  An in-situ neutron diffraction investigation of the annealing and healing of
dislocations in the bulk Pd-D2 system was carried out. Lattice misfit between
the alpha and beta hydride phases produces dislocations during the phase
transition in either direction, relieving elastic strain, which is reflected in
reduced pressure hysteresis compared to the spinodal hysteresis. The effects on
the dislocation density of annealing the metal under vacuum, of annealing in
the beta hydride phase, and of the phase transformation itself were
investigated by measuring diffraction peak breadths during annealing and
hydrogen cycling. During annealing under vacuum the dislocations were removed
at a lower temperature than was previously reported, but annealing in the beta
phase gave nearly the same result. However, when cycling hydrogen in and out of
the sample, the dislocation density decreased much faster with increasing
temperature compared to annealing. In other words the process of phase
transformation allows for healing of dislocations at lower temperatures than
would be required to anneal them purely by heating. This healing effect was
observed during both absorption and desorption. This result illuminates the
mechanism by which misfit dislocations can be healed at the same rate that they
are created in a sample undergoing absorption-desorption cycling, as proposed
in theoretical models of the origin of pressure hysteresis.
