Transgranular stress-corrosion cracking of disordered Cu-25Au in aqueous chloride and sulfate media
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INTRODUCTION
A principal goal in the study of stress-corrosion cracking is the development of refined models which can aid practical design to prevent or reduce the incidence of both transgranular (T-SCC) and intergranular (I-SCC) failure. Ford t has shown that from a knowledge of the involved mechanisms, it is possible to derive design criteria for practical systems undergoing I-SCC, e.g., cracking of sensitized 304 stainless steel in high-temperature water containing trace amounts of oxygen. The analysis is based on the slip dissolution model 2'3'4 or other models for which crack advance occurs primarily by Faradaic charge transfer due to bare-surface dissolution and/or film growth at a crack tip. Although such models may adequately describe I-SCC, they fail to take into account several established features of TSCC. These point to the decidedly non-Faradaic, discontinuous nature of the crack advance, which appears to occur by mechanical cleavage. The subject has been reviewed by Bursle and Pugh, 5 Pugh, 6 Sieradzki and Newman, 7 and by Meletis and Hochman. s The evidence includes (a) the distinctive "facet-step" morphology of the fracture surface, (b) the crystallographic uniqueness of steps and facets, (c) the exact matching of opposing fracture surfaces, (d) the appearance on the fracture surface of naturally occurring crack-arrest markings or of crack-front markings produced by load pulsing, and (e) the correlation between acoustic emission pulses and crack arrests or current transients. A theoretical treatment has also been offered to account for T.B. CASSAGNE, formerly with the Department of Mechanical and Materials Engineering, Vanderbilt University, is with The Johns Hopkins University. W. E FLANAGAN, Professor of Materials Science and Engineering, and B.D. LICHTER, Professor of Materials Science and the Management of Technology, are with the Department of Mechanical and Materials Engineering, Vanderbilt University, Nashville, TN 37235. Manuscript submitted March 6, 1987. METALLURGICALTRANSACTIONS A
these features. 7'9 Recently it was shown ~~ that copper-gold alloys, tested in aqueous FeC13, NaC1, and acid-sulfate media, display many of the characteristics which support the mechanical cleavage mechanism of T-SCC. Copper-gold alloys, in general, present an attractive model system in which to study the underlying mechanisms of SCC for single phase materials in aqueous media. Early work by Graf ~z'13on polycrystalline alloys containing up to 75 at. pct Au and by Gerischer and Rickert 14 for a copper18 at. pct Au alloy pointed to the significant role of selective dissolution in SCC and to the possible role of strainenhanced dissolution. Cracking was observed in several solutions, e.g., aqueous solutions of FeCI3, KC1, acidsulfate, HNO3, aqua regia, and H2CrO4. These early studies failed to make a clear distinction between T-SCC and ISCC. Subsequently studies in FeC13 o f C u 3 m n single crystals by Bakish and Robertson J5 and of single crystals containing up to 30 at. pct Au by Hardwick and Dodd ~6esta
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