Temperature-stimulated acoustic emission in the niobium-hydrogen system

Abstract

The hydride precipitation in the Nb‐H system has been studied by the acoustic‐emission technique varying the temperature between 330 and 225 K at hydrogen concentrations ranging from 3.8 to 5.8 at.%. Concomitantly, elastic‐modulus and internal‐friction measurements have been conducted in order to reveal the hydride‐precipitation temperatures and the plastic‐deformation phenomena occurring during the formation of the hydride misfitting precipitates. In the virgin material, acoustic emission is absent during evolution of the precipitation although the dislocation‐multiplication processes are operative. When, due to a severe precipitation, a certain degree of deterioration of the material has been reached, acoustic activity characterized by discrete bursts is then stimulated by the hydride formation and definitively persists also after the high‐temperature annealing. The temperatures of the onset of the acoustic emission are lower than those corresponding to the hydride precipitation and may fit a line parallel to solvus. The data clearly indicate that neither the phase transformation per se nor the plastic‐deformation processes accompanying the precipitation can be the sources of the observed acoustic emission. The results suggest that the measured emissivity may be attributed to the nucleation and growth of nonrecoverable defects. According to the proposed model, during hydride precipitation, acoustic‐emission activity should be detected when a progression of irreversible deterioration of the material is occurring.