The effect of hydrogen on fracture toughness of the Fe-Ni-Co superalloy IN903
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I.
INTRODUCTION
I T is well established that hydrogen significantly reduces the resistance to crack initiation and propagation in many alloy systems. As a result, determination of hydrogeninduced fracture susceptibility is of critical importance to many materials applications in hydrogen-producing environments. Two measures of crack growth resistance are rising load fracture toughness and in situ slow crack growth thresholds under sustained or decreasing (constant displacement) loads. An extensive data base exists on fracture toughness of bcc steels 1 5 in the absence of hydrogen and hydrogenaffected slow crack growth thresholds in bcc ~-H and fcc alloys. 12.13.14However, data on fracture toughness as a function of hydrogen concentration are sparse due to experimental difficulties. The relatively high effective diffusivities and low solubilities of hydrogen in many bcc alloys result in rapid offgassing from the near crack tip region. In contrast, the relatively low effective diffusivities and high solubilities of hydrogen in fcc alloys necessitates long term, high pressure hydrogen charging at elevated temperatures for saturation. Recent work ~5on the Fe-Ni-Co superalloy INO03 showed that hydrogen significantly reduced slow crack growth thresholds and fracture toughness values. The slow crack growth thresholds were lower than the fracture toughness values at similar hydrogen concentrations. Furthermore, the fracture modes differed. While fracture occurred by intergranular failure in slow crack growth samples, it occurred by microvoid coalescence in uncharged fracture toughness samples and predominantly by slip band fracture in the charged samples. Of the various fracture modes observed, hydrogen-induced slip band fracture was the most intriguing. While it has been observed in bcc steels 16'17'18and fcc alloys, t9"2~ slip band fracture has not been extensively
*IN is a trademark of the INCO family of companies. N.R. MOODY, R.E. STOLTZ, and M.W. PERRA are with Sandla National Laboratories, P O. Box 969, Livermore, CA 94550. Manuscript submitted November 13, 1986. METALLURGICAL TRANSACTIONS A
studied. A recent study 22 of hydrogen-induced slip band fracture in IN903 found that it is a ductile failure process characterized by microvoid formation at slip band intersections. Although the mechanisms of slip band fracture in tensile and fracture toughness samples appears similar, fracture toughness values cannot be directly predicted from the tensile results. In recent studies, 23.24fracture toughness models have been developed based on a continuum description of crack tip stress and strain fields and the micromechanisms of fracture. Early ductile fracture toughness models of McClintock 25 and Rice and coworkers 26'27employed the criterion that failure initiates when the intensely strained region directly ahead of a crack tip envelops the fracture initiation sites. This occurs when the crack tip opening displacement is approximately half the interparticle spacing of particles that govern the fracture process. However,
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