Norio Nunomura

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.

μSR study of Al-0.67%Mg-0.77%Si alloys

Zero-field muon spin relaxation measurements were carried out with Al-0.67%Mg- 0.77%Si alloys in the temperature range from 20 K to 300 K. Observed relaxation spectra were compared with the relaxation functions calculated by a Monte Carlo simulation with four fitting parameters: the dipolar width, trapping rate, detrapping rate and fraction of initially trapped muons. From the fitting, the temperature variations of the trapping rates reveal that there are three temperature regions concerning muon kinetics. In the low temperature region below 120 K, muons appeared to be trapped in a shallow potential yielded by dissolved Mg atoms, and thus little effect of heat treatment of the samples was observed, while in the mid and high temperature regions, the trapping rates clearly depended on the heat treatment of the samples suggesting muon-cluster and/or muon-vacancy interactions.

Iron-Water Interface under Electrochemical Condition

Spin polarized density functional theory calculations have been performed to characterize the structure of water molecules on iron surface under applied charges. It is found that water molecules of the contact layer take H-down configuration under the negative charge, on the other hand, under the positive charge, they adsorbed on a top site of iron atom, as the applied charge increases, the dissociation of water molecules proceed. In addition, we found that the energy shift of the Fermi level varies linearly in the range from-e to +e, while beyond this range it tends to saturate.

Effect of Chlorine Atoms for Development of Aluminum Corrosion

Computational density functional theory (DFT) model of the adsorption of chlorine atoms onto the perfect Al (111) surface has been performed. The structural and electronic properties of chlorine atoms adsorbed on the surface are investigated within a supercell approach for chlorine coverages of 0.25, 0.33, 0.5 and 1 ML respectively. It is found that the adsorbates prefer on-top sites over bridge, hcp and fcc sites in low coverage while fcc sites in high coverage, and the binding energy decrease with increase of coverage due to the interactions of chlorine atoms. The discussion of geometrical and electronic analysis by plotting differential charge density distribution and projected density of states (PDOS) are presented.

Electrochemical Evaluation for Rust Preventive Properties of Rust Preventive Oils Coated on Fe-Cu-C Sintered Steel

Since Fe-Cu-C sintered steels are easily rusted, they are coated with rust preventive oils. High viscosity of those rust preventive oils decrease workability, and low viscosity deteriorates rust preventive performance. Therefore, it is necessary to develop new rust preventive oils with contradictory properties of low viscosity and superior rust prevention. However, precise methodology to evaluate rust prevention ability has not been established. In this study, we developed new technique to quantitatively evaluate rust prevention ability by measuring the open circuit potential through thin corrosive solution on Fe-Cu-C sintered steels coated with a rust preventive oils. As a result, the ability for rust prevention can be measured quantitatively, and it decreases slowly over time, with repeating destruction and restoration. Furthermore, it was found that the deteriorating processes of rust prevention ability for rust prevention oils are composed of three characteristics steps respectively. That is, in the first step the great open circuit potential changes from 0V to-0.3V with repetition were observed where the excellent rust prevention ability was kept, in the second step it decreases slowly from-0.1V to-0.4V with oscillation of the small potential changes where the gradual decrease of rust prevention ability was recognized and in the third step it decreases monotonously in the lower potential than-0.4V where the rust was observed because of the remarkable deteriorating of the rust prevention ability.

First-Principles Study of H2O Adsorption on Oxygen-Covered Fe Surface

We present the results from first principle calculations of H2O adsorption on oxygen-covered Fe (100) surface. The calculations are based on a density-functional theory, surface modeling which uses supercell slab models. As a surface oxygen coverage increases, the surface is not activated, which makes the adsorption of water molecules on Fe surface unfavorable. It has been found that the surface covered oxygen exerts an influence on the adsorption of H2O molecule on Fe surface.

First-principles calculations of the water molecules and hydroxylated iron surface

ABSTRACT To provide atomic scale understanding of the initial corrosion process of Fe in aqueous environment, the electronic interaction of between water molecules and hydroxyl groups with the (110) and (100) surface of Fe is investigated in a density functional theory approach. Using periodic slab models, stable structures and electronic states are calculated, and the dynamical behavior of water molecules and hydroxylated iron surface under finite temperature is resolved by first-principles molecular dynamics. The position of hydroxyl groups on surface does not change from the initial bridge position, while water molecules move from the initial on-top to bridge site, and the molecular surface tilts from a parallel position, the hydrogen bonds are also formed between water molecules and hydroxyl groups. This result suggests that the interaction due to the hydrogen bonding is dominant.

Density Functional Theory Study of H2O Molecules on Cr2O3 Surfaces

In order to understand the reactivity of Cr2O3 surface towards H2O molecule, the optimized structure, electronic structure, and the behavior of adsorbates were examined using a first-principles calculation based on density-functional theory (DFT). H2O coverages varying from a quarter to two monolayers (MLs) were considered. At a low coverage, the oxygen atom of H2O adsorbs on the Cr atom of the outermost Cr2O3 surface layer, the entire H2O molecule is slanted at the direction of a hollow site, and a molecular plane is nearly parallel to the surface. The hydrogen bond is formed between the surface oxygen atom and the hydrogen atom of H2O molecule. From the optimized structure, the H2O dissociation mechanism which passes through a transition state is guessed. For 0.5ML coverage the obtained absorption energy is-82.5 kJ/mol. Our results are in good agreement with other reported theoretical and experimental results.

Magnetism and Electronic Structure Calculations of Pd-TM Alloys and Hydrogen Systems

The electronic and magnetic properties of Pd-TM (Fe, Co, and, Ni) alloys and their hydrides have been studied using first-principles calculations by Korringa-Kohn-Rostoke Coherent potential approximation (KKR-CPA) Green’s function method with density functional theory (DFT). The nature and the concentrations of the alloying elements and hydrogen have been examined. In addition, the magnetic properties associated with the density of states and the Fermi level in Pd-TM alloys and their hydrides are discussed. In particular, we find that the minority spin density of nickel (3d) has a sharp peak near the Fermi level. In the case of Fe and Co, the sharp peak exists above the Fermi level and this difference affects the spin polarization.

Electrochemical Impedance Characteristics of Sintered 7075 Aluminum Alloy under SSRT Test

Powder metallurgy (P/M) process has the advantage of better formability to fabricate complex shape products without machining and welding. And recently this P/M process has been applied to the production of aluminum alloys. The P/M aluminum alloys thus produced also have received considerable interest because of their fine and homogeneous structure. Many papers have been published on the mechanical properties of the aluminum alloys produced by P/M process while there have been few on their corrosion properties from the view point of electrochemistry. In this experiment, therefore, two kinds of 7075 aluminum alloys prepared by the conventional ingot metallurgy (I/M) process and P/M process were used, I/M material is commercially available, and their corrosion behavior were investigated through the electrochemical tests such as potentiodynamic polarization test, slow rate strain tensile (SSRT) test and electrochemical impedance spectroscopy (EIS) measurement under SSRT test in the corrosion solution and the deionized water.