Prior deformation effects on creep and fracture in inconel alloy X-750
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I.
INTRODUCTION
PRIORroom temperature deformation effects on creep and fracture have been studied in pure metals, single phase alloys, and engineering materials. J-5 These studies do not exhibit a clear systematic trend on the creep behavior. This is not surprising since there are many variables. These include material characteristics, microstructure, testing temperature, and testing procedures employed. For example, in a Ni-0.1 at. pct Au alloy, there was an improvement in creep properties at a high percentage of prestraining. J In engineering materials like Nimonic 80A, 2 prestraining had a deleterious effect on creep properties. In Inconel 617, 3 prestraining beyond the limit of 5 pct tends to decrease the rupture strength at 1000 ~ In Nimonic 105, a 5 pct prestrain had little4 effect on the rupture life, but marked reductions in creep life were found with greater amounts of prestraining and the detailed creep and fracture behavior were found to be dependent on the exact testing procedures adopted. In all the cases, creep fracture strain was always reduced. Dyson and Rodgers 2 and Dyson and Henn 5 showed that room-temperature prestraining nucleates grain boundary voids whose growth and interlinkage during creep testing led to premature intergranular fracture and reduction in creep fracture strain. The mechanism with regard to the incidence of low fracture strain is now somewhat understood6'7 although some important kinetic details remain to be clarified. Previous work s on lnconel alloy X-750 has shown that edge cracks are produced in the surface regions of the specimens during testing. The cavities or cracks were observed only near the fracture surface and not along the whole gauge length (Figure 1). Final failure appears to occur by plastic instability, although the role of testing environment cannot be ruled out in assisting the fracture. The objective of the present work was to investigate whether it was possible to alter the mechanism of failure by varying the amount of prior plastic deformation, which is an important issue both for ultimately understanding the creep fracture process and for practical consideration in engineering service. M.C. PANDEY, Postdoctoral Fellow, and A.K. MUKHERJEE, Professor, are with the Division of Materials Science and Engineering, Department of Mechanical Engineering, University of California, Davis, CA 95616. D. M. R. TAPLIN is Professor and Chairman, Department of Mechanical and Manufacturing Engineering, Trinity College, University of Dublin, Dublin-2, Ireland. Manuscript submitted November 7, 1983. METALLURGICALTRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
The nominal composition of Inconel alloy X-750 was (wt pct): 0.05 C, 14.81 Cr, 6.51 Fe, 2.68 Ti, 0.80 AI, 0.87 Nb, 1.0 max. Mn, 0.5 max. Si, 0.01 max. S, 0.5 max. Cu, and balance Ni. Bars of nominal size, 12.5 mm and 12.5 ram, were given a proprietory heat-treatment sequence 9 in air, viz., 4 hours at 1150 ~ air cool; 24 hours at 840 ~ air cool; 20 hours at 710 ~ air cool, which resulted in a mean linear intercept grain
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