Improved understanding of environment-induced cracking of sensitized 5XXX series aluminum alloys

Significance Statement

A number of issues facing the US navy in the last few years have resulted from aluminum-magnesium alloys becoming sensitized in their application to subsequently suffer from environmental-induced cracking. This has led to increased interest and research stimulated by scientific curiosity.  A clear assessment of the current knowledge in the phenomenon, with well-informed articles offering historical understanding is provided in a special issue of the National Association of Corrosion Engineers Journal, Corrosion publishe in February 2016 (co-edited by NJHH).

Critical surface defects are prerequisites for intergranular environmental-induced cracking initiation during slow strain rate testing of test pieces of commercial Aluminum-Copper-Magnesium (2XXX) and Aluminum-Zinc-Magnesium(-Cu) (7XXX) series aluminum alloys. Pre-exposure to saline conditions before straining in moist-air is confirmed to accelerate crack initiation and promote initial cracking propagation rates. Prompted by lack of sufficient data for commercial non-precipitate hardening of aluminum-magnesium based alloys, Henry Holroyd and colleagues conducted a research to improve the understanding of environment-induced cracking of sensitized 5XXX series aluminum alloys. Their work is published in journal, Materials Science & Engineering A.

When the authors examined using scanning electron microscopy failed samples they showed ductile failure by micro-void coalescence with typical small circular dimples. Comparison of the fracture surfaces produced in dry and humid air indicated the depth of intergranular corrosion formed during pre-exposure to sodium chloride increased with alloy sensitization treatment, increasing from about 70 micrometers for the as-received AA5083-H131 through to 110 micrometers for the H131 material.

In the presence of a suitable pre-existing stress-raiser, two types of environmental-induced cracking can occur in AA5083-H131 tensile samples subjected to rising-load conditions generated during slow strain testing in humid air.

Type-1 cracking is the classical form of intergranular stress corrosion cracking. It initiates at stress intensities around 4MNm-3/2 and is not generated in dry air irrespective of pre-exposure or sensitization. Type-2 cracking is enhanced by pre-exposure to saline conditions ahead of straining and further enhanced by sensitization. The authors successfully conducted high resolution 3D tomography and detailed scanning electron microscopy to distinguish the two different modes of environmental-induced cracking.

Dr Holroyd, Dr Seifi and Professor Lewandowski at Case Western Reserve University in Cleveland, Ohio and Dr Burnett at the University of Manchester in the UK continue their collaboration to elucidate the processes and mechanism’s controlling the environment-induced cracking of commercial aluminum and magnesium alloys.

 

Reference

N.J.H. Holroyd1,2, T.L. Burnett3, M. Seifi1, J.J. Lewandowski1. Improved understanding of environment-induced cracking (EIC) of sensitized 5XXX series aluminum alloys. Materials Science & Engineering A, volume 682 (2017), pages 613-621.

Show Affiliations
  1. Department of Materials Science & Engineering, Case Western Reserve University, Cleveland, OH, USA.
  2. Bolivar, MO, USA.
  3. Henry Mosely X-ray Imaging Facility, School of Materials, The University of Manchester, Manchester, UK.

 

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