Fatigue crack growth behavior in niobium-hydrogen alloys

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I.

INTRODUCTION

GROUP VB refractory metals and their alloys possess high elastic moduli, mechanical strength, and ductility over a wide range of temperatures.[1] The addition of hydrogen, however, decreases ductility, raises ductile-brittle transition temperature, and increases embrittlement of these metals. Numerous investigations have concentrated on the effect of hydrogen on the monotonic mechanical behavior of group VB metals.[2,3,4] Little attention, however, has been directed to the influence of hydrogen on their fatigue crack propagation behavior. Among the investigators in this field, Berger et al.[5] observed that the presence of hydrogen improved the high-cycle fatigue life and increased the tensile strength of titanium and a titanium-8 pct magnesium alloy. Chung and Stoloff[6] also found that hydrogen significantly improved the fatigue life of niobium. Owen et al.[7] showed that hydrogen in solution was the predominant cause for both strengthening and embrittlement in vanadium and vanadium-chromium alloys. Hydrides, however, appeared to make little or no contribution. Several crack closure models have been proposed to account for near-threshold crack growth behavior.[8,9,10] Kobayashi[11] found that the marked influence of load ratio (R) was due to oxide-induced crack closure. However, plasticity-induced crack closure became of importance to nearthreshold characteristics when oxide debris were excluded. Shih and Wei[12] showed that crack closure did occur in titanium alloys, but could not be regarded as the sole cause for the various phenomena observed in fatigue. Pendse and Ritchie,[13] on the other hand, found that the acceleration in fatigue crack growth rate (da/dN) due to hydrogen attack damage could be offset by an increase in roughness-induced crack closure. Similarly, Todd et al.[14] observed that inMARK CHING-CHENG LIN, formerly Graduate Student, Department of Mechanical Engineering, University of Houston, is Research Scientist, Materials Research Laboratories, Industrial Technology Research Institute, Hsinchu City, Taiwan, Republic of China. K. SALAMA, Professor, is with the Department of Mechanical Engineering, University of Houston, Houston, TX 77204-4792. Manuscript submitted May 17, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A

creased threshold stress intensity range (DKth) and decreased fatigue crack growth rate appeared to be a function of hydrogen embrittlement, causing metal wedges to form in the crack wake and contributing to crack closure. Eadie and Ellyin[15] modeled the stress of hydride precipitation in a delayed hydride crack of zirconium-2.5 pct niobium. The hydride was found to exhibit a higher fracture toughness at the crack tip in the solid solution alloy than in the bulk hydride. Recently, Smith[16,17] modeled the crack initiation and the crack growth of delayed hydride cracking and verified the model by experiments on a zirconium-hydrogen alloy. For a hydride region to fracture, the applied stress must override the compressive transformation stress generated within the

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