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

The precipitation behavior of omega(o) phase and creep properties of a Ti-45Al-9Nb-0.5B alloy at intermediate temperatures are investigated in this study. The prior beta(o) phase located at lamellar colony boundaries in the as-cast microstructure is entirely replaced by the omega(o) phase after creep tests. The imposed stress evidently contributes to the nucleation and growth of the omega(o) phase. The precipitation of omega(o) phase inside alpha(2) lamellae can be initiated in front of deformation twins located in adjacent gamma lamellae, which are impeded at the lamellar interface. Higher applied stress is expected to further enhance the alpha(2)->gamma phase transformation and to generate more deformation twins in the gamma 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 alpha(2) lamellae via alpha(2)->omega(o) and alpha(2)->gamma transformation reduces the creep resistance. The distribution and size of omega(o) precipitates have a pronounced impact on the cavity evolution at the tertiary creep stage. The omega(o) phase formation under the synergistic effects of the thermal exposure and applied stress accelerates creep failure. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study investigates the precipitation behavior of the omega(o) phase and creep properties of a Ti-45Al-9Nb-0.5B alloy at intermediate temperatures. It shows that the imposed stress contributes to the nucleation and growth of the omega(o) phase, which can reduce the creep resistance of the material. Additionally, dynamic recrystallization is identified as a primary cause of microstructural degradation.