arXiv subjects

Ryoji Sahara

Publications and source records attributed to Ryoji Sahara.

4 recordsLinked to original sources

arXiv2021

Order-Disorder Competitive Cooperation in Equiatomic 3d-Transition-Metal Quaternary Alloys: Phase Stability and Electronic Structure

We use high-throughput first-principles sampling to investigate competitive factors that determine the crystal structure of high-entropy alloys (HEAs) and the energetics dependence of the stable phase on the atomic configuration of fully ordered L1$_2$, D0$_{22}$, and random solid solution (RSS) phases of equiatomic quaternary alloys comprising four of the six constituent elements (Cr, Mn, Fe, Co, Ni, and Cu). Considering the configurational entropy, we demonstrate that valence electron concentration (VEC) and temperature are crucial to determine the phase stability of HEAs at finite temperatures, wherein the ordered phases are energetically more favorable than RSS phases. Some D0$_{22}$ phases with high VEC are energetically more stable than L1$_2$ phases, though both phases are metastable. Further, we explore magnetic configurations to identify the origin of the enthalpy term. The calculations reveal that ordered phases comprising antiferromagnetic atoms surrounded by ferromagnetic atoms are energetically stable. The quantitative structure--property relationship is also discussed.

cond-mat.mtrl-sci↗

arXiv2020

Data analytics approach to predict the hardness of the copper matrix composites

Copper matrix composite materials have exhibited a high potential in applications where excellent conductivity and mechanical properties are required. In this study, the machine learning models have been applied to predict the hardness of the copper matrix composite materials produced via powder metallurgy technique. Two particular composites were considered in this work. From experiments, we extracted the independent variables (features) like the milling time (MT, Hours), dislocation density (DD, 1/m2 ), average particle size (PS,nm), density (gm/cm3 ) and yield stress (MPa) while the Vickers Hardness (MPa) was used as the dependent variable. Feature selection was performed by calculation the Pearson correlation coefficient (PCC) between the independent and dependent variables. We employed six different machine learning regression models to predict the hardness for the two matrix composites.

physics.app-ph↗

arXiv2017

First principles study of oxidation of Si segregated α-Ti(0001) surfaces

The oxidation of α-Ti(0001) surface was studied using density functional theory. In order to enhance the oxidation resistance, we substituted Ti atoms with Si atoms in Ti(0001) surface. We observe that Si prefers to segregate at the surface layer of Ti(0001) compared to other sub-surface layers. The Si solubility on the Ti(0001) surface was estimated to be around 75 at.%. While Si segregation was found to reduce the binding between the oxygen atom and the Ti(0001) surface, the barrier for oxygen to diffuse into the slab increases. The dissociation of the oxygen molecule on the clean and Si segregated surfaces of Ti was found to be barrier-less. Overall, the Si segregation on the Ti(0001) surface was found to hinder the dissolution of oxygen in Ti.

cond-mat.mtrl-sci↗

arXiv2008

First-principles study of hydrogen storage over Ni and Rh doped BN sheets

Absorption of hydrogen molecules on Nickel and Rhodium doped hexagonal boron nitride(BN) sheet is investigated by using the first principle method. The most stable site for the Ni atom was the on top side of nitrogen atom, while Rh atoms deservers a hollow site over the hexagonal BN sheet. The first hydrogen molecule was absorbed dissociatively over Rh atom, and molecularly on Ni doped BN sheet. Both Ni and Rh atoms are capable to absorb up to three hydrogen molecules chemically and the metal atom to BN sheet distance increases with the increase in the number of hydrogen molecules. Finally, our calculations offer explanation for the nature of bonding between the metal atom and the hydrogen molecules, which is due to the hybridization of metal d orbital with the hydrogen s orbital. These calculation results can be useful to understand the nature of interaction between the doped metal and the BN sheet, and their interaction with the hydrogen molecules.

cond-mat.stat-mech↗