The effect of substitutional elements (Al, Co) in LaNi_4.5M_0.5 on the lattice defect formation in the initial hydrogenation and dehydrogenation

The formation of the vacancy and dislocation by the initial hydrogenation and dehydrogenation in LaNi_4.5M_0.5 (M = Al, Co, and Ni) was observed by means of the positron lifetime technique. The concentrations of vacancy introduced by these processes were 0.25, 0.13 and 0.01 at.% for LaNi₅, LaNi_4.5Co_0.5 and LaNi_4.5Al_0.5, respectively. Al substitution into LaNi₅ significantly prevented from vacancy formation, compared with LaNi₅ and LaNi_4.5Co_0.5. In LaNi_4.5Al_0.5, the increase of the hardness and the enhancement of the pulverization, i.e. enhancement of the formation of micro cracks compared with LaNi₅ were observed while the Co substitution had little effect on pulverization and hardness as well as vacancy formation. These results show that the formation of micro cracks became more active process by Al substitution than the formation of the lattice defects to release the strain energy generated by the hydride formation because of the higher formation energy of the lattice defects in LaNi_4.5Al_0.5, although both the formation of micro cracks and lattice defects were still observed in all alloys we studied.