Influence of Prolonged
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The authors would like to acknowledge the Texas Higher Education Coordinating Board for the financial support of this research through Grant No. 003661-018.
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6. S.K. Varma and B.G. LeFevre: Metall. Trans. A, 1980, vol. 11A, pp. 935-42.
Influence of Prolonged Thermal Exposure on Intergranular Fatigue Crack Growth Behavior in Alloy 718 at 650 ~ D. ZHENG a n d H . GHONEM Alloy 718 is a precipitation-hardening nickel-base superalloy which was developed to provide high creep resistance and high stress rupture at elevated temperatures. It has been extensively used in the fabrication of disks for gas turbines. Regions of these disks can experience thermal exposure in service condition up to 650 ~ and to durations extending 30,000 hours, tl,2,31 As have been pointed out by Radavich tS] and Molins et al., t2] the prolonged thermal exposure at 650 ~ of alloy 718 can introduce overaging effects that lead to certain microstructural alterations. These modifications, particularly the coarsening of ~" phase, were shown to have strong effect on the mechanical properties of this material, t2,a-TJ Furthermore, a number of investigators t4,8,91 have reported that the overaging heat treatment [927 ~ for 10 hours/air cooled (AC) + 732 ~ for 48 hours/ AC] or modified heat treatment [1093 ~ for 1 hour + 38 ~ to 538 ~ + He cooled to room temperature (RT) + 718 ~ for 4 hours + 38 ~ + 621 ~ for 16 hours + He cooled to RT] for alloy 718 appears to improve the high-temperature crack growth resistance when subjected to low frequency loadings in air environment. Floreen and Kane tSj speculated that homogenization of slip and, consequently, minimization of localized stress concentrations at the grain boundaries through changes in precipitate morphology are a way in which overaging heat treatment could increase the resistance to high-temperature crack growth. Smith and Michel t41 suggested that the improvement in crack growth resistance produced by the modified heat treatment is likely to be a result of the alteration of mechanisms of dislocation interactions with 7" particles and grain boundaries. Sadananda and Shahinian tgJ observed that the overaging heat treatment increases the tensile ductility of the alloy by nearly 50 pct and believed that this causes the crack tip to blunt readily, thus decreasing the crack tip sensitivity to environmental effects. In the work of Smith and Michel, t4~ however, severe embrittlement was evident in materials which had been subjected to the modified treatment then tested at 593 ~ The intent of the work reported here is to examine further the relationship between microstructural changes during prol
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