Effect of Microstructure on Cyclic Hydrogenation Properties in Ti-Cr-V Alloys

Abstract

The cyclic degradation of TiCrV hydrogen storage alloy has been studied by using SEM and TEM. Microstructural analysis indicated that the accumulation of lattice defects such as dislocations and strain field occurred due to volume expansion and shrinkage during cycles, led to a loss of the effective storage capacity. Annealing with minute particles in order to suppress the development of lattice defects during cycles resulted in increase in plateau width during hydrogenation but nothing improved during de hydrogenation. This is probably due to beta hydrides formed during hydrogenation, which is stable with lattice defects and strain field. Fine particles less than several hundred nano meter in size could have a good degradation resistance due to less accumulation of lattice defects and during cycles.

TiCrV hydrogen storage alloy studied by positron annihilation spectroscopy

“…Therefore, if the sample is pre-pulverized by mechanical means to have a particle size below that observed after a number of hydriding cycles, for example, less than 20 m, it is anticipated that the degradation of hydrogen storage capacity arising from embrittlement may be suppressed. A dramatic decrease in dislocation density near the surface of the particles is frequently observed by TEM in the pre-pulverized samples [11]. Also, from the SEM observation, pre-pulverized samples contain few cracks after an appropriate annealing.…”

“…Since the observed positron lifetime is well below that calculated for monovacancies, dislocations introduced by the volume expansion are probably responsible for positron trapping. In fact, TEM observation revealed that the dislocation density increases from 5 × 10 8 cm −2 to 1 × 10 10 cm −2 with the pressure swing cycle [11].…”

“…Melt-quenching [9], floating-zone-melting [10] and mechanical pre-pulverization [11][12][13] have been proposed as effective means of enhancing rechargeable hydrogen storage capacity. From a fundamental viewpoint, stable hydride formation, phase dissociation, strain accumulation, defect formation and surface contamination are regarded as the causes of degradation [14,15].…”