The effect of aging on hydrogen embrittlement of a nickel alloy
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THE application of thermomechanically strengthened Ni-base alloys in H-bearing environments has stimulated recent studies of the effects of the thermomechanical treatment (TMT) on embrittlement resistance? -7 One such alloy, HASTELLOY* Alloy C-276 *Registered trademark, Cabot Corporation.
(55Ni 16Cr 16Mo 5Fe 4W 2Co), has been found to be susceptible to hydrogen embrittlement (HE) following cold working and aging at relatively low temperatures (200 to 500 ~ where no obvious microstructural modification occurs. The HE of Alloy C-276 has been documented, but there has been no agreement as to its mechanisms.4-7 It has been suggested that either of two distinct aging induced phenomena play a primary role in embrittlement of Alloy C-276. Berkowitz and Kane 5 have suggested that the segregation of P to grain boundaries during aging of the cold worked alloy causes the increase in embrittlement. The P may retard the formation of molecular H 2 from reduced H ยง at the metal surface, thereby increasing H absorption and embrittlement along the grain boundaries. (Alloy C-276 typically contains about 0.04 wt pct P; the alloy described in this paper contained 0.042 pct P). Their proposal is based on two experimental observations: 1) as aging time at a given temperature increases, the grain boundary P content increases, as measured by Auger electron spectroscopy, and embrittlement also increases, and 2) decreasing the bulk P content results in both lower P segregation during aging and improved embrittlement resistance. Asphaphani, 6 however, has attributed the increase in embrittlement to a change in slip mode caused by aging induced ordering of the alloy. Ordering is believed to cause planar slip in the R. J. COYLE, JR., formerly Graduate Assistant, Dept. of Met. Eng. & Mat. Sci., University of Notre Dame, is now with the Laser Study Group, Western Electric Co., P.O. Box 900, Princeton, NJ 08540. J. A. KARGOL is Assistant Professor and N. F. FIORE is Chairman, Department of Metallurgical Engineering and Materials Science, University of Notre Dame, Notre Dame, IN 46556. Manuscript submitted July 14, 1980. METALLURGICAL TRANSACTIONS A
alloy which may enhance crack nucleation at sharp surface steps, increase internal stress concentrations at dislocation pileups, or promote accelerated transport of H by a dislocation drag mechanism. To date, neither of the models has been shown to account completely for all aspects of embrittlement in Alloy C-276. For example, the segregation model can not account for the mixed mode fracture reported under certain test conditions.6 The slip mode hypothesis, on the other hand, has not been able to account for embrittlement in samples aged for such short times that order is not observed? The purpose of this investigation was to isolate the major factors that cause an increase in embrittlement of cold worked Alloy C-276 following aging. In previous studies it was not possible to separate the effects of segregation and ordering on embrittlement. This paper describes the results of a series of tests whic
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