Impact of β-phase in TiAl alloys on mechanical properties after high temperature air exposure
Résumé
In this paper, the influence of high temperature air exposure on the tensile properties at room temperature and on the fatigue strength at high temperature of two TiAl alloys, is studied. The alloys, Ti-48Al-2Cr-2Nb (Ti-48-2-2), and Ti-44Al-4Nb-1Mo-0.1B (TNM-B1), both exhibit near-gamma microstructures, but the TNM-B1 alloy contains significant amounts of beta phase. Air exposure at high temperature (650-700°C, 500 h) induces significant loss of ductility, and decrease of fatigue strength at high temperature, in the case of the beta-containing TNM-B1 alloy, but not for the gamma-Ti-48-2-2 alloy. This indicates a potential influence of the presence of beta phase for embrittleme nt. Because embrittlement is believed to be the consequence of microstructure modifications in the sub-surface, first attempts have been performed to investigate these local phenomena. For this purpose, SEM, FIB, STEM-EDX and ACOM techniques have been employed. Destablilization of beta and, to a lesser extent, of gamma into alpha2, has been observed in the sub-surface of the TNM-B1 alloy. We interpret this as a consequence of oxygen diffusion during the high temperature air exposure, which would promote formation of alpha2 phase in which oxygen solubility is high, at the expenses of the beta and gamma phases of lower oxygen solubility.
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