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.

The effects on fatigue life of high temperature exposures simulating service conditions were considered for two disk superalloys. Powder metallurgy processed, supersolvus heat treated Udimet 720 and ME3 fatigue specimens were exposed in air at temperatures of 650 to 704 C, for times of 100 h to over 1000 h. They were then tested using conventional fatigue tests at 650 and 704 C, to determine the effects of exposure on fatigue resistance. Cyclic dwell verification tests were also performed to contrast the effects of intermixed exposures and fatigue cycles. The prior exposures reduced life by up to 70% and increased the scatter in life, compared to unexposed levels. Cyclic dwell tests reduced lives even more. Fractographic evaluations indicated the failure mode was shifted by the exposures and cyclic dwells from predominantly internal to often surface crack initiations. The increased scatter in life was related to the competition between internal crack initiations at inclusions or large grains producing longer lives, and surface crack initiations at an environmentally affected surface layer producing shorter lives.

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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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