The Thermal Stability Of A High Nb+Hf Containing TiAl Based Alloy Subjected To A Long-Term Exposure At 700℃ In Air

A fully lamellar TiAl alloy Ti-44Al-4Nb-4Hf-0.2Si-1B was exposed at 700℃for up to 10000 hours in air to assess the thermal stability in the present research. The changes in microstructure and phase structure were characterized using both transmission electron microscopy and scanning electron microscopy. The corresponding changes in mechanical properties were examined by tensile testing and S-N fatigure limit tesing at room temperature.It was found that the alloy produced from hot isostatic pressing and isothermal forging in a single phase region was a fully lamellar alloy, with small amunts of equiaxialγgrains and B2(ω) phase at colony boundaries. The volume fraction of B2(ω) phase was about 3%. There were some borides spreading widely in the alloy.It was found that both the decomposition ofα2 lamellae throughα2â†'γand the formation of B2(ω) throughα2+γâ†'B2(ω) occurred to some degree inside theα2+γlamellar colonies. After 10000-h exposure, the average thickness ofα2 lamellae was roughly halved while the area fraction of B2(ω) with micron/submicron size range was increased from～3% to 8.4%.As a result, tensile ductility at room temperature was reduced by one third after 10000-h exposure at 700℃. However, no detrimental effect on tensile strength and high cycle fatigue limit was produced by the long-term thermal exposure. After 10000-h exposure, the proof stress was still at a level of 600MPa, while the S-N fatigue limit somehow managed to increase noticeably, although the empirical data was quite scattered. The results indicate that the Nb-Hf containing TiAl alloy demonstrates a higher thermal stability than its Nb-containing counterparts due to reduced influence of the "oxygen-release induced embrittlement" and "B2+ω-formation induced embrittlement"...