Microstructure characterization, mechanical properties and toughening mechanism of TiB2-containing conventional cast TiAl-based alloy

Effects of TiB2 addition on microstructure and mechanical properties of Ti-48Al-2Cr-2Nb+(0.72,1.62)wt% TiB2 alloys fabricated by the induction skull melting (ISM) process were investigated. Results showed that the TiB2-induced microstructure was characterized by randomly orientated fully lamellar colonies and both the colony size and lamellae spacing were refined (100 μm and ~185 nm, respectively) by TiB2 addition. The borides were identified to be TiB2 with plate, needle and block morphologies, determined by different growth stages during solidification. At room temperature and 700 °C, the TiB2-containing alloys exhibit non-deteriorated fracture toughness and superior tensile properties than that of the as-cast and heat-treated matrix alloys. Furthermore, the fracture toughness anisotropy was eliminated due to the randomly orientated lamellar microstructure induced by TiB2 addition. The fine TiB2 particles with special morphology (plate and needle) and the easy-to-deform ligament bridges induced by the refined microstructure can account for the notable fracture toughness of the studied TiB2-containing alloys. The main toughening mechanism was analyzed and discussed in light of the microstructure characterization, size and morphology of borides and the deformed ligament bridges.