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.