The Effect of Stress State on Internal Hydrogen-Induced Crack Growth in Ti-6AI-6V-2Sn
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INTRODUCTION
H Y D R O G E N embrittlement occurs in many alloy systems under a variety of environmental conditions. From the large volume of hydrogen embrittlement data published, no single failure mechanism has been found to account for all the observed behavior. ~ Current thinking is that hydrogen embrittlement in steels is due to a lattice decohesion process) In a//3 titanium alloys, fracture of strain-induced hydrides is responsible. 3"4 Even though the failure mechanisms are different for steel and titanium alloys, hydrogen embrittlement in both is dependent on hydrogen concentration, microstructure, yield stress, and temperature, s-9 Hydrogen may be provided internally from processing or externally from the environment. In either case the fracture process is the same although the kinetics may differ considerably. ~'m~ Williams and Nelson H conducted the first definitive study on the kinetics of crack growth in an a / / 3 titanium alloy tested in a gaseous hydrogen environment. Their results led to the conclusion that the diffusion of hydrogen through the continuous /3 matrix was the rate controlling process. Recent studies on similar titanium alloys have examined crack growth as a function of internal hydrogen, m2'13 The rate controlling process was found to be the same as that determined in a gaseous hydrogen environment, but crack growth rate behavior differed. In a gaseous hydrogen environment, the major portion of crack growth occurred independent of stress intensity. When hydrogen was provided internally, the rates were very dependent on the applied stress intensity. Early studies on high strength steels showed that embrittlement was also sensitive to the stress state.5'14 Of all the variables which influence hydrogen-induced crack growth, stress state is the least understood. Steel and titanium alloys tested under plane strain conditions have exhibited crack growth rates up to 400 times faster than samples tested under mixed stress state conditions. 15'~6When threshold stress inN. R. MOODY, formerly a Graduate Student at the University of Minnesota, is now with the Materials Development Division, Sandia National Laboratories, Livermore, CA 94550. W.W. GERBERICH, Professor, is with the Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455. Manuscript submitted August 14, 1981. METALLURGICAL TRANSACTIONS A
tensities were determined, they exhibited a lower limiting value under plane strain conditions and increased appreciably as plane stress conditions were approached, m8 The variables influencing hydrogen embrittlement interact making it difficult to isolate individual effects. A recent study 13 on Ti-6A1-6V-2Sn determined both thresholds and crack growth rates as a function of temperature under plane strain conditions. This study was extended in the present work to determine the effect of thickness, and therefore stress state, on both thresholds and crack growth rates due to the internal hydrogen concentration. The threshold model previously devel
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