| An investigation was conducted to understand the electrochemical and metallurgical processes governing the transition from localized corrosion attack to SCC initiation involved in Al-Li-Cu alloy AA2096 as a function of isothermal aging heat treatment. SCC screening conducted on bluntly notched specimens in 0.6 M NaCl alternate immersion revealed two distinct and separate windows of IGSCC susceptibility as a function of temper. Stress corrosion failure in susceptible tempers involved pitting, pit coalescence to form fissures, intergranular corrosion within fissures and stress corrosion cracking.; Diagnostics were performed to elucidate whether differences in localized corrosion behavior and/or IGSCC susceptibility as a function of temper influenced the overall susceptibility to SCC initiated from smooth specimens. Serial time exposures in the alternate immersion environment enabled documentation of localized corrosion site evolution in terms of corrosion type and resultant geometry. The evolution consisted of several distinct stages: pit initiation at constituent particles, coalescence of corrosion sites along bands of constituent particles forming corrosion fissures, and intergranular/intersubgranular corrosion at the base of these fissures in SCC susceptible tempers. Stress corrosion cracking occurred at the base of these corroded regions. However, sharp intersubgranular cracks did not form until fissures reached a certain depth by an intergranular/intragranular corrosion process. All tempers pitted or formed cathodic trenches at constituent particles and, subsequently, formed elongated corrosion fissures, but the transition to corrosion at metallurgical interfaces/intergranular cracking was controlled by a complex combination of metallurgical susceptibility, speculated occluded site chemistry development, and stress.; Electrochemical testing defining critical potentials of alloy phases (i.e., second phase precipitates, solute depleted zones) as a function of temper was performed to explain the differences in localized corrosion site evolution as a function of temper.; This thesis contributes to the understanding of stress corrosion cracking in AA2096, sheds light on the metallurgical culprit(s) responsible for SCC in Al-Li-Cu-Ag alloys, and provides detailed information regarding the transition from localized corrosion to stress corrosion cracking in machined notched specimens in AA2096. (Abstract shortened by UMI.)... |
