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An in-situ neutron diffraction investigation of the annealing and healing ofdislocations in the bulk Pd-D2 system was carried out. Lattice misfit betweenthe alpha and beta hydride phases produces dislocations during the phasetransition in either direction, relieving elastic strain, which is reflected inreduced pressure hysteresis compared to the spinodal hysteresis. The effects onthe dislocation density of annealing the metal under vacuum, of annealing inthe beta hydride phase, and of the phase transformation itself wereinvestigated by measuring diffraction peak breadths during annealing andhydrogen cycling. During annealing under vacuum the dislocations were removedat a lower temperature than was previously reported, but annealing in the betaphase gave nearly the same result. However, when cycling hydrogen in and out ofthe sample, the dislocation density decreased much faster with increasingtemperature compared to annealing. In other words the process of phasetransformation allows for healing of dislocations at lower temperatures thanwould be required to anneal them purely by heating. This healing effect wasobserved during both absorption and desorption. This result illuminates themechanism by which misfit dislocations can be healed at the same rate that theyare created in a sample undergoing absorption-desorption cycling, as proposedin theoretical models of the origin of pressure hysteresis.

Dislocation healing during hydrogen absorption and desorption in palladium