Fractography of Stress Corrosion Cracking of Mg-Al Alloys
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I.
INTRODUCTION
OUR review of the stress corrosion cracking (SCC) of Mg alloys showed that there exists a considerable amount of research outlining the phenomenology of transgranular stress corrosion cracking (TGSCC) of Mg alloys. This TGSCC is the inherent mode of the SCC of Mg alloys.[1] It is generally accepted that the mechanism for the TGSCC of Mg alloys is a form of hydrogen embrittlement (HE); however, the specific nature of the HE mechanism remains equivocal. The HE models that may be applicable for Mg alloys are hydrogen-enhanced decohesion (HEDE), hydrogen-enhanced localized plasticity (HELP), adsorption-induced dislocation emission (AIDE), and delayed hydride cracking (DHC). The AIDE[2] and DHC[3–6] models have been proposed for the TGSCC of Mg alloys; however, the evidence for both mechanisms is limited. The HEDE and HELP models also remain possible mechanisms. Detailed reviews of these mechanisms are provided in Birnbaum,[7] Lynch,[8] and Gangloff.[9] A brief review of these mechanisms with respect to the experimentally [1]
NICHOLAS WINZER, formerly PhD Candidate, Materials Engineering Department, The University of Queensland, is Postdoctoral Research Fellow, Joining Technology, Bundesanstalt fu¨r Material Forschung und-pru¨fung, D-12200 Berlin, Germany. Contact e-mail: [email protected] A. ATRENS, on sabbatical at the Swiss Federal Laboratories for Materials Science and Technology (EMPA), Uberlandstrasse 129, CH-8600 Dubendorf, Switzerland, is Head of Materials Engineering, Materials Engineering Department, The University of Queensland. W. DIETZEL, Head of Corrosion Research Group, and K.U. KAINER, Head of Institute for Materials Research, are with the Institute for Materials Research, GKSS– Forschungszentrum Geesthacht GmbH, D-21502 Geesthacht, Germany. G. SONG, formerly with the CAST Cooperative Research Centre, Materials Engineering Department, The University of Queensland, Brisbane 4072, Australia, is with the GM Technical Center, Warren, MI 48093. Manuscript submitted September 30, 2007. Article published online March 7, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
measurable characteristics of SCC in Mg alloys is given in Winzer et al.[10] The fractographic aspects of these mechanisms are reviewed here. A. Fractography of HE The HEDE mechanism involves a reduction in the electron charge density between metal atoms in the region ahead of the crack tip, where H accumulates by stress-assisted diffusion. This causes weakening of the bonds between and eventually the tensile separation of adjacent metal atoms. Fracture may be intergranular or transgranular. In the case of transgranular HEDE, fracture is expected to occur by cleavage, resulting in river markings (per conventional cleavage).[7–9] The DHC mechanism involves repeated stages of the following: (1) the stress-assisted diffusion of H to the region ahead of the crack tip; (2) hydride precipitation, as the local H concentration exceeds the local solvus; and (3) brittle fracture through the hydride. Previous proposals for DHC in Mg
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