THE INFLUENCE OF THERMAL TREATMENTS ON INTERFACIAL PROPERTIES AND FATIGUE-CRACK GROWTH RESISTANCE IN SCS-6 TI BETA-21S COMPOSITES

The mechanical properties of fibre-reinforced composites are dependent on the properties of the fibres, the matrix and the fibre/matrix interface. The latter two were investigated in a metastable beta Ti matrix (Ti beta21 s) reinforced with silicon carbide (SCS-6) fibres by heat treatment for 2 and 6 h at 910-degrees-C (solution treatment) and 8 h at 540-degrees-C (ageing). Characterization of the microstructure using scanning and transmission electron microscopies revealed that the interfacial reaction layer had become thicker in the solution-treated specimens. This variation did not result in any significant change in interfacial shear strength, determined by a fibre push-out technique, nor in the fatigue crack growth resistance in three-point bending. Conversely, ageing at 540-degrees-C resulted in the precipitation of 34% volume fraction of alpha plates with negligible change to the fibre/matrix reaction layer thickness. This resulted in a large increase in both the interfacial debonding and frictional shear strength, which was consistent with local volume changes in the matrix on precipitation of alpha. The fatigue crack growth resistance of the peakaged composite was also reduced under some testing conditions employed. In particular, the number of cycles to failure was decreased markedly after ageing.