Metallurgical Factors and Changes Driving Susceptibility to Environment Assisted Cracking in Aluminum-Magnesium AlloysThursday (07.11.2019) 09:15 - 09:35 Part of:
Metallurgical Factors and Changes Driving Susceptibility to Environment Assisted Cracking in Aluminum-Magnesium Alloys
David Schrock, Allison Akman, Jenifer S Locke
The Ohio State University, Department of Materials Science and Engineering, Fontana Corrosion Center
Aluminum-magnesium alloys, designated as AA5xxx, are used readily in naval applications. In order to impart sufficient strength, Mg additions are typically higher than the solubility limit of ~3.5%. At room temperature a Mg supersaturated solid solution is kinetically stable; but it has been well established that exposure to elevated temperatures, even those typically experienced in-service, for extended periods of time leads to precipitation of a Mg rich β phase on grain boundaries. The literature establishes that the precipitation of β phase on grain boundaries leads to a severe degradation in corrosion and stress corrosion cracking resistance. Because marine structures are exposed to corrosive seawater environments while simultaneously experiencing cyclic stresses from 1) ship machinery vibrations, 2) diurnal thermal variations leading to thermal expansion and contraction, 3) wind and wave action (hog/sag motion), and 4) combat/operational loading events; understanding the impact of β phase on corrosion fatigue (CF), the simultaneous interaction of corrosion and fatigue, resistance is of interest. Fracture mechanics based studies, corrosion experimentation, and high resolution SEM are utilized to understand and quantify the effect of the formation of β phase along grain boundaries on CF of high Mg AA5xxx alloys. Specifically, results show the resistance to CF is degraded under low frequency loading, and the deleterious effect of grain boundary β phase is magnified as fatigue loading frequency decreases. Additionally, comparison of stress corrosion cracking (SCC) resistance via K1SCC, the threshold below which SCC does not occur, with CF results shows that severe degradation in CF resistance is triggered by loading conditions which promote SCC. Examination of differences between lab sensitized and in-service sensitized material is underway.
This work is supported by the Office of Naval Research through a Young Investigator Award managed by Dr. Airan Perez under award number N00014-16-1-2756. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Office of Naval Research.