Hydrogen embrittlement of iron-nickel alloys
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INTRODUCTION
IN many alloy systems, hydrogen embrittlement under relatively high fugacity conditions occurs by intergranular fracture. One of these is the Fe-Ni alloy system which exhibits intergranular hydrogen-related fracture when tested with a high supersaturation of solute hydrogen or while being cathodically charged with hydrogen. tl,21 The results obtained for this alloy system indicate that the hydrogen embrittlement susceptibility decreases as the iron concentration of the alloy increases.it,2] While there is no known basis for this phenomenon, it is of significance, as the Fe-Ni system is the basis of many commercially important alloy systems. These early measurements contained no information about the grain boundary chemistry, and it is known that the effects of hydrogen are generally sensitive to the concentrations of other elements at grain boundaries. One particular element, S, is known to increase the susceptibility of Ni and its alloys p 71 and Fe and its alloys 18,9,1~ to hydrogen embrittlement, although the mechanism of this increased susceptibility is not known. Yoshino and McMahon I81 suggested that the cohesive energy of the prior-austenitic grain boundaries in steel were decreased by the presence of both S and H and that the plasticity which accompanies fracture is also decreased. I~tl While a direct demonstration of the decrease of the cohesive energy by either segregated S or H has not been obtained, the decrease of plasticity which accompanies S segregation at grain boundaries in Ni has been directly observed. 171Similar conclusions were drawn for pure iron by Shin and Meshii,[I~ who suggested that the effects of each element on the fracture were independent and additive. Both of these experiments utilized solute hydrogen for the embrittlement. In the case of hydrogen introduced from an aqueous solution, it was suggested I3~ that the presence of metalloids such as S at the grain boundaries caused intergranular embrittlement by facilitating the entry of hydrogen, with the metalloids acting as hydrogen recombination poisons at the metal surface, implying that the effects are synergistic rather than simply additive. It is difficult to determine whether the effects of the segregated S and hydrogen are additive or synergistic on W.Y. CHU, Research Associate, is with the Department of Metallurgy, Carnegie Mellon University, Pittsburgh, PA 15213. H.K. BIRNBAUM, Director, is with the Materials Research Laboratory, University of Illinois, Urbana, IL 61801. Manuscript submitted April 29, 1988. METALLURGICAL TRANSACTIONS A
the basis of the available results. The measurements of Bruemmer eta/. [4,5,9] clearly show that the hydrogen fugacity during cathodic charging, which is necessary for intergranular fracture of Ni and Fe, decreases as the amount of S segregated at the grain boundaries increases. Similar effects have been shown for solute hydrogen in Ni, where the amount of hydrogen required for intergranular fracture is greatly decreased by even partial monolayer S segregation, t5'61 These eff
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