Hydrogen trapping at dislocation cores at room temperature in deformed Pd

Small-angle neutron scattering (SANS) measurements of hydrogen segregation at dislocations in heavily deformed single crystal Pd have been performed at very low solute concentration (PdH_0.0016) at equilibrium with respect to hydrogen gas at 295 K. The net (the without-hydrogen measurement subtracted from with-hydrogen measurement) absolute differential macroscopic scattering cross section has been fit with a cylindrical form factor to represent the Cottrell atmosphere, yielding local trapped concentration δ ~ 0.06 [H]/[Pd], local volumetric dilatation f ~ 1.01, and trapping radius R ~ 4 Å of the segregated hydrogen. This measurement augments SANS results below ambient temperature [B.J. Heuser, H. Ju, Phys. Rev. B 83 (2011) 094103]. The temperature dependence of the measured radius is confirmed by a Fourier transform of hydrogen occupation at dislocations based on an elastic continuum treatment [Trinkle et al., Phys. Rev. B 83 (2011) 174116]. The measured trapping parameters are consistent with a depopulation of weak long-range dislocation strain fields at ambient temperature; hydrogen binding to stronger core dislocation sites persists at 295 K and results in the measured net scattering. The local solute concentration and trapping radius (less than two Burgers vectors in Pd), are both too small to support optic mode dispersion due to inter-hydrogen interactions. This result supports the conclusion that trapped hydrogen undergoes a hydride to solid solution phase transformation between 200 and 300 K based on hydrogen vibration density of states measurements using incoherent inelastic neutron scattering [Trinkle et al., Phys. Rev. B 83 (2011) 174116, Ju et al., Nucl. Instr. Meth. Phys. Res. A 654, (2011) 522, and Heuser et al., Phys. Rev. B 78 (2008) 214101].