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

Atom probe field ion microscopy investigations of α2+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 α2/γ interfaces. The solubility of boron in the γ and α2 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 α2 matrix phases was found to be 0.16±0.02 at.% W and 0.33±0.06 at.% W, respectively. Several borides including TiB2, TiB as well as a (Ti,Cr)2B phase were detected. No appreciable evidence of boron segregation to γ/γ or α2/γ interfaces was observed. The interfacial coverage of tungsten at α2/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 α2 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 α2+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.