The role of sulfur in the air embrittlement of nickel and its alloys
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INTRODUCTION
THE embrittlement of nickel by sulfur segregated to grain boundaries has been extensively reported in the literature. ~-6 At high temperatures of testing, where the bulk solubility is large, no embrittlement is observed to quite high levels of sulfur. 4 At intermediate temperatures, where the grain boundary segregation is marked and the kinetics of segregation still rapid, severe embrittlement is seen even at moderate sulfur concentrations, say 10 to 20 ppm. 3'4 At low temperatures, ductility remains high at these moderate sulfur levels, but not at higher concentrations. 3'4 This may simply be reflective of the slower kinetics at these temperatures allowing for less segregation to the boundaries, as it is also known that the embrittlement seen at low temperatures is a function of cooling rate from the higher.6 The faster quenches produce less segregation and more ductile alloys. It is also expected that the embrittlement will be a sensitive function of the amount of impurity elements present, as those which form stable sulfides will effectively reduce the free sulfur levels in the alloy. Indeed, this characteristic was employed over one hundred years ago to produce the first malleable nickel by additions of magnesium 7 or manganese, 8 although at the time the mechanism was not understood. Today, many grades of commercially available "pure" nickel contain sufficient sulfur to render them brittle at all testing temperatures below the very highest; however, good ductilities are achieved by controlled additions of the type described above. For example, Ni200 (International Nickel Company, 99.6 pct Ni) contains about 50 ppm sulfur but also 2000 ppm of the sulfide former manganese. In addition, such sulfur levels are often encountered in many nickel-base superalloys, but again the sulfur is chemically combined as sulfides or carbo-sulfides. Sulfides have generally lower heats of formation than the corresponding oxides, the exceptions being potassium, ruR.H. BRICKNELL and D. A. WOODFORD are both Staff Members, Metallurgy Laboratory, General Electric Corporate Research and Development Center, Schenectady, NY 12301. R . A . MULFORD, formerly with the General Electric Corporate Research and Development Laboratory, Schenectady, is now with General Electric, Knolls Atomic Power Laboratory, Schenectady, NY 12301. Manuscript submitted October 26, 1981. METALLURGICAL TRANSACTIONS A
bidium, and cesium. Moreover, the heat of formation of most oxides is in excess of that of sulfur dioxide, and hence, in the situation where there is a limited supply of oxygen, the reaction MS + [O] ---->MO + S, where M is a metal, would be expected to occur. Recently it has been reported that a range of nickels 9 and nickel alloys ~~ embrittle after high temperature exposure in oxygen containing environments, and that this embrittlement is related to the penetration of grain boundaries by this species. 9'tt't3 Here is precisely that situation of the limited oxygen supply envisioned above, and it is possible that the reaction sugges
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