The early stages of AA2096 stress-assisted corrosion and environmental crack initiation from machine notched specimens, lacking any pre-fatigued cracks, were investigated. An under-aged (UA) temper of this alloy was found to be extremely susceptible to SCC failure. Double-notched tensile specimens of under-aged (i.e., 4-hour artificially aging at 160ºC) Al-Li-Cu-X alloy AA2096 with a 3% pre-stretch were investigated in an alternate immersion (AI) NaCl environment. Tensile specimens were exposed under an applied remote stress, without applied stress, and during interrupted tests where stress was applied and removed after corrosion fissures had developed. The aim of the present work was to use a novel focused ion beam (FIB) sectioning-scanning electron imaging method and confocal scanning laser microscopy to study and document the earliest stages of pit, intersubgranular corrosion, corrosion fissure, and intersubgranular crack development in the overall SCC process. Previous results have indicated that SCC in machine notched AA2096 involved several stages: trenching of the alloy matrix along bands of constituent particles, constituent induced pit or trench coalescence, elongated corrosion fissure growth initially by general dissolution on the length scale of subgrains and the transition to extensive intersubgranular attack, followed by the initiation/transition to intergranular SCC. In this work, FIB sectioning to surgically produce cross-sections between constituent particles in stressed alloys revealed that early trenching between particles did not
produce any form of intersubgranular attack. Intersubgranular attack occurred within developed corrosion fissures after 100 hours of AI exposure but was concurrent with considerable general, intragranular dissolution when corrosion fissures reached a critical depth...
Keywords: aluminum-lithium-copper alloys, environmental cracking, stress corrosion cracking initiation, corrosion fissures