Takanori Suda

Hydride formation and fracture of vanadium alloys

Abstract The effects of hydrogen in vanadium, V–5Cr and V–5Ti have been evaluated by the change in surface and in microstructure and by the aspect of fractured surfaces. Hydrogen accumulation promotes local concentration of hydrides and assists the generation of stacking faults. Under increased stress, crack propagation occurs by the successive formation of crack fronts.

Temporal fluctuation and its power law in the crystalline-to-glass transition during electron irradiation

Temporal nanostructural fluctuations brought about by transient metastable atom-cluster formation during radiation-induced amorphizing transformation in the intermetallic compound NiTi, observed using a combination of high-resolution high-voltage electron microscopy and molecular dynamics simulation, were characterized in terms of power-law responses of non-equilibrium energy-dissipative systems. Within the framework of the multi-Lorentzian picture, the resultant power law also describes the multirelaxation time (i.e. cluster lifetime) distribution. In addition, a unified relation for the autocorrelation functions for such fluctuation phenomena is discussed.

Development of Environmental Cell for Gas Reaction of Nano-Size Particles

“Environmental cell” microscopy was applied for surveying gas reaction of hydrides in magnesium base alloys, which are candidates for hydrogen storage materials in advanced hydrogen energy systems. In order to clarify the mechanism of hydrogenation process, in-situ experiment has been carried out by using a 200 kV transmission electron microscope (TEM) equipped with a newly developed environmental cell, which is capable to 0.1 MPa in the temperature range between R.T. and 200°C. When hydrogen gas reacted with magnesium powders, straightening of surface steps (60~70 nm in height) was observed, indicating that volume expansion occurred. In addition, the formation of MgH2 was indicated in selected-area-diffraction patterns (SADP). The precise study on this in-situ experiment, as well as its improvement, will be continued, with using transparent films.

Dynamic and static hydrogen effects on mechanical properties in Vanadium alloys

To understand hydrogen behavior in V and V‐based alloys, two kinds of tensile tests were carried out for hydrogen‐charged miniature specimens: One is with hydrogen charging prior to testing (static charging), and the other is with hydrogen charging during straining (dynamic charging) with the continuous and intermittent ways. Static hydrogen‐charging effect: From the results of gas emission and microstructure, it was indicated that hydrogen can be trapped by lattice defects, dislocations, vacancies and voids, which were effective up to 500 C. The static charging prior to straining generally resulted in hydrogen‐induced hardening. Dynamic hydrogen‐charging effect: The deformation stress dropped just after starting the dynamic charging, and returned to the original level after stopping the charging, which can be called as a hydrogen‐induced softening. The significant softening is attributed to fast diffusion and interactions of hydrogen with mobile dislocations. The activation volume from different strain r...

Atomistic Analysis of Stress-induced Local Amorphization in NiTi Alloy

In situ tensile straining experiments have been carried out on thin films of the ordered intermetallic compound NiTi (B2 structure) in a high-voltage electron microscope. Real-time observations of stress-induced local amorphization of moving crack tips were followed by high-resolution transmission electron microscopy examination of nanocrystalline regions formed near the crack tip and in the region between the amorphous and crystalline regions. We have also independently confirmed the occurrence of local amorphization by ultra-high speed deformation at an atomistic level in bulk samples of the same NiTi sample. A fast Fourier transformation (FFT) image analysis technique was used to determine the root mean square displacement (RMSD) parameter within the nanocrystalline regions. The RMSD parameter increases to a critical fraction of the nearest neighbor distance of 0.12±0.01 as the amorphous region is approached, and this finding agrees with the generalized Lindemann melting criterion for amorphization proposed by Okamoto and Lam [1,2]. The present study shows that FFT-real-space analysis technique can provide local measurements of the RMSD parameter in an inhomogeneous system.

In Situ High-Resolution Observation for Decomposition of NaAlH4

NaAlH4 has a theoretical hydrogen capacity of 5.6 wt. % with two-step reaction, and the control of the reaction temperature and reversibility is a critical issue for onboard application. To clarify nano-structural details of decomposition of NaAlH4, the in-situ annealing experiment was carried out in a high resolution microscope. It was confirmed that NaAlH4 decomposed at between 200 and 300°C, resulted in formation of many gas bubbles at interface between the particle and oxide film. A reactive intermediate, Na3AlH6, may decompose in this temperature range. Sodium alanate particle was originally agglomeration of small nano-sized crystal with the size of 10 – 20 nm, and the crystal grain grew to 110 nm in diameter after completing decomposition at around 400°C. This is the first step for examination of the microstructural response of catalysts on hydrogen storage materials.