Improvement of microstructure, mechanical properties and hot workability of a TiAl-Nb-Mo alloy through hot extrusion

Abstract

γ-TiAl alloys are excellent candidate materials for high-temperature applications, although poor hot workability limits their widespread application. In the present study, the microstructure, mechanical properties and hot workability of TiAl-Nb-Mo alloy in the as-cast and as-extruded conditions were investigated. The as-cast TiAl alloy consisted of γ/α2 lamellae, γ and β phase distributed at colony boundaries and on the lamellar interfaces. The tiny particles of ω0 phase were formed, transformed from β phase during HIP treatment, which dissolved after hot extrusion. The TiAl alloy exhibited improved mechanical properties and excellent hot workability after hot extrusion because of grain refinement, the introduction of β phase and the disappearance of ω0 phase. The anomalous yield stress phenomenon occurring in as-cast material disappeared in the as-extruded condition, which was ascribed to grain refinement that reduced the resistance of grain boundaries to intragranular slip at elevated temperature. The findings demonstrate the benefits of introducing small amounts of β phase to improve hot workability of TiAl-Nb-Mo alloys, particularly when followed by heat treatment to eliminate the β phase and increase the mechanical properties.