Hydride precipitation in Nb and some properties of NbH

Abstract

The precipitation of the niobium β-phase hydride from the solid solution has been studied using a variety of techniques including T.E.M., S.E.M., internal friction, and resistivity methods. It was shown that the morphology of the β-phase depended on the cooling rate and on the degree of under cooling. The “equilibrium” morphology was β plates on the {100}c, with the size of the plates being dependent on the cooling rate. “Rapid” cooling caused a transition to a massive form of the hydride whilst precipitation on large undercooling resulted in a “dendritic” morphology.The approximately 12% volume change which accompanies the hydride precipitation resulted in plastic deformation of the matrix. T.E.M. studies of the hydride precipitates showed both interfacial dislocations and prismatic loops. The elastic and plastic accommodation of the volume change had a major effect on the equilibrium between the β- and α-phases. This was studied using resistivity measurements. During the first cooling cycle, the α-β solvus temperature was significantly lower than on subsequent cooling after reversion of the precipitated β-phase. On each cooling cycle, the β-phase precipitated at the positions at which the β-phase had previously formed. Reversion of the β-phase on heating required elastic and plastic accommodation of a sign opposite to that observed on cooling. These accommodation terms resulted in a large thermal hysteresis, of the order of 100 K. The magnitude of these accommodation terms and the effects of external stress on these terms have been calculated.Experimental observations of the stress effects on β-phase precipitation have been carried out. Hydride can be formed at temperatures above the stress-free solvus by the application of elastic tensile stresses. Plastic deformation prior to hydride precipitation has little effect. However, deformation during precipitation has a major effect on the formation of the hydride, the solvus temperature being raised and the hydride forming preferentially along slip bands. These effects will be related to the volume accommodation terms.The properties of the β-phase have been studied using single crystals. As previously reported, these contain “domains” which are twin-related and have the {100}c and {110}c as the twin and boundary planes. These domains are highly mobile under external stresses, as expected from the twin relationship. The mobility is, however, expected to be temperature dependent as it is determined by the redistribution of the hydrogen among the interstitial tetrahedral sites. The β crystals are extremely brittle and fail by cleavage along the {100}c and {110}c planes.