Na Partitioning During Thermomechanical Processing of an Mg-Sn-Zn-Na Alloy
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THE need for lightweight alternative materials has led to an increased interest in the use of magnesium alloys, which have the lowest density of any commonly used structural material. Precipitation hardening is one method used to increase the strength of these alloys. In Mg alloys, the precipitates that form during elevated temperature exposure are often plate or rod/lathshaped, and are often oriented either parallel to the basal plane, or normal to it.[1] Strengthening is obtained when these precipitates impede dislocation motion during deformation, whether through shearing of the precipitates or by forcing the dislocations to bow around the obstacles.[2] The Mg-Sn binary system was developed as a promising precipitation-hardenable alloy. Sn is highly soluble in Mg at elevated temperatures, with a maximum solubility of 14.5 wt pct (3.35 at. pct) at the eutectic temperature of 834 K (561 C),[3] while the solubility decreases markedly at temperatures below ~573 K (300 C). The precipitates that form are the Mg2Sn phase (Fm 3m space group; a = 0.68 nm),[4] which has a relatively high melting temperature of ~1043 K
JESSICA R. TERBUSH, formerly Research Fellow with ARC Centre of Excellence for Design in Light Metals, Department of Materials Engineering, Monash University, Clayton, VIC 3800, Australia, is now Senior Research Specialist with Missouri University of Science and Technology, Rolla, MO. Contact e-mail: [email protected] NICOLE STANFORD, Senior Research Academic, and MATTHEW R. BARNETT, Associate Professor, are with the ARC Centre of Excellence for Design in Light Metals, Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia. JIAN-FENG NIE, Professor, is with the ARC Centre of Excellence for Design in Light Metals, Department of Materials Engineering, Monash University. Manuscript submitted November 26, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
(770 C).[3] These precipitates usually form as laths on the basal plane of the Mg matrix.[5–7] Although a relatively high volume fraction of precipitates can theoretically form in this system (~10 vol pct), a low age hardening response was observed because of the coarse size and distribution of the precipitates that did form.[7] No metastable phases have been observed during aging.[1] The age hardening response of Mg-Sn alloys can be increased through ternary or higher-order alloying additions. One popular ternary addition is Zn,[7–12] which leads to the precipitation of Mg2Sn on both basal and nonbasal (prismatic and pyramidal) planes during aging. The increase in hardness observed after Zn addition was attributed to the increased precipitate– dislocation interaction (because of the increased number density of precipitates on nonbasal planes) as well as a refinement in the precipitate size.[7,8] Additional elements have been added to Mg-Sn-Zn ternary alloys to further improve the age hardening response, often in the form of microadditions, and include such elements as Al,[10,12,13] Ag, Ca, Cu, and rare-earth-containing misc
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