Elastic constants of a Pd-H crystal with coherent interphases

Modeling hydrogen-induced embrittlement in metals requires a clear understanding of the effects of hydrogen on the elastic constants of the metal host and of the hydride phases that may form at the crack tip. We have used an ultrasonic spectroscopy technique to measure the three independent elastic constants of Pd as a function of the concentration of interstitial H. A notable result is that within the mixed (α + β) twophase range, C ₄₄ decreases monotonically with increasing hydrogen concentration, whereas C′ = (C₁₁ - C₁₂)/2 shows a concave parabolic dependence on hydrogen concentration. It is shown that this difference is due to Zener-type anelastic relaxations involving μm-size coherent lenticular-shape precipitates (β-hydride precipitates in α-Pd(H) matrix, and α-hydride precipitates in β-hydride matrix). In the β-phase, C₄₄, C' and B decrease with increasing hydrogen content.