Gas phase embrittlement of nickel by sulfur
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I.
INTRODUCTION
THE history of nickel metal is punctuated by its relationship with sulfur. In the 1870s, it was first learned that the otherwise brittle metal could be made malleable by additions o f manganese and magnesium. These elements were, in fact, combining with the embrittling sulfur and rendering it innocuous, although this was not understood until about 1925. m Subsequently, it was found that even properly melted nickel could become embrittied if it later came in contact with sulfur sources, such as lubricants, dirt, and sulfurous fuel vapors, during any of the heating operations of the manufacturing process. ~2~ It became axiomatic to avoid sulfur exposure during processing. More recently, it has become apparent that hightemperature nickel alloys will be operating in increasingly corrosive environments, especially those containing sulfur. Most of the emphasis to date, in terms of the interaction of high-temperature alloys with sulfurcontaining environments has been concerned with surface chemical reactions and metal loss due to hot corrosion. However, in recent years, there has been increased awareness that such sulfur-containing environments may lead to pronounced enhancement in crack growth rates in both cyclic r3,41 and sustained loading, ts,61 There have been few attempts to understand the mechanism of this crack growth enhancement, although there are clear similarities with the less dramatic oxygen enhancement of fatigue crack propagation, t7,81 In the latter situation, the effect is most likely due to crack tip penetration and embrittlement of the material ahead of the growing crack, t9,~~ The kinetics of such gas phase emJ.P. BECKMAN, formerly with the Materials Engineering Department, Rensselaer Polytechnic Institute, is Director of Technical Services, Crucible Services Centers, Camillus, NY. D.A. WOODFORD, Professor of Materials Engineering, is with the Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. Manuscript submitted January 16, 1990. METALLURGICAL TRANSACTIONS A
brittlement (GPE) for oxygen-containing environments have been measured by exposing specimens of nickel t~z] and nickel-base alloys r9] to various partial pressures of oxygen prior to subsequent mechanical property measurements. It appears probable that GPE by sulfur would show similar characteristics. Indeed, some preliminary tests have demonstrated that such exposure embrittles both nickel and a nickel-base superalloy (IN738). [~3] This study concerns the embrittlement of nickel caused by sulfur vapor. Its premise is that the intergranular penetration of sulfur from the environment should lead to the same reduction in grain boundary cohesion as that commonly caused by intergranular sulfur segregation from the matrix. Accordingly, pre-exposures of sufficient time and temperature in sulfur-containing environments should embrittle the grain boundaries and result in a reduction of ductility and fracture resistance during subsequent mechanical testing in air. The depth of the intergranular mode
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