| The mechanism of transgranular stress corrosion cracking (T-SCC) was studied using oriented Cu-30wt%Zn single crystals in deoxygenated cuprous-ammonia solutions. The results of potentiodynamic polarization showed that there was no significant passivation behavior in this system, indicating that general corrosion rather than selective dissolution occurred. However, potentiostatic polarization tests demonstrated that subtle dealloying existed as the current decayed exponentially with time after application of a small anodic overpotential. On the other hand, a large anodic overpotential caused a high background current, indicating the occurrence of general dissolution. Slow strain rate tensile tests were performed using 1 x 3 mm rectangular samples aligned with their (110) plane perpendicular to the tensile axis. The results showed that T-SCC readily occurred on {dollar}{lcub}110{rcub}{dollar} planes both at the open circuit potential and under anodic polarization, but stopped immediately once cathodic polarization was applied, indicating electrochemical dissolution is necessary for T-SCC in this system. Macroscopically, the crack propagated continuously at a rate of approximately 0.15 {dollar}mu{dollar}m/s for the cross-head extension rate of 4.2 {dollar}times{dollar} 10{dollar}sp{lcub}-5{rcub}{dollar} mm/s. However, the crack grew discontinuously on a microscale, with facets being blunted by slip on {dollar}{lcub}111{rcub}{dollar} planes, as evidenced by load transients and the presence of faint parallel microcrack-arrest markings perpendicular to the {dollar}langle 001rangle{dollar} crack growth direction. These microcrack-arrest markings with a spacing of approximately 1 {dollar}mu{dollar}m or less were correlated with the slip bands on the side surface. Also, the average crack growth rate was independent of applied anodic potential, but highly dependent on strain rate. SEM and stereoscopic analysis revealed that, generally, the micromorphology of the fracture surface consisted of parallel {dollar}{lcub}110{rcub}{dollar} facets and serrated {dollar}{lcub}111{rcub}{dollar} shear steps which were perpendicular to the facets, and that two modes of secondary crackings occurred on the main {dollar}{lcub}110{rcub}{dollar} fracture surface, depending on local stress state. A model based on the concept of Corrosion-Assisted Cleavage is proposed to explain the morphology and kinetics of T-SCC of {dollar}alpha{dollar}-brass, as well as the competitive relationship between SCC and deformation (slip). |
