Abstract: Microstructure and phase transformation of Ti46Al8Nb0.5B0.2C alloy have been investigated. X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results show that as-cast and hot isostatic pressing (HIP) alloy mainly composed of γ and α₂ phase have fully lamellar microstructure with point-like or ribbon-like TiB₂ distributing in lamellar colony or at grain boundary. The mean size of lamellar colony is about 150 and 450 μm for as-cast and HIP alloy, respectively. The lamellar spacing is about 550 and 600 nm for as-cast and HIP alloy, respectively. It has been found that cooling rates and quenching temperatures have significant effect on phase transformation of Ti46Al8Nb0.5B0.2C alloy. When the alloy is treated at 1380 °C for 1 h and cooled from α domain, water cooling leads to complete α→α₂ transformation, oil cooling leads to predominant α →α₂ and part α→γ_m transformation, air cooling leads to α →α + γ_p2 →L(α + γ) →L(α₂ + γ) transformation, and furnace cooling leads to α→α + γ_p3→L(α +γ) →L(α₂ + γ) transformation. However, when the alloy is treated at 1400 °C for 1 h and cooled from α domain, water cooling leads to predominant α →α₂ and part α →α +γ_p4 →γ_m transformation, oil cooling leads to α→α +γ_p5 →γ_m transformation, air cooling leads to α→ α +γ_p6→L(α + γ) →L(α₂ + γ) transformation, and furnace cooling leads to α→α + γ_p7 →L(α +γ) →L(α₂ +γ) transformation. Microstructural evolution of the alloy during various heat treatments has been examined and the phase transformation mechanisms have been elucidated. Based on the experimental observation, schematic CCT diagrams for the alloy have been given.

Key words: TiAl-based alloys, Heat treatment, Microstructure, Phase transformation