On the failure mechanism of chemically embrittled Cu₃Au single crystals

Abstract

In order to distinguish between “true” transgranular stress-corrosion cracking (T-SCC) and the “chemical embrittlement” previously described for Cu₃Au single crystals (Bakish,AIME Trans., 1957), copper-25 atomic percent gold single crystals were subject to constant deflection (a) while immersed in aqueous ferric chloride or (b) in air after undergoing stress-free corrosion for 10 and 30 days in aqueous ferric chloride. In the conventional stress-corrosion testing mode carried out at the corrosion potential and at applied anodic and cathodic overpotentials, SEM observation and microprobe analysis revealed that characteristic T-SCC fracture surfaces were produced without the occurrence of massive dealloying, at least over a 0.1 micrometer depth. For bending tests in air following stress-free corrosion at the corrosion potential, a 30-day sample was completely converted into a brittle, virtually pure gold “sponge” while retaining the external shape and orientation of the original alloy single crystal. The fracture surface of this sample revealed relatively flat facets separated by irregularly serrated steps, as are seen in conventional cleavage. However, the surface is porous with a mean pore size of ∼0.1 micrometer. For the 10-day sample, bending produced multiple cracks in a massively dealloyed layer (gold-rich sponge). Several of these cracks propagated into the unattacked, normally ductile alloy for distances up to ∼20 micrometers. Interpretation of these results leads to an alternative explanation for the “chemical embrittlement” previously observed and offers significant new insights on the mechanisms of T-SCC.