THE EFFECT OF THE INTERFACE ON THE THERMAL-CONDUCTIVITY OF TITANIUM-BASED COMPOSITES

Thermal conductivity measurements have been made on composites of unalloyed Ti reinforced with short fibre SiC, particulate SiC and particulate TiB2, and Ti-6Al-4V reinforced with long fibre SiC. Measurements were also made on the unreinforced matrices and on the reinforcements themselves. The results have been compared with predictions from the Hasselman and Johnson model for spherical and long fibre composites and from the Eshelby model for ellipsoids of any aspect ratio. It is shown that, while an enhancement of the thermal conductivity is expected to arise from the presence of both types of particulate, this effect is in fact produced only with the TiB2 reinforcement. This is explained in terms of the large thermal resistance of the SiC/Ti interface, which is in turn related to the nature of the reaction layer formed there during processing. The transverse conductivity of the Ti-6Al-4V reinforced with long SiC fibres was also impaired by the high thermal resistance of the interface, but in this case the effect was less significant because of the larger diameter of the reinforcement, which reduces the frequency with which transport of heat across an interface must occur. Finally, similar characteristics were exhibited by the short SiC fibre/Ti composite, except that the axial conductivity was rather lower than expected. This effect is attributed to matrix porosity localized at the fibre ends.