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

AbstractTwo 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%.</jats:p

Crack initiation during environment-induced cracking of metals: current status

Environment-induced cracking (EIC) research spanning the last 80 years for ferrous and non-ferrous metals in aqueous environments at ambient and elevated temperatures has concentrated on crack propagation. Studies clearly reveal EIC involves two differentiable processes, one controlling initiation and the other propagation. Utilization of advanced high-resolution electron microscopy over the last 20 years has enabled more focused studies of crack initiation for stainless steel and nickel-based alloys at elevated temperatures exposed to environments associated with the nuclear industry. More recently, when coupled with advanced in-situ experimental techniques such as time-lapse X-ray computed 3D-tomography, progress has also been made for aluminum alloys suffering EIC at ambient temperatures. Conventional wisdom states that chemical processes are typically rate-controlling during EIC initiation. Additionally, experimental evidence based on primary creep exhaustion ahead of the introduction of an aggressive environment indicates that time-dependent mechanically-driven local microstructural strain accommodation processes (resembling creep-like behavior) often play an important role for many metals, even for temperatures as low as 40 % of their melting points (0.4 Tm). EIC studies reveal initial surface conditions and their associated immediate sub-surface alloy microstructures generated during creation (i.e. disturbed layers) can dictate whether or not EIC initiation occurs under mechanical loading conditions otherwise sufficient to enable initiation and growth. The plethora of quantitative experimental techniques now available to researchers should enable significant advances towards understanding EIC initiation

The role of crack branching in stress corrosion cracking of aluminium alloys

Abstract Stress corrosion cracks of all types are characterised by extensive crack branching, and this is frequently used as the key failure analysis characteristic to identify this type of cracking. For aluminium alloys, stress corrosion cracking (SCC) is almost exclusively an intergranular failure mechanism. For plate and extruded components, this had led to the development of test procedures using double cantilever beam and compact tension precracked specimens that rely on the pancake grain shape to constrain cracking, so that fracture mechanics can be applied to the analysis of stress intensity and crack velocity and the evolution of a characteristic performance curve. We have used X-ray computed tomography to examine in detail SCC in aluminium alloys in three dimensions for the first time. We have found that crack branching limits the stress intensity at the crack tip as the applied stress is shared amongst a number of cracks that are held together by uncracked ligaments. We propose that the plateau region observed in the v-K curve is an artefact due to crack branching, and at the crack tips of the many crack branches, cracking essentially occurs at constant K almost irrespective of the crack length. We have amplified the crack branching effect by examining a sample where the long axis of the pancake grains was inclined to the applied stressing direction. Our results have profound implications for the future use of precracked specimens for SCC susceptibility testing and the interpretation of results from these tests.</jats:p