Composite of Ti6Al4V and SiC fibres: evolution of fibre-matrix interface during heat treatments

The composite of Ti6Al4V alloy, reinforced by unidirectional SiC fibres coated with graphite, is a promising material for aeronautical applications including turbine parts that operate at middle temperatures of 500 - 600 °C. For these components, the stability of mechanical properties is one of the most important requirements. Because this stability is mostly determined by fibre-matrix interface, the interface modifications after prolonged working time must be strictly controlled. In order to reduce the production time and costs, a new fabrication process of Roll Diffusion Bonding (RDB) has been recently developed at the C.S.M. laboratories. In the present work, the characteristics of the composite, fabricated by RDB, are compared with those of identical composite, prepared by the conventional process of Hot Isostatic Pressing (HIP), which has been already investigated [1-2]. Micro-hardness tests and XRD measurements have been carried out on the cross-sectioned samples of RDB and HIP composites. In order to extend the sampling zone for micro-chemical analysis, the samples with the surface forming a very low angle (about 2°) with fibre axis were prepared by mechanical polishing. Micro-chemical characterization was focussed on the interface between carbon (coating of SiC fibers) and metal (Ti6Al4V alloy). The composition of this interface was investigated by means of multipoint AES (VG Escalab MkII) and photoemission spectro-microscopy (ESCAmicroscopy beamline at the ELETTRA Synchrotron Light Source). Before these analyses, the surface contamination was removed in situ by Ar ion sputtering. The results of micro-chemical analyses revealed very similar composition of the fibre-metal interface in RDB and HIP composites. The micro-structure of RDB samples exhibits smaller grains and higher dislocation density with respect to that of HIP samples. These features togehther with noticeably higher hardness guarantee better mechanical properties of composite fabricated by new RDB technique. Furthermore, obtained results permit to explain the anelastic behaviour of this material.