Relationship Between Microstructure and Fatigue Behavior of β-Phase Containing TiAl Alloys Prepared by Metal 3D Printing

TiAl alloys are expected to be used for low-pressure turbine blades in aircraft jet engines because of their light weight and excellent high-temperature strength, which contributes to improved fuel efficiency through weight reduction. In recent years, β-containing TiAl alloys that contain an ordered β phase (B2 structure) at service temperatures have been developed and have attracted attention as next-generation heat-resistant materials. In our previous study, we have reported that it is possible to obtain β-containing TiAl alloys with a nano α₂/γ lamellar structure in which grain boundaries are covered with a fine α₂/γ cellular structure using the electron beam powder bed fusion (EB-PBF), one of the metal additive manufacturing techniques. Furthermore, we also found that the alloys with this unique microstructure have excellent high-temperature strength-ductility balance. However, the fatigue properties of this alloy and the effects of each microstructure on the fatigue behavior have not yet been investigated. In this study, the morphologies of the nano-lamellar and cellular structures were controlled by post heat treatment, and the effects of each microstructure on the fatigue behavior were investigated, focusing particularly on the crack initiation and propagation behaviors. As a result, we found that the fatigue strength of the alloys at 750°C was superior to that of the cast Ti-48Al-2Cr-2Nb alloy even without a hot isostatic pressing process. This is considered to be attributed to the suppression of fatigue crack propagation by the α₂ phase in the fine cellular structure. In addition, ductile γ phase in the fine cellular structure suppresses crack initiation, resulting in excellent fatigue properties.