Effect of Al and Gd Solutes on the Strain Rate Sensitivity of Magnesium Alloys
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INTRODUCTION
MAGNESIUM alloys are receiving renewed research interest as the need for light weight vehicles becomes more pressing. Magnesium is the lightest of the engineering metals. However, magnesium suffers from two deficiencies in its mechanical behavior: it suffers from poor room-temperature ductility and has a relatively low tensile strength.[1] Consequently, there has been significant research into the deformation behavior of this system. One aspect of deformation behavior that has not been extensively examined in magnesium is its strain rate sensitivity (SRS). This property is important because a reduction in the SRS is known to reduce the ductility of alloys such as aluminum, e.g., Reference 2. In magnesium, the commonly used alloy AZ31 shows a strong strain rate dependence on the room-temperature ductility.[3] The room-temperature behavior of Mg alloys is particularly susceptible to strain rate effects, because room temperature is approximately 0.4 of the homologous temperature. For these reasons, the present work investigates the SRS of magnesium alloys. For this study, we chose to examine pure magnesium, and used this as a benchmark to examine two binary alloys, Mg-1 wt pct Al and Mg-1.4 wt pct Gd. These were chosen because aluminum is a common alloying N. STANFORD, Senior Researcher, and M.R. BARNETT are with the Centre of Excellence for Design in Light Metals, Deakin University, Victoria, Australia, 3217. Contact e-mail: stanford@ deakin.edu.au I. SABIROV, Senior Researcher, is with the Instituto Madrilen˜o de Estudios Avanzados de Materiales (IMDEAMateriales), E.T.S. de Ingenieros de Caminos, 28040, Madrid, Spain. G. SHA, Senior Researcher, A. LA FONTAINE, Atom Probe Engineer, and S.P. RINGER, Director of Electron Microscopy Unit, are with the Australian Key Centre for Microscopy and Microanalysis, The University of Sydney, NSW 2006, Sydney, Australia. Manuscript submitted June 23, 2009. Article published online January 5, 2010 734—VOLUME 41A, MARCH 2010
addition to magnesium and exists in solid solution. We chose to examine Gd as an alloying addition, because rare earth additions are known to increase ductility of magnesium alloys.[4] The Gd exists in solution and does not form precipitates,[5] and is also known to have a transition temperature above which it produces a similar extrusion texture to the other two alloys.[4] A similar texture is desirable, because the SRS is known to be highly texture dependant.[6] Care was also taken to ensure that the grain size was similar for the alloys, because a recent article showed a significant grain size effect on the SRS in magnesium-based alloys.[7]
II.
EXPERIMENTAL METHOD
Three alloys were examined: pure Mg, Mg-1 wt pct Al, and Mg-1.4 wt pct Gd. The full composition analysis for these alloys is given in Table I. The alloys were prepared in 2 kg batches in a steel crucible under an argon atmosphere. Billets with a diameter of 30 mm and height of 20 mm were machined from the as-cast material. The billets were solution treated for 3 hours at 530 C and then 5
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