Mitsuhiro Itakura

Solution softening in magnesium alloys: the effect of solid solutions on the dislocation core structure and nonbasal slip

There is a pressing need to improve the ductility of magnesium alloys so that they can be applied as lightweight structural materials. In this study, a mechanism for enhancing the ductility of magnesium alloys has been pursued using the atomistic method. The generalized stacking fault (GSF) energies for basal and prismatic planes in magnesium were calculated by using density functional theory, and the effect of the GSF energy on the dislocation core structures was examined using a semidiscrete variational Peierls-Nabarro model. Yttrium was found to have an anomalous influence on the solution softening owing to a reduction in the GSF energy gradient.

Effect of atomic scale plasticity on hydrogen diffusion in iron: Quantum mechanically informed and on-the-fly kinetic Monte Carlo simulations

We present an off-lattice, on-the-fly kinetic Monte Carlo (KMC) model for simulating stress-assisted diffusion and trapping of hydrogen by crystalline defects in iron. Given an embedded atom (EAM) potential as input, energy barriers for diffusion are ascertained on the fly from the local environments of H atoms. To reduce computational cost, on-the-fly calculations are supplemented with precomputed strain-dependent energy barriers in defect-free parts of the crystal. These precomputed barriers, obtained with high-accuracy density functional theory calculations, are used to ascertain the veracity of the EAM barriers and correct them when necessary. Examples of bulk diffusion in crystals containing a screw dipole and vacancies are presented. Effective diffusivities obtained from KMC simulations are found to be in good agreement with theory. Our model provides an avenue for simulating the interaction of hydrogen with cracks, dislocations, grain boundaries, and other lattice defects, over extended time scales, albeit at atomistic length scales.

Mobile effect of hydrogen on intergranular decohesion of iron: first-principles calculations

Atomistic mechanisms of hydrogen-induced cracking along a bcc Fe Σ3(111) symmetrical tilt grain boundary (GB) have been studied by first-principles calculations. The mobile and immobile effects of hydrogen on the GB decohesion are analyzed by calculating the dependence of hydrogen segregation energy on the coverage relevant to the repulsive interaction among segregated hydrogen atoms at the GB and on its fracture surfaces, together with generalizing McLeans formula. It was found that the segregation of combined mobile and immobile hydrogen atoms from the bulk and/or GB on the fracture surfaces causes much stronger reduction (70–80%) in the GB cohesive energy. It can occur even at a very low bulk hydrogen content of about 10−9 atomic fraction during slow cracking. This is in contrast to only 10–20% decohesion induced by immobile hydrogen at much higher hydrogen content during fast cracking. The mobile effect of hydrogen, giving rise to a profound reduction in the GB cohesive energy, is a key factor controlling the mechanism of hydrogen-induced GB cracking.

Material design for magnesium alloys with high deformability

Magnesium alloys are very promising structural material, especially because of their low density, but their poor deformability at room temperature is making their commercial application impractical. Some alloying elements have been shown to improve the ductility dramatically, and a search for the yet better alloying elements is now a pressing task. In this study, we investigated the effects of several alloying elements on the mechanical and deformation behaviour of magnesium alloys using both first-principles calculations as well as experiments. The first-principles calculations indicate that the influential key factor for the activation of non-basal slips is the electronegativity and atomic radius. The experimental result proves that the alloys with a similar electronegativity as magnesium and a little larger atomic radius than that of magnesium, such as Ca, Sr and some rare earth elements, show superior ductility due to activation of non-basal dislocation slips. These results propose a promising design principle for the alloys with the improved deformability at room temperature ranges.

Mathematical modeling of radioactive contaminants in the Fukushima environment

Abstract Significant amounts of radioactive materials were released to the atmosphere from the Fukushima Daiichi nuclear power plant after the accident caused by the major earthquake and devastating tsunami on March 11, 2011. Accurate and efficient prediction of the distribution and fate of radioactive materials eventually deposited at the surface in the Fukushima area is of primary importance. In order to make such a prediction, it is important to gather information regarding the main migration pathways for radioactive materials in the environment and the time dependences of radioactive material transport over the long term. The radionuclide of most concern in the Fukushima case is radioactive cesium. Previous surveys indicate that the primary transportation mechanisms of cesium are either soil erosion and water transport of sediment-sorbed contaminants or transport of dissolved cesium in the water drainage system such as by rivers. A number of mathematical models of radioactive contaminants, with particular attention paid to radiocesium, on the land and in rivers, reservoirs, and estuaries in the Fukushima area are developed. Simulation results are examined while simultaneously implementing field investigations. For example, the orders of magnitude of the radiocesium concentration on the flood plain of the Ukedo River by model prediction and field investigation results were both 105 Bq/kg. Microscopic studies of the adsorption/desorption mechanism of cesium and soils have been performed to shed light on the mechanisms of macroscopic diffusive transport of radiocesium through soil. The maximum exchange energy between cesium and prelocated potassium in the frayed edge site was simulated to be 27 kJ/mol, which reproduces the corresponding value previously achieved by experiments. These predictions will be utilized for assessment of dose from the environmental contamination and proposed countermeasures to limit dispersion of the contaminants.

Atomic scale HAADF-STEM study of η′ and η 1 phases in peak-aged Al–Zn–Mg alloys

The microstructures of precipitates in Al–Zn–Mg alloys in peak-aged condition have been studied using scanning transmission electron microscope. The same thermo-mechanical treatment was applied in all alloys. Investigation of peak-aged samples revealed that the most commonly found phases were η′ and η1 with their respective habit planes on {111}Al and {100}Al. η′ phases under [110]Al were analyzed and compared with η′ structure models. Furthermore, a close inspection of η1 phase as the second most found precipitate revealed that it incorporates an anti-phase resembling boundary, not observed in other orientation relationships that precipitates create with Al matrix, in addition, differences in matrix-precipitate interfaces between η′/η2 and η1 phases were noticed. This paper addresses the first part to the analysis of η′ phase. Next part is extended to the analysis of the η1 phase.

Atomistic study on the cross-slip process of a screw dislocation in magnesium

The cross-slip process of a screw

Multiscale thermodynamic analysis on hydrogen-induced intergranular cracking in an alloy steel with segregated solutes

dislocation from the basal to the prismatic plane in magnesium was studied using the density functional theory and the molecular dynamics calculations. An atomistic method for calculating the total Peierls energy map has been devised to track the transition path of a dissociated and/or constricted screw