MPF) method, is introduced. Next, a hot-working multi-scale model, where the macro deformation is simulated by the finite element (FE) method and the microstructure evolution is simulated by the MPF-DRX method, is presented.

Regularly-formed three-dimensional gold nanodot array with controllable optical properties

A new efficient and cost-effective method to fabricate a three-dimensional ordered nanodot array is developed. Using this method, firstly, a two-dimensional nanodot array is fabricated by a combination of nano-plastic forming patterning of an Au layer coated on the substrate and thermal dewetting-induced self-organization. Then, a multilayer ordered Au nanodot array is fabricated by repeating SiO2?deposition and Au deposition, followed by thermal dewetting. The extinction spectra of the fabricated ordered Au nanodot arrays show strong localized surface plasmon resonance. It is found that the extinction spectra of a single-layer nanodot array can be tuned by controlling fabrication conditions such as the thermal dewetting time. It is demonstrated that the optical properties of double-layer nanodot arrays can be tuned by controlling the spacer thickness and pitch setting. It is also found that the intensity of the extinction spectrum increases linearly with an increase in the number of layers in multilayer nanodot structures. This enhanced extinction gives rise to great potential for use in biosensing devices.

Phase-Field Simulation during Spherulite Formation of Polymer

The establishment of the coupled numerical model which enable to simulate the spherulite formation and its mechanical behavior continuously is our final goal. In this paper, we have developed Phase-field model for spherulte growth of polymer by generalizing the model proposed by Granasy et. al.. The numerical simulations for single spherulite and multi-sperulites have been performed with isotropic interface energy.

Microstructure-Based Multiscale Analysis of Hot Rolling of Duplex Stainless Steel Using Various Simulation Software

We proposed a microstructure-based multiscale simulation framework using various commercial simulation software and applied it to analyze the hot rolling of duplex stainless steel. According to the Integrated Computational Material Engineering (ICME) concept, we established a procedure to bridge various simulation software from nano- to macroscopic length scales. Using our framework, first, microstructure evolutions by multiphase field (MPF) simulations coupled with the Calculation of Phase Diagrams (CALPHAD) database were performed. In our application, we simulated the columnar and equiaxed solidification during the continuous casting of duplex stainless steel. In the MPF simulations, the temperature field in the slab was calculated by heat conduction analysis using a finite element method (FEM). Then, the macroscopic elastic and plastic mechanical properties of the microstructure obtained by the MPF simulations were estimated by the virtual material test using a nonlinear FEM based on the homogenization method. Because the elastic constants of single δ-ferrite and γ phases in the microstructure are necessary for the virtual material test, they were calculated by molecular dynamics and first principle calculations. Furthermore, the plastic stress–strain properties of the single phases were estimated on the basis of the results of nano-indentation and uniaxial tensile tests. Subsequently, the hot rolling of the slab was simulated using an elastoplastic FEM with the mechanical properties obtained by the virtual material test. Finally, the static recrystallization in the rolled slab was again simulated by the MPF method.

High Throughput Method to Fabricate Ordered Nano Dot Array on Various Plastic Films

In this paper, we propose a high throughput method to fabricate ordered metal nano dot array on a plastic film by combination of patterning by nano plastic forming, coating, annealing, and transferring to a plastic film. The effects of process parameters such as indentation load, annealing temperature on the formation of gold nano dot array and dot transfer ratio to a PDMS film are investigated. The results show that an ordered gold nano dot array is successfully formed on the pre-patterned substrate. The transfer of an ordered gold nano dot array to PDMS film is demonstrated.

Phase-Field Modeling and Simulation of Nucleation and Growth of Recrystallized Grains

The novel coupling recrystallization model is proposed in this study. First, the deformation microstructure was simulated by the finite element method based on the strain gradient crystal plasticity theory. The calculated dislocation density and crystal orientation were transferred to the recrystallization phase-field simulation. The initial subgrain structures used in phase-field simulation were determined by a relationship between dislocation density and subgrain size with the dislocation density distribution calculated by crystal plasticity simulation. The so-called KWC phase-field model, which can introduce both subgrain rotation and grain boundary migration, was employed, and spontaneous nucleation and grain growth were simulated simultaneously.

Numerical biaxial tensile test for sheet metal forming simulation of aluminium alloy sheets based on the homogenized crystal plasticity finite element method

The simulation of the stretch forming of A5182-O aluminum alloy sheet with a spherical punch is performed using the crystal plasticity (CP) finite element method based on the mathematical homogenization theory. In the simulation, the CP constitutive equations and their parameters calibrated by the numerical and experimental biaxial tensile tests with a cruciform specimen are used. The results demonstrate that the variation of the sheet thickness distribution simulated show a relatively good agreement with the experimental results.