Effects of heat treatment on the microstructures and mechanical properties of as-cast Ti-45Al-2Nb-2Cr-(0.2, 0.5) C alloys

Heat treatment is an effective post-processing method to modify the microstructure and mechanical properties of TiAl alloys. Thus, we report the influences of a heat treatment at 1553 K for 90 min followed by air cooling on the microstructure evolutions, strength and fracture strain of the as-cast Ti-45Al-2Nb-2Cr-(0.2, 0.5)C alloys. It reveals that the heat treatment has little effect on the phase constituents, but can cause lattice shrinkage of γ phase due to the compressive stress of solid-state phase transition α→α + γ. The microstructure changes from the as-cast nearly lamellar structure to the heat-treated duplex structure, and the growth of massive γ phase is at the expense of the (α2 +γ) lamellae. Compared with the as-cast samples, there is a significant increase in the fracture strain of the heat-treated samples, but the yield/ultimate strength shows an opposite trend. The cracks existing in the as-cast samples propagate along the lamellar interface and across the lamellae, while the cracks of the heat-treated samples are basically distributed in the massive γ phase. Dislocations multiplication and slip within the γ lamellae govern the plastic deformation of the as-cast sample, while the plastic deformation of the heat-treated sample is mainly carried by the massive γ phase.