Effect of Heat Treatment on the Workability of Hot Isostatically Pressed TNM-B1

Titanium aluminides (TiAl) hold attractive material properties for a range of industries. Specifically, hot forged TiAl turbine blades are seeing implementation in commercial aircraft engines due to their reduced weight and similar strength as compared to the conventional nickel-based blades. Usually, these blades are hot forged from cast ingots, which have undergone hot isostatic pressing (HIP) to remove casting porosities. However, the low workability of the HIPed microstructure can cause long processing times and excessive wear on the costly molybdenum tools due to high forging stresses. These factors lead to a high unit cost for TiAl forgings. This unit cost can be reduced by introducing a heat treatment step to the isothermal forging process. In this work, a new two-step heat treatment strategy (1300 degrees C for 1 h, 1100 degrees C for 5 h) is developed with the aim of softening the HIPed microstructure to increase workability of the TiAl alloy TNM-B1 for hot forging. The samples were deformed inside the industrial forging window of TNM-B1 at the temperatures 1125, 1150, 1175 and 1200 degrees C, with a strain rate of 0.005 s(-1). The microstructure characteristics and deformation behavior of the heat-treated samples were analyzed and compared to their HIPed counterparts. The results revealed a significant reduction in both peak stresses and steady state stresses of the heat-treated samples. SEM images indicate that the heat treatment resulted in a reduction of both; the number of the less deformable lamellar colonies and the relative fraction of lamellar colonies. In addition, a higher degree of precipitation of the highly deformable beta-phase was observed in the heat treated samples. The main conclusion drawn from this work is that the heat treatment strategy can result in higher hot workability for TNM-B1, which in turn can allow for accelerated forging speeds or reduced forging temperatures.