On the failure mechanism of chemically embrittled Cu 3 Au single crystals
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I.
INTRODUCTION
IN a paper published in 1957, Bakish 1 described a process of chemical embrittlement in which a fully annealed single crystal of C u 3 A u failed in a distinctly cleavage-like manner when a low bending stress was applied to the crystal after its removal from 2 pct aqueous FeCI~ in which it had been immersed, stress-free, for one month. Using optical microscopy, Bakish was able to show that the fracture surface consisted of flat facets separated by serrated steps. From Laue back-reflection photograms, the surface appeared to have a {110} orientation with little asterism, again suggesting that fracture occurred by cleavage. This result, only recently rediscovered, 2'3 bears directly on attempts to understand the mechanisms of transgranular stress-corrosion cracking (T-SCC) of metals and alloys. This is so because in "true" SCC (which requires the simultaneous presence of stress and corroding environment) crack propagation produces a cleavage-like fracture surface which is alleged to occur by a series of discontinuous cleavage events 3-~~or by continuous, adsorption-induced decohesion." Moreover, SEM-observed transgranular fracture surfaces also display the faceted-serrated step morphology, for example: alphabrass/aq, ammonia; 6 AI-Zn-Mg/aq. NaC1; s 304 and 310 stainless steels/boiling MgC12;9 and Cu-Au/aq. FeC13 or NaC1 or acid-sulfate. 3 For all of these systems (except 310 stainless steel) {110} fracture facets have been reported. Bakish 1 proposed that "the cleavage on bending is due to T.B. CASSAGNE, formerly with the Department of Mechanical and Materials Engineering, Vanderbilt University, Nashville, TN, is now with the Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, MD. W. E FLANAGAN and B.D. LICHTER, Professors, are with the Department of Mechanical and Materials Engineering, Vanderbilt University, Nashville, TN 37235. Manuscript submitted April 15, 1985.
METALLURGICALTRANSACTIONS A
the presence of a very thin layer of brittle gold-rich sponge along the {110} planes. This sponge is formed as a result of selective leaching of copper from the alloy along highly active {110} planes." In recent metallographic studies 3 we have shown that polycrystalline copper-25 at. pet Au undergoes T-SCC in three ionic media (aq. FeC%, aq. NaC1, and acid-sulfate) under constant deflection (bending) and slowstrain-rate loading modes. SEM observations of fracture surfaces produced in all three solutions reveal characteristic cleavage-like features with matched opposing fracture surfaces and crack-arrest markings, suggesting discontinuous crack advance. However, no evidence of massive dealloying on the fracture surface could be observed in the SEM at high resolution, despite the fact that massive gold "sponge" formation could be seen at low magnification in the vicinity of occasional surface scratches. A subsequent microprobe analysis of the fracture surface, using energy-dispersive detection, failed to show any dealloying to within a resolution of 1 micrometer.~2'~3 Th
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