Accelerated hot deformation and heat treatment of the TiAl alloy TNM-B1 for enhanced hot workability and controlled damage

Abstract

The titanium aluminide alloy TNM-B1 displays a strong softening behavior (i.e. flow stress reduction) during hot deformation. By taking advantage of this, the deformation speed can be increased according to the rate of softening. Thus, the deformation process can be accelerated, leading to reduced costs for formed TNM-B1 parts. However, accelerated deformation can lead to increased workpiece damage. Therefore, a heat treatment prior to deformation was utilized in order to both produce a desirable microstructure for accelerated deformation conditions and to prevent increased damage. In this work, a cast and hot isostatically pressed (HIPed) TNM-B1 alloy was investigated and compared to a HIPed and heat treated version under constant strain rate conditions at the temperatures 1150, 1175 and 1200 °C. The developed method of applying accelerated deformation was based on a damage model and a material model that were fitted to titanium aluminides. For constant strain rate conditions, the heat treatment improved both the hot workability and the damage tolerance of the HIPed material state. Therefore, only the heat treated state was investigated under accelerated strain rate conditions at the same constant temperatures. The results show that, depending on deformation temperature, the processing times can be reduced by around 51.4–59.6% by applying accelerated deformation with an initial strain rate of 0.0013 s−1 and by around 56.1–69.3% for an initial strain rate of 0.005 s−1, compared to deformation at the corresponding constant strain rates. Furthermore, the amount of damage observed for accelerated deformation was comparable to constant strain rate deformation. The microstructure characteristics were similar for accelerated and constant strain rate deformation at 1200 °C. However, increased lamellar fractions occurred at 1150 and 1175 °C for accelerated deformation compared to constant strain rate deformation. © 2020 Elsevier B.V.