Effect of stress orientation on microstructural evolution during creep of near-lamellar Ti-47Al-2Cr-2Nb

Microstructural evolution was studied in a near-lamellar two phase (α2+γ) Ti-47Al-2Cr-2Nb alloy under high temperature creep and exposure conditions. The aim of this study was to probe the role of stress orientation, with respect to lamellar plates, on microstructural changes during primary creep. Creep testing was complemented with SEM and TEM based microstructural characterization. It was observed that retention of excess α2 resulted in an unstable microstructure. Under stress and temperature, excess α2 was lost and Cr-rich precipitates formed. Depending on stress orientation, the sequence of precipitates formed was different. α2 loss was accompanied by formation of the non-equilibrium C14 Laves phase when lamellar plates were oriented parallel to the stress axis. In contrast, α2 loss did not result in formation of the C14 phase in perpendicular samples. It was concluded that C14 formed preferentially in certain test orientations because of its effectiveness in relieving residual stresses in α2 that arose from lattice misfit and modulus mismatch.