Seeded growth of Ti–46Al–8Nb polysynthetically twinned crystals with an ultra-high elongation

Large size polysynthetically twinned crystals of Ti–46Al–8Nb alloy with a parallel lamellar microstructure were successfully prepared using a Ti–43Al–3Si seed by our new operation. A large amount of columnar B2 phase paralleling to the growth direction was found in the final lamellar microstructure. Higher growth rate (>30 mm/h) led to the failure of seeding process. Based on these results, a new mechanism is proposed to describe the seeding process of the hypo-peritectic TiAl alloys. The peritectic α phase is suggested to directly nucleate from the melt, and then act as nucleus for transformed α phase in the subsequent β to α transformation. At the higher growth rate, the appearance of β phase secondary dendrites and homogeneous nucleation lead to the failure of seeding process. High Nb addition leads to a large amount of residual β phase, and these β dendrites finally evolve into B2 phase. The room temperature tensile elongation was measured to be 11.9–18.5% for Ti–46Al–8Nb PST crystals, which is the highest ever reported value for TiAl based alloys.