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A study was conducted to determine the processes which govern hold time crack growth behavior in the LSHR disk P/M superalloy. Nineteen different heat treatments of this alloy were evaluated by systematically controlling the cooling rate from the supersolvus solutioning step and applying various single and double step aging treatments. The resulting hold time crack growth rates varied by more than two orders of magnitude. It was shown that the associated stress relaxation behavior for these heat treatments was closely correlated with the crack growth behavior. As stress relaxation increased, the hold time crack growth resistance was also increased.The size of the tertiary γ' in the general microstructure was found to be the key microstructural variable controlling both the hold time crack growth behavior and stress relaxation. No relationship between the presence of grain boundary M 23 C 6 carbides and hold time crack growth was identified which further brings into question the importance of the grain boundary phases in determining hold time crack growth behavior.

A study was performed to determine the variables which influence hold time crack growth resistance of Alloy 10, a new powder metallurgy (P/M) superalloy. In a well controlled study, both the effect of compositional changes and variation in heat treatments were investigated. The results indicate that significant changes in the alloy's niobium, tantalum and cobalt content did not have an appreciable effect on hold time crack growth resistance. In contrast to the composition study, the heat treatments evaluated produced up to an order of magnitude changes in the crack growth resistance. Quantitative image analysis was performed to analyze the microstructural features produced by each heat treatment. It was found that the cooling precipitate size distribution is closely related to the measured hold time crack growth behavior. The larger the mean size of the cooling precipitates, the better is the resistance to hold time crack growth. It is proposed that the size and distribution of the precipitates might play an important role in determining the extent of crack tip relaxation which occurs through creep-type processes. The differences in the stress relaxation rate influence the crack driving forces and thus crack growth resistance.

Five heat treat options for an advanced nickel-base disk alloy, LSHR, have been investigated. These included two conventional solution heat treat cycles, subsolvus/oil quench and supersolvus/fan cool, which yield fine grain and coarse grain microstructure disks respectively, as well as three advanced dual microstructure heat treat (DMHT) options. The DMHT options produce disks with a fine grain bore and a coarse grain rim. Based on an overall evaluation of the mechanical property data, it was evident that the three DMHT options achieved a desirable balance of properties in comparison to the conventional solution heat treatments for the LSHR alloy. However, one of the DMHT option, SUB/DMHT, produced the best set of properties, largely based on dwell crack growth data. Further evaluation of the SUB/DMHT option in spin pit experiments on a generic disk shape demonstrated the advantages and reliability of a dual grain structure at the component level.

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