| Summary: | The effects of thermal exposure on the bulk microstructure and surface properties of a duplex gamma alloy with a nominal composition of Ti-45AI-8Nb-0.2C (atomic percent) have been studied at temperatures of 750°C for exposure times of 500 and 1000 hrs. The microstructure of this alloy was found not to be stable under such exposure. Additional phases were observed to precipitate. Tensile properties were, however, not altered by the formation of these additional phases, although, these additional phases appear to have some influence on the room temperature fracture toughness and fatigue crack growth resistance. There was no significant change in the fracture stress before and after exposure and it is suggested that the fracture micromechanism of this material is best described by a mixed stress and strain failure criterion, regardless of thermal exposure. The formation of an oxide layer (less' than a micrometer) was observed to change dramatically the surface roughness and hardness. The present study concludes that fatigue strength (at 107 cycles) decreases after thermal exposure but that the formation of the oxide layer at the surface itself has no impact on the fatigue strength. This observation is consistent with the general observation that fatigue crack initiation is sub-surface on all testpieces which approach lives of up to 107 cycles.
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