EFFECT OF DEFORMATION ON LOW-TEMPERATURE DEUTERIDE PRECIPITATION IN SINGLE-CRYSTAL NIOBIUM

Small-angle neutron scattering (SANS) measurements of low concentratio n, 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 m atrix was consistent with a spherical particle morphology, as expected from a large body of published TEM studies. In addition, a significan t 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 requi red to nucleate the incoherent phase. Deformation produced by unidirec tional 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 partic le morphology. Metallographic analysis of deformed specimens electropo lished below the solvus temperature confirmed the presence of large de uteride plates with an orientation consistent with the anisotropic SAN S response. Deformation also eliminated the temperature hysteresis obs erved in the undeformed material. A cellular dislocation substructure was found in TEM characterization of the deformed Nb single crystal ma terial. 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 temperat ure hysteresis is consistent with an interaction between the deuterium and regions of high dislocation density facilitating heterogeneous nu cleation.