Grain refinement through borides is known to be suppressed when TiAl is welded with a laser beam. As β grains do not primarily nucleate on boride at a high cooling rate, a mixture of nitrogen and argon is applied as a protecting gas for the formation of TiN during solidification. The phase transformation is changed correspondingly from Liquid → Liquid + β → β → α + β → α + γ+ β → α2 + γ + B2 to Liquid → TiN + Liquid → β+ TiN → α + γ + TiN → α2 + γ+ TiN. It is found that β grains prefer to nucleate heterogeneously on the suspending TiN in the melt with orientation relationship {111}TiN//{110}β, leading to refined β grains. α2 colonies that were thus modified into fine non-dendritic grains. The effects of nitrogen as a shielding atmosphere on the microstructure evolution of TiAl are elaborately studied.
