Evaluation of elastic strain energy associated with the formation of hydride precipitates in LaNi₅

The elastic energy associated with coherent precipitation of hydrides with various shapes in LaNi₅ has been calculated by the Eshelby method with the single-crystal elastic constants experimentally determined by a rectangular parallelepiped resonance method. The calculated elastic energy amounts to about 70% of the change in chemical energy that occurs upon hydride formation. This implies the difficulty for coherent hydrides to nucleate in the perfect lattice of LaNi₅. This is in contrast to the case of titanium or zirconium, in which the associated elastic energies are only about 15% of the chemical energy change. Hydride precipitation in LaNi₅ is thus suggested to occur preferentially at free surfaces and lattice defects such as dislocations and grain boundaries.