Microstructural characterization of segregation and precipitation in α2+γ titanium aluminides

Abstract

Abstract Atom probe field ion microscopy investigations of α 2 + γ TiAl alloys doped with B and W indicate that a significant proportion of the boron is concentrated in boride precipitates and the tungsten segregates to both γ / γ and α 2 / γ interfaces. The solubility of boron in the γ and α 2 matrix phases was found to be 0.011±0.005 at.% B and 0.003±0.005 at.% B, respectively. The solubility of tungsten in the γ and α 2 matrix phases was found to be 0.16±0.02 at.% W and 0.33±0.06 at.% W, respectively. Several borides including TiB 2 , TiB as well as a (Ti,Cr) 2 B phase were detected. No appreciable evidence of boron segregation to γ / γ or α 2 / γ interfaces was observed. The interfacial coverage of tungsten at α 2 / γ and γ / γ interfaces was estimated to be ∼1–5%. Chromium was also found to segregate to certain γ / γ interfaces. The observation of partitioning and interfacial segregation supports the previous result that tungsten additions stabilize the α 2 lamellae against growth and coarsening. There is no experimental evidence that boron significantly affects the rates of growth and coarsening of the lamellar structure in α 2 + γ TiAl alloys due to interfacial segregation; however, boron additions dramatically affect the initial lamellar spacing, possibly through a modification of the γ phase nucleation site density.