Tomohito Tsuru

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.

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.

Effect of Sn and Nb on generalized stacking fault energy surfaces in zirconium and gamma hydride habit planes

We have investigated the effects of Sn and Nb on dislocation properties in a Zr lattice to elucidate the role of these alloying elements in hydride nucleation processes. According to experimental observations, γ-hydride habit planes are close to the prismatic plane in pure Zr and close to the basal plane in Zircaloy. Dislocation loops are observed around hydride precipitates, implying they play a part in hydride formation. Our ab initio generalized stacking-fault energy calculations showed remarkable effects of Sn on unstable-stacking energy and stacking-fault energy: these parameters for basal slip were considerably reduced while those for prismatic slip were increased in the presence of Sn. These results suggest selective stabilization and enhancement of dislocation spreading in the basal plane, promoting possible elementary processes of hydride precipitation with basal habit plane, i.e. screw-dislocation spreading and edge-dislocation emission in the basal plane.

Effects of Carbon Impurity on Microstructural Evolution in Irradiated α-Iron

Abstract It is known that the presence of even a small amount of impurity in interstitial positions can, depending on temperature, have a drastic influence on the one-dimensional (1-D) motion of self-interstitial atom (SIA) loops, and thus, on the accumulation of radiation damage in materials. In this study, atomic-scale computer simulations based on a recently developed optimization technique have been performed to evaluate the binding energies of SIA loops with interstitial carbon, a vacancy-carbon (V-C) complex, and a vacancy as a function of loop size in α-iron. While weak and strong attractive interactions are found when an interstitial carbon atom and a vacancy, respectively, are located on the perimeter of an SIA loop, the interactions for both quickly weaken approaching the loop center. In contrast, for a wide range of loop sizes, significantly higher binding energies are obtained between an SIA loop and a V-C complex located within the habit plane of the loop. A cluster dynamics model was developed by taking into account the trapping effects of V-C complexes on 1-D migrating SIA loops, and preliminary calculations were performed to demonstrate the validity of the assumed trapping mechanism through a comparison of the microstructural evolution with experimental data in neutron-irradiated α-iron.

Fundamental role of Σ3(1¯11) and Σ3(1¯12) grain boundaries in elastic response and slip transfer

The techniques of grain boundary engineering are rapidly gaining significance microstructural design. To understand individual grain boundary characteristics, the influence of grain boundaries on the elastic and plastic deformation behaviors of copper bicrystals with Σ3(1¯11) twin and Σ3(1¯12) grain boundaries were investigated by large scale molecular statics simulation. These grain boundaries were chosen as examples of coherent and incoherent grain boundaries. Nanoindentation tests perpendicular to the grain boundary plane were used to investigate local deformation properties. Our results showed that an incoherent boundary experiences a reduction in elastic resistance due to the increase in excess free volume and structure-dependent local indentation modulus, while a coherent boundary has little effect on the elastic deformation. The propagation of plastic deformation is strongly blocked by the dissociation into a displacement shift complete (DSC) lattice dislocation which explains the superficial absor...

Electronic Modification of C60 Monolayers via Metal Substrates

The structural and electronic modifications of C60 monolayer on Cu(111) and Pt(111) have been studied by scanning tunneling microscopy. On Cu(111), controlled experiments demonstrated that the electronic structure of the C60 layer changes markedly with increasing extent of interaction between the C60 layer and the substrate. The most strongly interacting monolayer exhibited a metallic density of states at the Fermi energy. On Pt(111), where the interaction between the C60 layer and the substrate is stronger, the annealing of the monolayer caused the decomposition of the C60 layer, resulting in graphene formation instead of polymerization.

Nonempirical prediction of impurity segregation in α-Fe from first principles

The segregation and clustering of impurities in α-Fe were investigated by first principle density functional theory calculations. The segregation tendencies of various elements observed in reactor pressure vessels were considered and the interaction characteristics between Fe and each impurity element were estimated by mean field approximation. Stable N-atom impurity clusters were subsequently chosen to evaluate the changes in free energy for clustering. These calculations show that Cu and Mn impurities embedded in α-Fe are more stable when they are in the segregated state. Conversely, Nb and Ta are stable in the separately solute state. The present estimates provide reliable suggestions for the segregation characteristics, and the tendencies are in good agreement with the recent atom probe observations. We suggest that the segregation tendency is derived from the d-orbital interaction and that the solubility limit is not necessarily correlated with the tendency of clustering formation.

Atomistic modeling of He embrittlement at grain boundaries of α-Fe: a common feature over different grain boundaries

He atoms introduced into materials may lead them to fracture intergranularly, and understanding such an effect is a key issue in the design of future fusion reactors. In the current study, we investigated the decrease of grain boundary (GB) strength caused by He segregation at several kinds of α-Fe GBs by exploiting first principles calculations and a set of empirical potentials. We found enough evidence to support the notion that the GB cohesive energy, a critical measure of GB strength, approximately scales with the He concentration at the GB surface, regardless of the GB type.

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