HYDROGEN INCORPORATION IN TI-BASED METAL-MATRIX COMPOSITES FABRICATEDBY VACUUM PLASMA SPRAYING AND VACUUM HOT-PRESSING

In this paper we investigate the incorporation of hydrogen as a tempor ary alloying element during vacuum plasma spraying (VPS) of Ti-based m etal-matrix composites and show that it has probable beneficial effect s for secondary processing by vacuum hot pressing (VHP), The composite s investigated included both commercial purity Ti and Ti-6Al-4V matric es, which were sprayed onto DRA Sigma SiC fibres. The as-sprayed matri x microstructures were investigated by transmission electron microscop y 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 i ncreased 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 phas es, but particularly beta, contained needle-shaped b.c.t. hydride prec ipitates. It has been found that monotapes produced by VHP can be cons olidated 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 ph ase and the very small grain sizes inherent in VPS. However, the disso lved hydrogen and the potentially embrittling hydride precipitates are removed during VHP, The presence of hydrogen after primary VPS may th erefore have an overall beneficial effect.