Film-Induced Cleavage of Ag-Au Alloys

Single-shot fracture experiments have been carried out in a dealloying environment (perchloric acid) on Ag-20 at. pct Au and Ag-23 at. pct Au alloys in the form of thin wire and foil, respectively. Such environments produce a nanoporous metallic layer on the surface. Transgranular fracture was promoted in some of the wire experiments by inducing a bamboo grain structure. Using the wires, it was shown that complete brittle intergranular or transgranular fracture could be obtained, either by applying a very low tensile stress during dealloying, by rapidly straining the wire after dealloying without stress, or by straining to failure after first dropping the potential to a value where little or no further faradaic reaction was possible. Extremely low fracture stresses were found, especially for intergranular fracture, and all fracture events were sudden with no secondary substrate fracture. The work with foils showed that some irreproducibility noted in the response of the wires was associated with premature self-fracture of the dealloyed layer. By optimizing the condition of the dealloyed layer, extremely reproducible and complete film-induced fractures were obtained. These dealloyed layers showed properties not previously seen, such as an ability to inject deep substrate cracks even after a 5-minute immersion in deionized water. Recent studies showing extraordinary mechanical properties of dealloyed layers, such as near-theoretical strength in compression, give more credibility to this still-mysterious fracture mechanism. A detailed rebuttal of the claims of the alternative surface mobility'' model is presented.