Micro-structural damage and mechanical response of TiAl alloy with lamellar microstructure during thermal shock progress

Abstract

Considering the service safety for TiAl alloy under thermal shock, the effect of thermal shock temperature and cycles on micro-structural degeneration and mechanical properties were studied. The results show that the vertical decomposition of α₂→β₀ phase transition occurs in α₂ lamellae if the single thermal shock temperature increases to 1000 °C or the thermal shock cycle increases to 10 times at 900 °C. Cracks form during the thermal shock progress as the single thermal shock temperature increases to 1000 °C or the number of thermal shocks is 5 times at 900 °C. Simultaneously, the flexural strength of samples decreases to 70% of the initial flexural strength under the same condition. It indicates that Ti–45Al–4Nb–1Mo-0.1B alloy fails as the single thermal shock temperature is above 1000 °C or the number of thermal shocks exceeds 5 times at 900 °C, and cracks generated during the thermal shock progress cause the dramatic deterioration in flexural strength. Meanwhile, the formed cracks are coarse, and the propagation path is relatively straight, which further deteriorates the flexural strength of the Ti–45Al–4Nb–1Mo-0.1B alloy. Besides, the nucleation and propagation of cracks change with the thermal shock temperature and cycles were analyzed. When the single thermal shock temperature is 1000 °C, the nucleation of cracks is not only in lamellar colony boundaries but also in α₂/γ lamellar interfaces and the propagation path of cracks is relatively straight compared with the formation of cracks after 5 thermal shocks at 900 °C, because the thermal stress increases, and then cracks directly pass through lamellar colonies when the angle between lamellar orientation and crack propagation direction is about 90°.