The response of SiC fibres to vacuum plasma spraying and vacuum hot pressing during the fabrication of titanium matrix composites

Abstract

Vacuum plasma spraying (VPS) and vacuum hot pressing (VHP) have been used to fabricate Ti-6Al-4V matrix composite material reinforced longitudinally with DERA Sigma C coated SiC 1140+ fibres. VPS of Ti-6Al-4V onto Sigma 1140+ SiC fibres caused no fibre/matrix interfacial reaction. During VHP a fibre/matrix reaction occurred, producing a mixture of fine (< 50 nm) TiCx (x </= 1) adjacent to the fibre coating and coarse-grained (0.3-0.5 &mgr;m) equiaxed TiCx adjacent to the Ti matrix. A decrease in C concentration with increasing distance from the C coating is proposed, and is consistent with the evidence presented. A similar thickness and morphology of reaction product arose from conventional foil-fibre foil processing, but the matrix coated fibre/hot isostatic pressing process led to a slightly thicker reaction layer. The TiCx reaction product acted as a diffusion barrier, inhibiting further reaction more effectively than in experiments on earlier SiC fibres having a C coating. Surface damage was shown to be a factor in lowering 1140+ SiC fibre failure stress. Surface damage to 1140+ fibres resulted from both VPS and VHP, the former causing a slight reduction in mean ultimate tensile strength (UTS), and a large reduction in the bend strain to failure Weibull modulus. This damage was caused by both fibre winding and by deposition of metal during VPS, giving rise to coating flaws, and is not in itself considered to be a major problem. Surface damage increased after VHP, reducing the mean UTS and tensile Weibull modulus, and the mean bend strain to failure. This damage arose from bending and flattening of the rough monotapes, and from the fibre/matrix reaction caused by thermal exposure. The level of damage to 1140+ SiC fibre from VHP was reduced by modification of the process path. Increasing the VHP temperature and lowering the pressure ramp rate reduced fibre damage sufficiently to enable a macroscopic composite UTS of 95% of the theoretical maximum to be achieved.

Mechanical Properties of Sigma 1140+ Sic Fibres Prior and after Composite Processing

“…3, together with the experimental results and the theoretical fit. The Weibull parameters for the as-fabricated fibres were equivalent to those reported in previous investigations [5]. In addition, it should be noted that the as-fabricated fibres which were immersed in the HF solution prior to testing did not show any change in strength.…”

Interfacial shear strength behaviour of Ti/SiC metal matrix composites at room and elevated temperature

“…This is important, because the SCS-6 fibre fracture strength has been previously observed to fall sharply from $5-6 GPa [17,19] to $1.2-2 GPa after the initial fibre fracture event [17,18], presumably owing to the introduction of surface flaws. Similarly, the failure strength of free Sigma SiC fibres has also been observed to fall from 3.2 to 1.3 GPa, along with a significant drop in Weibull modulus, when pristine fibres are processed [20]. For SCS-6 fibres, this drop in failure strength is accompanied by a change in failure mode from initiating at the core [19,21] to failures originating from the surface, leading to multiple 'wedge' cracks being observed by X-ray tomography for subsequent fractures [21], in agreement with scanning electron microscope observations [16,22].…”

Tensile Failure Behavior of SiC/Ti-6Al-4V Composites Manufactured by Plasma Spraying Route

“…However, the impact of high temperature-high velocity droplets can cause physical damage and thermal shock to fiber. 14,15) This paper describes the longitudinal tensile properties of SiC fiber reinforced Ti-6Al-4V composites manufactured by plasma spraying route and subsequent vacuum hot pressing. Two different sizes of Ti-6Al-4V feedstock powders were used in plasma spraying to form matrix.…”