A comparative study of the embrittlement of monel 400 at room temperature by hydrogen and by mercury
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INTRODUCTION
S E V E R A L authors have remarked upon similarities between hydrogen embrittlement (HE), liquid metal embrittlement (LME), and/or stress corrosion cracking.~ 9 A concise summary of resemblances and differences, which includes temper embrittlement also, is given by Stoloff.~~ Lynch" ~5 has recently made comparisons between hydrogen and mercury embrittlement in nickel single crystals, an AI-6Zn-3Mg alloy, Ti-6AI-4V, and a high strength steel, D6-ac. He has suggested that a common mechanism exists for HE and LME involving enhanced dislocation nucleation at crack tips, with the crack growing by an alternative slip sequence. He has observed small microvoids ( - 0 . 1 /zm diameter) on intergranular fracture surfaces, and believes that his model is applicable to intergranular, as well as transgranular, cracking. This paper reports comparative observations upon the embrittlement of Monel 400 (UNS N04400) by hydrogen and by mercury at room temperature. Tensile tests were performed in both environments, with strain rate and grain size as variables. Some testing also was done upon cold worked specimens. The intent was to study a simple polycrystalline fcc alloy. Monel 400 was chosen because it was known to be much affected by both hydrogen and mercury. ~6-2~ It is a nickel-copper solid solution alloy with a nominal Ni/Cu ratio of 2/1. 21 This alloy is more embrittled than either constituent. Thus, h y d r o g e n e m b r i t t l e s nickel22-27 but not copper, 2~ while mercury embrittlement of copper has been observed by some, but not all, investigators. 29-33 Mercury embrittles nickel but to a lesser extent than does hydrogen. 34 There has been one other study of hydrogen and mercury embrittlement in Monel 400, by Funkenbusch, Heldt and Stein. 2~Their main interest was in phosphorous segregation to grain boundaries, which decreased the embrittlement. This, they reasoned, was due to an increased packing efficiency at grain boundaries that reduced the number of adsorption sites. They used sheet specimens 1.3 mm thick, C.E. PRICE is Professor, School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK 74078. R.S. FREDELL is Project Engineer, Air Force Wright Aeronautical Laboratories, Wright-Patterson Air Force Base, OH 45433. Manuscript submitted June 8, 1984.
METALLURGICAL TRANSACTIONS A
and obtained overwhelmingly intergranular fractures under all test conditions. They varied the charging rate but not strain rate or grain size. There are many models of LME extant but all have limitations. 3"~~The most favored model is that the adsorption of atoms at the metal surface causes a reduction in the tensile cohesive strength. This model was advanced by Stoloff and Johnston, 35 Westwood, 36 and Westwood and Kamdar. s7 The model of Lynch, ~ by contrast, is one of enhanced shear. For HE, one question has always been whether it is a surface or volume phenomenon. Hydrogen can diffuse readily in bcc steel at room temperature and there are many internal traps of varied effectiveness. In
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