Pre-exposure embrittlement of a commercial Al-Mg-Mn alloy, AA5083-H131

Two distinct modes of environment-induce cracking (EIC) can initiate in AA5083-H131 during slow strain rate testing (SSRT) in laboratory air (50% RH) at nominal strain rates around 10−6/s, for material either sensitized in distilled water (DW) at 80°C or when material sensitized in dry air is subsequently pre-exposed to DW at room temperature. Type-1 EIC is the “classic” form of intergranular stress corrosion cracking (IGSCC), which during SSRT in laboratory air, initiates at intergranular corrosion (IGC) sites promoted during exposure to DW and provides the prerequisite local stress intensity factor of around 5 MNm−3/2 required for crack initiation. When Type-1 EIC cracks become sufficiently deep for local plane-strain stress intensity factors to exceed 12–15 MNm−3/2, Type-2 EIC initiation triggers a series of sudden large load-drops and SSRT failures with extremely low fracture stresses (30–65 MPa). Pre-exposure to DW at room temperature can enhance propensity to both Type-1 and Type-2 EIC. SSRT in dry air, as opposed to humid laboratory air, exhibits failure consistently as a 45° slant failure via a predominantly ductile microvoid coalescence fracture mode, with Type-1 EIC very rarely initiating and any Type-2 EIC restricted to isolated internal patches that only form when relative elongations exceed ~9%.