Small-angle neutron scattering measurements of deuteride „hydride... formation and decomposition in single-crystal Pd

The deuteride ~hydride! precipitation and decomposition microstructure in single-crystal Pd has been investigated in a series of in situ small-angle neutron scattering ~SANS! measurements. The particle morphology along the absorption and desorption branches of the 353-K pressure-composition isotherm are consistent with a loss of particle coherency, leading to the formation of large, micron-thick plates. The loss of coherency coincides with the system entering the miscibility gap, an observation that suggests irreversible dislocation formation in part drives the hysteretic behavior of the Pd-D ~-H! system. SANS analysis further indicates that the decomposition process is characterized by a much higher particle dispersion, with a factor of 40 greater surface-to-volume ratio of the precipitating phase. This we attribute to a more heterogeneous transformation process, presumably at dislocations formed during initial deuteride formation.