Effect of the filament nature on fatigue crack growth in titanium based composites reinforced by boron, B(B₄C) and SiC filaments

Abstract

Crack propagation testing has been applied to synthetic metal matrix composites (MMC) in order to compare failure mechanisms in Ti-6Al-4V alloy reinforced by uncoated boron, B(B₄C) and chemical vapour deposition (CVD) SiC filaments. The impeding effect of the fibres leads to low crack growth rates, compared to those reported for the unreinforced Ti-6Al-4V alloy and to higher toughness despite the presence of the reinforcing brittle phases. After long isothermal exposures at 850° C, the MMC crack growth resistance is reduced mainly due to fibre degradation, fibre-matrix debonding and an increase in matrix brittleness. However, for short-time isothermal exposures (up to about 10 h for B/Ti-6Al-4V, 30 h for B (B₄C)/Ti-6Al-4V and 60 h for SiC/Ti-6Al-4V) the crack growth resistance is significantly increased. This improvement is related to the build up of an energy-dissipating mechanism by fibre microcracking in the vicinity of the crack tip. This damaging mechanism allowing matrix plastic deformation is already effective for boron and B(B₄C) in the as-fabricated state, but occurs only after 10 h of thermal exposure at 850° C in the case of SiC/Ti-6Al-4V composites.