Changes in microstructure and absorption-desorption pressures during hydrogen cycling in some intermetallic compounds

Changes in microstructure and their effects on hydrogen sorption properties have been investigated as a function of the number of hydrogen sorption cycles in LaNi5 and FeTi. For LaNi5, while the desorption pressure does not significandy depend on the number of cycles, the absorption pressure drastically decreases between the first and second cycles. In addition to severe cracking and pulverization, numerous edge dislocations with Burgers vectors of the l/3<1210>-type are introduced during the first absorption cycles. Both the introduction of numerous lattice defects and the formation of severe cracks seem to be completed during the first cycle, resulting in the reproducible P (pressure)-C (composition) characteristics for the subsequent cycles. In contrast, FeTi exhibits ever-changing P-C characteristics with cycle number. The introduction of dislocations with Burgers vectors of the <100>-type in the form of tangles and cracks occurs during each of absorption cycles with the extent to which this occurs being considerably smaller in each of absorption cycles of FeTi than in the first absorption cycle of LaNi5. The difference in material response to hydrogénation seems to be closely related to the difference in ductility of these two intermetallics.