Remarkable improvement in tensile strength of a polycrystalline γ-TiAl-based intermetallic alloy by deformation nanotwins

Abstract

Strengthening of TiAl-based intermetallic alloy is vital for its broader application, however, microstructural refinement through conventional processes is difficult to further strengthen this intermetallic alloy. Although high-density deformation nanotwin is expected to effectively strengthen it, there is still lack of in-depth understanding about the strengthening mechanism of high-density deformation nanotwin in TiAl-based alloy. Herein the microstructures, room-temperature (RT) tensile properties, tensile fracture behavior, and deformation mechanism of the Ti-45.5Al–4Cr-2.5Nb (at.%) master alloy (M. A.) as well as its continuous casting (C. C.) alloy and the heat-treated alloys were systematically investigated. After the C. C. alloy was annealed at 1250 °C for 2 h, the volume fraction of (B2+γ) coupled structures in the original grains reached the minimum, the interlamellar spacing was markedly refined; besides, substantial Shockley partial dislocations and stacking faults were generated in γ phase. During RT tension, both the dominant deformation mechanisms of the M. A. and C. C. alloy were dislocation slip, while that of the 2 h heat-treated alloy was changed into deformation twinning. High-density deformation nanotwins were generated in the γ phase (especially the γ lamellae) of the 2 h heat-treated alloy, which improved the tensile strength of the M. A. by 79%. High-density deformation nanotwins can further strengthen TiAl-based alloy to a large extent, which is mainly attributed to the fact that their batch-to-batch generation in the polycrystalline TiAl-based alloy during tension could considerably relief the stress concentration in addition to effectively improving the work-hardening rate, thus making the work-hardening rate keeping stable.