Processing-structure-properties relations in titanium carbide-titanium boride composites fabricated by transient plastic phase processing

A systematic study of processing-structure-properties relations was carried out for a class of titanium carbide-titanium boride composites produced by Transient Plastic Phase Processing (TPPP), novel processing technique capable of in situ net-shape manufacturing fully dense refractory ceramic composites at relatively low homologous temperatures. These fully dense TiC{dollar}sb{lcub}rm x{rcub}{dollar}-TiB{dollar}sb2{dollar}-Ti{dollar}sb3{dollar}B{dollar}sb4{dollar} composites were fabricated starting from Ti/B{dollar}sb4{dollar}C, TiC{dollar}sb{lcub}0.5{rcub}{dollar}/TiB{dollar}sb2,{dollar} Ti/C/TiB{dollar}sb2{dollar} and TiC{dollar}sb{lcub}0.5{rcub}{dollar}/B{dollar}sb4{dollar}C mixtures of different molar ratios with the resulting different morphologies of the final phases. Plastic flow of TiC{dollar}sb{lcub}0.5{rcub}{dollar} (the starting and/or intermediate phase) combined with particle rearrangement and comminution of porous reaction product are believed to be the operating mechanisms for the most part of densification process responsible for increase of relative density to {dollar}sim{dollar}95%. Reaction paths and microstructural evolution in the two starting compositions with 4:4:1 ratio of Ti:B:C, namely: 4:1 Ti/B{dollar}sb4{dollar}C and 1:1 TiC{dollar}sb{lcub}0.5{rcub}{dollar}/TiB{dollar}sb2,{dollar} were examined in detail. In the former, the Ti{dollar}sb3{dollar}B{dollar}sb4{dollar} phase nucleated and grew as platelets, whereas it exhibited roughly equiaxed morphology in the latter. The presence of TiB as an intermediate phase in "platelet" composition appeared to determine the morphology of the Ti{dollar}sb3{dollar}B{dollar}sb4{dollar} phase. Based on XRD and SEM results from interrupted runs, a model is proposed according to which the faster diffusion of C, relative to B, is instrumental in the microstructural evolution of the "platelet" composite. The microstructural evolution of the "equiaxed" composite is less complicated in that there are no intermediate phases. The resultant microstructure is believed to develop by a displacive reaction that only involves the diffusion of B and C. The flexural strength in the temperature range 25{dollar}spcirc{dollar}C-1400{dollar}spcirc{dollar}C, plane strain fracture toughness in the temperature range 25{dollar}spcirc{dollar}C-1000{dollar}spcirc{dollar}C, room temperature hardness and microhardnesses of individual phases, thermal shock susceptibility, wear and oxidation resistance were measured for a variety of titanium carbide-titanium boride composites with different microstructures. The effects of various microstructural parameters such as the length scale of microstructure, the morphology of the Ti{dollar}sb3{dollar}B{dollar}sb4{dollar} phase, and the volume fraction of borides on these properties were identified. The relatively high strength and thermal shock resistance of the TPPP composites, and their competitive wear behavior combined with economic processing requirements make this class of materials an excellent candidate for cutting tools.