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

Atom probe field ion microscopy investigations of alpha(2) + gamma 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 gamma/gamma and alpha(2)/gamma interfaces. The solubility of boron in the gamma and alpha(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 gamma and alpha(2) 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)(2)B phase were detected. No appreciable evidence of boron segregation to gamma/gamma or alpha(2)/gamma interfaces was observed. The interfacial coverage of tungsten at alpha(2)/gamma and gamma/gamma interfaces was estimated to be similar to 1 - 5%. Chromium was also found to segregate to certain gamma/gamma interfaces. The observation of partitioning and interfacial segregation supports the previous result that tungsten additions stabilize the a, 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 alpha(2) + gamma TiAl alloys due to interfacial segregation; however, boron additions dramatically affect the initial lamellar spacing, possibly through a modification of the gamma phase nucleation site density. (C) 1997 Elsevier Science S.A.