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

Thermal conductivity measurements have been made on composites of unal loyed Ti reinforced with short fibre SiC, particulate SiC and particul ate TiB2, and Ti-6Al-4V reinforced with long fibre SiC. Measurements w ere also made on the unreinforced matrices and on the reinforcements t hemselves. The results have been compared with predictions from the Ha sselman 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 i n 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 d uring processing. The transverse conductivity of the Ti-6Al-4V reinfor ced with long SiC fibres was also impaired by the high thermal resista nce of the interface, but in this case the effect was less significant because of the larger diameter of the reinforcement, which reduces th e frequency with which transport of heat across an interface must occu r. Finally, similar characteristics were exhibited by the short SiC fi bre/Ti composite, except that the axial conductivity was rather lower than expected. This effect is attributed to matrix porosity localized at the fibre ends.