Hydrogen incorporation in Ti-based metal-matrix composites fabricated by vacuum plasma spraying and vacuum hot pressing

In this paper we investigate the incorporation of hydrogen as a temporary alloying element during vacuum plasma spraying (VPS) of Ti-based metal-matrix composites and show that it has probable beneficial effects for secondary processing by vacuum hot pressing (VHP),The composites investigated included both commercial purity Ti and Ti-6Al-4V matrices, which were sprayed onto DRA Sigma SiC fibres. The as-sprayed matrix microstructures were investigated by transmission electron microscopy and X-ray diffractometry, and were found to consist in both cases of a fine mixture of(1) hexagonal alpha'-Ti martensite plates, with an increased lattice parameter relative to the usual value, presumably due to dissolved hydrogen; and (2) an increased proportion of equiaxed b.c.c. beta grains, probably stabilized by dissolved hydrogen. Both phases, but particularly beta, contained needle-shaped b.c.t. hydride precipitates.It has been found that monotapes produced by VHP can be consolidated more easily by secondary VHP than those produced by electric are spraying, leading to reduced adverse interfacial reactions between fibre and matrix. This improved processability is believed to be due to the combination of a greater proportion of the more ductile beta phase and the very small grain sizes inherent in VPS. However, the dissolved hydrogen and the potentially embrittling hydride precipitates are removed during VHP, The presence of hydrogen after primary VPS may therefore have an overall beneficial effect