Environment-Induced Deformation Localization During Transgranular Stress Corrosion Cracking

Abstract

The nucleation and evolution of deformation patterns occurring during transgranular stress corrosion cracking (TGSCC) were studied in an attempt to produce new alternatives for addressing the nature of the embrittlement process. Flat, tensile α-brass (72% Cu-28% Zn) specimens were tested in 5 M aqueous ammonia (NH 4 OH) solution at a strain rate of 1 x 10 -5 /s. Slip-band spacing (SBS) and slip-band height (SBH) were measured as a function of strain by conducting interrupted experiments in the SCC environment and were compared with those developed during laboratory air experiments. The presence of the TGSCCcausing environment during straining promoted localized plastic deformation at the near-surface region and, more importantly, produced an entirely different deformation pattern from that developed in laboratory air. The deformation evolved in the presence of the TGSCC electrolyte was highly localized, exhibiting small SBS but large SBH. Also, a periodicity was exhibited by the crack initiation process. The amount of localized strain developed at the specimen near-surface region before nucleation of stress corrosion cracks was found to be equivalent to the strain required for ductile fracture of the material in air, suggesting the existence of a fundamental fracture criterion. Grounds for an environment-induced deformation localization mechanism were introduced to explain the phenomenology of TGSCC initiation and propagation.

Deformation evolution during initiation of transgranular stress corrosion cracking

“…It has been shown that enhanced corrosion takes place at emergent slip bands, but corrosion alone cannot account for the crack initiation or propagation. The dissolution rates calculated for the present system were by almost two orders of magnitude lower than the experimentally observed initiation rate of 0.1 #m/s [ 12,13].…”

“…Similarly to the T-SCC causing electrolyte, the presence of hydrogen has been found to ease dislocation motion causing strain localization. In a recent study [12,13], we attributed the evolved enhanced dislocation activity to an environment-induced deformation localization mechanism. It was suggested that corrosion causes dislocation generation and injection from the surface at loads well below those required for normal yielding through a vacancy-dislocation interaction process.…”

“…when a crack has not yet been formed. The characteristics of the nucleation and evolution of deformation patterns occurring during T-SCC of the a-brass/NHnOH system were recently investigated [12,13], and these results are summarized below. In view of these observations, the motivation of the present work was to elaborate on the initial stages of the stress-environment interaction in an effort to shed more light on the nature of the embrittlement process.…”

Contribution of Hydrogen Embrittlement to SCC Process in Excess Si Type Al-Mg-Si Alloys

“…Regarding a substance of the interaction, some models explaining the localized plastic deformation promoted by anodic dissolution have been proposed primarily from the viewpoint of TG-SCC in fcc alloys. Among those, EIDEL(Environment-induced deformation localization) mechanism introduced by Meletis et al 9) seems available to the present SCC. Namely, based originally on the AVG (Anodic vacancy generation) model by Uhlig et al, 18) the mechanism is principally grounded on the enhanced dislocation mobility, accompanied by the following repeated processes at the surface layer; anodic dissolution-vacancy generation-dislocation nucleation-formation of planar dislocation arrangement-{111} micro-cracking-anodic dissolution.…”

“…SCC mechanisms have been conventionally debated through classifying into two categories of anodic dissolution (AD) and hydrogen effects, but experimental results and new models presented [7][8][9][10] in recent years indicate that the above discussion is not sufficient; they demand that SCC process should be understood as the interactions under applied stress between AD and plastic deformation (dislocation mobility) 8,9) or among AD, plastic deformation and hydrogen effects. 10) Regarding HE also, in spite of quite many systems of materials and environments studied and a variety of mechanisms proposed, incomplete understanding is still obtained.…”