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The influence of microstructure, in particular secondary gamma primeprecipitates, γ’S, on intergranular cracking of the P/M IN100 is investigated. A series ofheat treatments using coupons made of as-received material is carried out in order toidentify conditions leading to minimum and maximum variation in the size and volumefraction of secondary gamma prime, γ’S. These specific heat treatments are applied tocompact tension specimens to regenerate the corresponding γ’S statistics. In addition tothese microstructures, specimens subjected to extended aging time as well as longthermal exposure at 650°C have been generated and characterized. Fatigue crackgrowth experiments are performed on compact tension specimens having as received aswell as modified microstructures at both 650°C and 700°C in air. The loading cycleincludes a dwell time ranging from 100 seconds to 7200 seconds and superimposed atthe maximum load level. Results of the as received material show that the length of thedwell time does not influence the crack growth rate. Results of the heat treatedmicrostructures at 650°C showed that a reduced volume fraction of γ’S enhances thegrowth rate. The influence of the volume fraction of γ’S on altering the intergranularcrack growth rate is explained in terms of the crack tip stress relaxation.