Creep-induced ωo phase precipitation and cavity formation in a cast 45.5Ti-45Al-9Nb-0.5B alloy

The precipitation behavior of ωo phase and creep properties of a Ti-45Al-9Nb-0.5B alloy at intermediate temperatures are investigated in this study. The prior βo phase located at lamellar colony boundaries in the as-cast microstructure is entirely replaced by the ωo phase after creep tests. The imposed stress evidently contributes to the nucleation and growth of the ωo phase. The precipitation of ωo phase inside α2 lamellae can be initiated in front of deformation twins located in adjacent γ lamellae, which are impeded at the lamellar interface. Higher applied stress is expected to further enhance the α2→γ phase transformation and to generate more deformation twins in the γ lamellae. A stress exponent of n = 4.2 is determined for the present creep tests. The microstructural degradation is predominantly induced by dynamic recrystallization (DRX). The decomposition of α2 lamellae via α2→ωo and α2→γ transformation reduces the creep resistance. The distribution and size of ωo precipitates have a pronounced impact on the cavity evolution at the tertiary creep stage. The ωo phase formation under the synergistic effects of the thermal exposure and applied stress accelerates creep failure.