Processing and High Temperature Properties of Nb/Nb5Si3 Laminates

Abstract

Abstract : The primary objective of this project was to study the processing and high-temperature deformation of laminated Nb/Nb5Si3 composites. Our initial goal was to control the grain size and porosity present in the Nb5Si3 silicide phase of the composite by developing improved processing techniques. For this part of the project we investigated four processing routes including hot pressing during annealing, vapor deposition onto a heated substrate, annealing under a large thermal gradient, and hot pressmg elemental Nb/Si microlaminates. The final processing method proved most promising and demonstrated that full dense Nb/Nb5Si3 microlaminates can be processed with large grains and no metastable phases. Our subsequent goal was to characterize the deformation of Nb/Nb5Si3 microlaminates at high temperatures and determine the controlling deformation mechanisms in both phases. We demonstrated that the creep rate of the silicide phase at 1000 degrees centigrade and 1100 degrees centigrade has a nonlinear dependence (or weak power law dependence) on stress and a very strong dependence on grain size, larger than the 1/d3 dependence expected for Coble creep, which is the predicted deformation mechanism at these temperatures and stresses (10-100MPa). For the Nb phase at 600 degrees centigrade power law creep is observed under high stresses with no grain size dependence, a expected. Under lower stresses, diffusional creep is observed, also as expected. However, unlike conventional diffusional creep, this data set shows a weak, 1/d, grain size dependence and an approximately linear stress dependence (1.0). The data support an earlier hypothesis that the rate of vacancy generation and annihilation controls diffusional creep in fine-grained samples, not the rate of vacancy diffusion.