Effect of deformation on low-temperature deuteride precipitation in single crystal niobium

Small-angle neutron scattering (SANS) measurements of low concentration, single crystal Nb-D alloys have been performed to characterize the effect of deformation on deuteride phase formation. The SANS response from the deuteride phase formed at low temperature in the undeformed matrix was consistent with a spherical particle morphology, as expected from a large body of published TEM studies. In addition, a significant hysteresis was observed (Delta T similar to 100 degrees) in the SANS response versus temperature from the undeformed sample material. The hysteresis is attributed to the irreversible plastic deformation required to nucleate the incoherent phase. Deformation produced by unidirectional rolling to 50% thickness reduction altered the deuteride phase transformation characteristics in a fundamental way. The SANS response was highly anisotropic, indicative of an oriented, anisometric particle morphology. Metallographic analysis of deformed specimens electropolished below the solvus temperature confirmed the presence of large deuteride plates with an orientation consistent with the anisotropic SANS response. Deformation also eliminated the temperature hysteresis observed in the undeformed material. A cellular dislocation substructure was found in TEM characterization of the deformed Nb single crystal material. The plate-like particle morphology in the deformed material is attributed to an interaction between the precipitating deuterium and the cellular dislocation substructure. The elimination of the temperature hysteresis is consistent with an interaction between the deuterium and regions of high dislocation density facilitating heterogeneous nucleation.