Precipitation of two kinds of γ laths in massive γ coexisting with γ lamellae in as-cast Ta-containing TiAl-Nb alloys

The relationship between γ_m phase (diffusionless transformation) and γ lamellae (diffusion transformation) and the precipitation of γ laths in γ_m were experimentally characterized in a Ta-containing as-cast TiAl-Nb alloy. Unlike the irregular and curved boundary of γ_m obtained by completely diffusionless transformation, γ_m phase formed in this work presents a particular orientation relationship with γ lamellae which is confirmed to be 120° rotational relationship. It indicates that γ lamellae can grow through the existed γ_m phase due to diffusion-controlled growth. Meanwhile, two kinds of γ laths shown true-twin and pseudo-twin relationship with γ_m respectively can be found, which can relax the stress in γ_m phase. The precipitation mechanisms of these laths are discussed.