Kinetics-controlled reaction pathway and microstructure development of Ti3SiC2-TiC composite processed through reactive spark plasma sintering

Abstract

Reactive spark plasma sintering of ternary Ti-Si-C system was performed using three different powder precursors systems 3Ti/Si/2C, 3Ti/SiC/C and 2Ti/TiC/Si, to explore the fundamental physics behind Ti₃SiC₂ MAX phase formation, its stability and microstructure development, and, finally linked with its hardening and contact induced damage tolerance. Phase evolution in Ti-Si-C system is a complex phenomenon, and, present experimental conditions never yield a phase pure Ti₃SiC₂ MAX phase, rather results in varying volume fractions of Ti₃SiC₂-(Ti_x,Si_1-x)C solid solution due to non-equilibrium processing conditions exerted by SPS processing which restricts coherent site specific diffusional jumps and promotes the formation of (Ti, Si)C solid-solution instead of well reported non-stoichiometric TiC_x. 3Ti/SiC/C precursor was the best candidate for processing composite with highest yields of Ti₃SiC₂. Phase evolution is guided by the free energy of formation of different phases and chemical affinity amongst the constituent elements rather than the equilibrium phase diagram of the Ti-Si-C system. Presence of free carbon, low temperature liquid phase and slow heating rate are the key requirements for forming phase pure Ti₃SiC₂, where excess free carbon reduces the stability of Ti₃SiC₂ via decarburization. Non-equilibrium processing conditions impart nano-precipitation of coherent hexagonal Ti₃SiC₂ precipitates within a cubic (Ti, Si)C matrix with a distinct orientation relation of (220)_matrix ║(0004)_precipitate and <114>_matrix ║<2–1–10>_precipitate that has never been reported, instead of growing highest density plane of hcp-on-fcc matrix. Coherency strain and fine interlocking microstructure of the as-processed composite experiences ≈36 % of enhancement in hardness followed by an improved contact damage for the as-processed composite.