Initiation and short crack growth behaviour of environmentally induced cracks in AA5083 H131 investigated across time and length scales

Visweswara C Gudla,N J Henry Holroyd,James Carr,Timothy L Burnett,John J Lewandowski,Philip J Withers,Parmesh Gajjar,Malte Storm,Alistair Garner,Benjamin C Palmer

doi:10.1515/corrrev-2019-0044

Abstract

Abstract Environmentally induced cracking (EIC) in a sensitized high-strength AA5083 H131 alloy has been investigated using time-lapse synchrotron X-ray computed tomography combined with post-mortem correlative characterization. Small corrosion features deliberately introduced in a pre-exposure step were found to be the site of initiation for over 95% of the 44 EIC cracks that developed under slow strain rate testing. Detailed analysis using three-dimensional electron backscatter diffraction and energy-dispersive spectroscopy analysis of a single crack confirmed the intergranular nature of the cracks from the start and that the pre-exposure corrosion was associated with an α-AlFeMnSi particle cluster. It also appears that several cracks may have initiated at this site, which later coalesced to form the 300-μm-long crack that ultimately developed. Of further note is the fact that initiation of the EIC cracks across the sample started below the yield strength and continued beyond the ultimate tensile strength. The most rapid crack propagation occurred during sample extension following a period of fixed displacement.

Highlights

Aluminium alloys based on the Al-Mg alloy system are widely used in marine applications owing to their high strength, low density and good corrosion resistance to saline environments (Vargel, 2004; Polmear, 2006)
Detailed analysis using three-dimensional electron backscatter diffraction and energy-dispersive spectroscopy analysis of a single crack confirmed the intergranular nature of the cracks from the start and that the pre-exposure corrosion was associated with an α-AlFeMnSi particle cluster
Of further note is the fact that initiation of the Environmentally induced cracking (EIC) cracks across the sample started below the yield strength and continued beyond the ultimate tensile strength

Summary

Introduction

Aluminium alloys based on the Al-Mg alloy system are widely used in marine applications owing to their high strength, low density and good corrosion resistance to saline environments (Vargel, 2004; Polmear, 2006). The EIC in structural components was attributed to high susceptibility to intergranular stress corrosion cracking (IGSCC; Holroyd & Scamans, 2016). High-strength AA5xxx alloys (>3 wt.% Mg) rely on a combination of solid solution strengthening and cold working for their strength. These alloys are not precipitation hardenable but do undergo ageing or precipitation at temperatures as low as 40°C due to the low solubility of Mg in Al at room temperature (RT; Scamans et al, 1987; Zhang et al, 2016). Intergranular corrosion (IGC) and IGSCC are both promoted by dissolution of the anodic β-phase, leading to acidification of the crack tip/corrosion feature and generating H, which is taken up by the surrounding Al which under an applied stress is believed to create the conditions needed to

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Conclusion