On the role of weak interface in the high cycle fatigue damage mechanism at elevated temperature in forged TNM-TiAl alloy

Abstract

As intermetallic alloy with complex three-phase microstructure, TNM-TiAl alloy offers significant advantages for replacing superalloys in the fabrication of aero-engine turbine blades. However, a deep understanding of the damage evolution behavior under high cycle fatigue (HCF) at high service temperatures is still lacking. Therefore, we propose an idea whether we can systematically clarify the deformation coordination mechanism and weak interface of the complex structure in tensile, and then deeply analyze the local plastic deformation and cracking behavior of HCF. Here, tensile tests at different strain rates were conducted at 550 °C, 650 °C, 750 °C, and 850 °C. Multiscale characterizations were performed on the fracture surfaces and cross-sections. Subsequently, HCF tests were conducted and the secondary cracks during the initiation and propagation were analyzed. Results indicate that 750 °C is the tensile ductile-to-brittle transition temperature. The deformation capability of each phase increases with temperature, and the γ-phase and β-phase are the primary microstructures responsible for coordinated deformation. The γ-phase interfaces are weak regions susceptible to cracking due to dislocation accumulation, resulting from the strength-plastic mismatch between the γ-phase and α-phase or lamellar colonies. At 850 °C, a grain refinement layer forms on crack initiating-surface, leading to that the endurance property at 850 °C is abnormally high.