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ABSTRACT Dealloying of a binary noble alloy produces a porous layer rich in the more noble element. Application of a tensile load initiates a brittle intergranular (IG) crack in the dealloyed layer that advances into the unattacked material. This study showed that the crack penetration depth (Cd) is proportional to the thickness of the dealloyed layer (t). For a given value of t, the grain-boundary crack penetration distance was shown to decrease as the dealloying potential increased. The dependence of Cd on t and the dealloying potential, as opposed to the applied potential at the time of fracture, supported the film-induced cleavage model.
INTRODUCTION Alloys that undergo selective dissolution also are susceptible to stress corrosion cracking (SCC). The combination of stress and selective dissolution has been proposed as a dissolution mechanism for SCC.1 In this mechanism, cracks progress when porous regions formed by dealloying fail under applied stress. However, recent work has shown that stress and dissolution need not be applied simultaneously to produce brittle cracking in binary face-centered cubic (fcc) alloys.2-3 Cracks can advance in the absence of simultaneous dissolution. Instantaneous crack velocities of 5 mm/s to 40 mm/s (0.2 in./s to 1.6 in./s) have been measured.3-4 With time, a dealloyed layer capable of initiating a brittle crack may lose its ability to propagate a crack into the bulk alloy.2-3
Binary noble alloys have been used by a number of researchers to study the SCC phenomena.1-9 These alloys, when polarized to potentials greater than the critical potential (Ec), corrode by selective dissolution. 10 The removal of the less noble element of the alloy by selective dissolution produces a porous structure rich in the more noble element.11
Film-induced cleavage refers to the injection of a brittle crack into an otherwise ductile material by a surface film or dealloyed layer.5,12 As originally proposed, the model applied to transgranular (TG) SCC, but experimental studies have shown that this mechanism also applies to brittle intergranular (IG) cracking.3,7,13 In this case, film-induced ?cleavage? is a misnomer, and in this work, this mechanism is referred to as film-induced cracking (FIC).
The failure mode (brittle or ductile) of porous Au foils formed by selective dissolution may be dependent on the pore size distribution and average pore size.14 A narrow distribution and a small average pore size relative to film thickness are expected to enhance the brittle response of a porous material.15 The coarsening, or increasing of pore size, in dealloyed layers (for a constant layer thickness) is believed to cause the reversibility in cracking.15 The coarsening kinetics of dealloyed layers have a t1/4 time dependence indicative of a surface diffusion-controlled process.16-17
In the present study, the propensity of dealloyed layers to cause FIC was examined for Ag-Au alloys in perchloric acid (HClO4) solutions. This system was chosen to simplify the electrochemistry and metal-
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