The Effect of Mn-rich Precipitates on the Strength of AZ31 Extrudates
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TION
The magnesium alloy system is gaining renewed interest in the research community due to its high specific strength.[1,2] Although there exist a large number of alloys, and their properties are readily available in the literature,[1–3] there is a surprisingly small number of papers that examine the basic behaviors of the secondphase particles that exist in the most common wrought magnesium alloy: AZ31. Under equilibrium conditions, and at room temperature, the ternary system Mg-3Al-1Zn can form the Mg17Al12 phase known as c, along with a third phase known as /, Figure 1(a). The commercial alloy is slightly more complicated than the ternary basis. Commercial alloys typically contain a Mn addition to improve the corrosion resistance,[2] and the commercial designations for AZ31 with a manganese addition of between 0.2 and 1 wt pct Mn are either AZ31B or AZ31C.[3] The thermodynamic predictions shown in Figure 1 indicate that in the solid state, Mn and Al are able to form several intermetallic compounds in AZ31, Figure 1(b). It is apparent from these predicted phase diagrams that there is the potential for a quite complicated interaction among second-phase particles, phase transformations, and thermo-mechanical processing. NICOLE STANFORD, Senior Research Scientist, and DALE ATWELL, Research Engineer, are with the Faculty of Science and Technology, Institute for Frontier Materials, Deakin University, Pigdons Road, Geelong, VIC 3217, Australia. Contact e-mail: [email protected] Manuscript submitted December 17, 2012. Article published online June 6, 2013 4830—VOLUME 44A, OCTOBER 2013
Although the formation of secondary phases is fairly well studied in cast AZ31,[e.g., 4,5] there is a considerable lack of information about the behavior of second phases in wrought magnesium alloys, with only a small number characterizing the particles that are present or investigating the effect of manganese.[6,7] More importantly, the particles with the biggest effect on strengthening and grain size refinement are in the sub-micron size range, and their characterization through transmission electron microscopy in AZ31 is rarely reported. The aim of the present paper therefore is to clarify which particles are present in a commercially available, Mncontaining AZ31 alloy after thermo-mechanical processing. The impact of these precipitates on the extrudability and microstructural development will also be examined. At the end of the paper, the precipitation and microstructure are correlated with the measured mechanical properties.
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EXPERIMENTAL METHOD
The alloy used in this study was purchased from a commercial supplier in the as-cast form. The composition is detailed in Table I. The as-cast material was sectioned into billets that were cylindrical in shape, had a height of 20 mm, and a diameter of 30 mm. These billets were given a series of solution treatments prior to extrusion, and these are detailed in Table II. The solution treatments were carried out in a tube furnace under a flowing argon atmosphere and were water quenched at
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