LSU Historical Dissertations and Theses

1995

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Engineering Science (Interdepartmental Program)

E. I. Meletis

Abstract

The objective of the present work was to provide an insight to the nucleation and evolution of deformation patterns occurring during transgranular stress corrosion cracking (TGSCC) and produce new alternatives for addressing the nature of the embrittlement process. Flat, tensile $\alpha$-brass (72Cu-28Zn) specimens were tested in 5 M NH$\sb4$OH, 0.1 M CuSO$\sb4$ and 1 M NaNO$\sb2$ solutions at a strain rate of $1\times 10\sp{-5}$ s$\sp{-1}.$ Slip band spacing (SBS) and slip band heights (SBH) were measured as a function of strain by conducting interrupted experiments in the SCC environments and were compared with those developed during laboratory air experiments. The presence of the TGSCC-causing environment during straining was found to promote localized plastic deformation at the near-surface region, induce strain hardening and more importantly to produce an entirely different deformation pattern compared to that developed in laboratory air. The deformation evolved in the presence of the TGSCC electrolytes was highly localized, exhibiting a dense SBS but coarse SBH. Also, a periodicity was exhibited by the crack initiation process. The amount of localized strain developed at the specimen near-surface region prior to 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. In view of the present observations, an environment-induced deformation localization mechanism is introduced to explain TGSCC initiation and propagation. The main elements of the proposed mechanism are: (i) strain localization due to corrosion instability and periodicity; (ii) vacancy-induced dislocation emission at the surface region and (iii) vacancy-dislocation interaction localizing deformation and modifying dislocation arrangement.

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