The Effects Of Long-term Air Exposure On The Microstructures And Mechanical Properties For W+Nb Containing Fine-grained Lamellar TiAl Alloys

The segregation of B2+ωblocks at grain boundaries was investigated for a fine-grained Nb+W containing cast TiAl alloy and the ability to change the segregated B2+ωthrough changing the hot isostatic pressing (HIP) temperature (from 1260℃to 1340℃) was assessed. It was found that the 4 vol. % B2+ωblocks retained after 1260℃HEPping were almost eliminated totally by raising the HIP temperature to the single a phase field (1340℃).The main objective of the research was to determine the effects of long-term air exposure, in the temperature of 700℃for up to 10000h, on the stability of the microstructure and mechanical properties of the W+Nb containing TiAl alloy. It was found that after long-term high temperature exposure, considerable decomposition of metastableα₂ laths occurred in the direction parallel to theα₂/γlamellar interface, while little decomposition occurred along the direction perpendicular to theα₂/γlamellar interface. In contrast to decomposedα₂ laths,γlaths were observed to be quite stable after 10000h exposure. No grain recovery, recrystallization and decomposition of y laths were recorded. On the other hand, Oswald coarsening ofωparticles in retained B2+ωblocks was observed during the thermal exposure process, during which some of B2 was still retained. It was interesting to find that a small amount of the B2+ωblocks which were eliminated originally by 1340℃HIPping can re-form in theα₂+γlamellae near the grain boundaries through theα₂+γ→B2+ωphase transformation. This was attributed to a higher degree of enrichment in Nb and W in such lamellar regions, near which dissolving of retained B2+ωblocks once occurred. After a long-term thermal exposure, B2+ωcould be induced from the Nb+W enriched areas. It was noted that such a phase transformation from lamellae packets at later exposure stage was not a widespread phenomenon, and therefore there were no significant detrimental effects on the mechanical properties of the alloy. The tensile properties and S-N fatigue limit were assessed at room temperature systematically for the fine-grained Nb+W containing alloy before and after exposure. (1) For the alloy before exposure, the tensile and S-N fatigue properties obtained were not quite changed with increasing HIPping temperature (from 1260℃to 1340℃). This suggests that the 4 vol. % B2+ωblocks segregated along grain boundaries did not affect the mechanical properties considerably. (2) For the alloy after thermal exposure, regardless of HlPping temperature, no noticeable deterioration in the tensile and S-N fatigue properties were observed throughout the prolonged exposure process. This indicates that the W+Nb containing fine-grained cast TiAl alloy is relatively stable in terms of its resistance against thermal deterioration.