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In this study, the microstructural evolution of Inconel alloy 740 during solution treatment and aging was characterized using optical and scanning electron microscopy. During double solution heat treatment, carbon is liberated from the dissolution of MC carbides during the first solution treatment at 1150°C, and fine MC carbides are precipitated on gamma grain boundaries during the second solution treatment at 1120°C. Due to the concurrent decrease in carbon solubility and the increase in the contribution of grain boundary diffusion at lower temperatures, the MC carbides on the gamma grain boundaries provide a localized carbon reservoir that aids in M 23 C 6 carbide precipitation on gamma grain boundaries during exposure at 760°C. The c¢ phase, which is the key strengthening phase in alloy 740, is incorporated into the alloy microstructure during aging at 850°C. The main source of microstructural instability observed during exposure at 760°C was the coarsening of the c¢ phase.

The effect of the addition of 5 at. pct boron on the microstructure and creep behavior of a nominally Ti-22Al-26Nb (at. pct) alloy was investigated. The boron-modified alloy contained boride needles enriched in titanium and niobium, and because to these borides, this material was considered to be a discontinuously reinforced metal matrix composite. These needle-shaped borides made up to 2 pct of the volume and were up to 158-lm long and 22-lm wide. The effect of boron on the mechanical properties was evaluated through in-situ creep testing and tensile testing at room temperature (RT) and 650°C. Overall, the addition of 5 at. pct boron proved to be detrimental to the tensile and creep behavior. The composite exhibited a brittle failure and lower elongations-to-failure than the monolithic material. The in-situ tensile and creep experiments revealed that the deformation process initiated in the boride needles, which cracked extensively, and significantly greater primary creep strains and creep rates were exhibited by the composite.