Solidification Pathways of Alloys in the Mg-Rich Corner of the Mg-Al-Ba Ternary System
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DUE to their high-specific strengths, magnesium (Mg) alloys show potential as a replacement for conventional steels and aluminum (Al) alloys in transportation applications. Although Mg alloys have seen limited integration into automotive components, their inadequate creep performance hinders further incorporation.[1–5] The presence of thermally stable second phase particles in Mg alloy microstructures has been shown to improve creep resistance through grain boundary and dislocation pinning.[5–10] Mg-Al-Sr alloys have shown particular promise as nonrare earth containing die cast components for elevated temperature applications due to their good creep and corrosion resistance, in addition to castability.[9,11] The microstructures of these alloys consist of a-Mg grains surrounded by a eutectic structure with Al4Sr intermetallic particles.[11–13] However, further alloy development beyond the Mg-Al-Sr system is needed to achieve similar elevated temperature performance as current commercial Al alloys.[14] Previous research has shown that Mg-Al alloys with Ba additions exhibit improvements in both elevated temperature strength and creep resistance.[15–18] The individual binary phase diagram topologies in the Mg-Al-Ba and Mg-Al-Sr systems exhibit numerous topological similarities, which indicate that the cast microstructures may also be similar.[19–21] Further improvements in alloy creep ZACHARY L. BRYAN, formerly Graduate Student with the Materials Science and Engineering Department, University of Florida, Gainesville, FL 32611, is now Test Design Engineer with the Exactech, Gainesville, FL. Contact e-mail: [email protected]fl.edu RYAN J. HOOPER, Graduate Student, HUNTER B. HENDERSON, Post-doctoral Scholar, and MICHELE V. MANUEL, Associate Professor, are with the Materials Science and Engineering Department, University of Florida. Manuscript submitted August 3, 2014. Article published online 7 January 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
performance may be realized in the Mg-Al-Ba system because of higher intermetallic melting temperatures and lower enthalpies of formation.[20–24] This could lead to more effective particle dispersions and reduced thermal softening. Although there are disadvantages regarding the use of Ba, namely its high reactivity and toxicity,[25] its effect on microstructural development and mechanical properties can provide further insight into high-temperature alloy design. Currently, there is no published literature on the ternary Mg-Al-Ba phase diagram or the microstructural evolution during casting, which ultimately hinders further alloy development in this system. In the present work, an experimental investigation of five alloys with varying Al concentration in the Mg-Al-Ba system was completed. Since solidification behavior is critical in die and conventional cast processes, the preliminary heat flow curves and the corresponding cast microstructures were examined. To enable identification of the phases that were associated with the observed solidification reactions, an alloy with a highe
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