Thermodynamics and Constitution of Mg-Al-Ca-Sr-Mn Alloys: Part I. Experimental Investigation and Thermodynamic Modeling of Subsystems Mg-Ca-Sr and Al-Ca-Sr

Journal of Phase Equilibria and Diffusion, Feb 2009

Ternary Mg-Ca-Sr and Al-Ca-Sr phase transitions were experimentally investigated by differential thermal analysis; phase formation in slowly solidified samples was analyzed with scanning electron microscopy and electron probe microanalysis. Significant mutual ternary solid solubilities of pertinent intermetallic phases are revealed and quantitatively introduced in thermodynamic descriptions that were developed for the Mg-Ca-Sr and the Al-Ca-Sr systems. Calcium and strontium are two chemically rather similar alloying elements. Even so, nontrivial ternary phase equilibria occur in these systems, and these alloys should not be simplified as “binary” Mg-(Ca + Sr) or Al-(Ca + Sr) alloys. Experimental work was limited, and, thus, reported results are affected by a relatively high level of uncertainty.

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Thermodynamics and Constitution of Mg-Al-Ca-Sr-Mn Alloys: Part I. Experimental Investigation and Thermodynamic Modeling of Subsystems Mg-Ca-Sr and Al-Ca-Sr

0 A. Janz and R. Schmid-Fetzer, Institute of Metallurgy, Clausthal University of Technology , Robert-Koch-Str. 42, D-38678 Clausthal- Zellerfeld, Germany . Contact Ternary Mg-Ca-Sr and Al-Ca-Sr phase transitions were experimentally investigated by differential thermal analysis; phase formation in slowly solidified samples was analyzed with scanning electron microscopy and electron probe microanalysis. Significant mutual ternary solid solubilities of pertinent intermetallic phases are revealed and quantitatively introduced in thermodynamic descriptions that were developed for the Mg-Ca-Sr and the Al-Ca-Sr systems. Calcium and strontium are two chemically rather similar alloying elements. Even so, nontrivial ternary phase equilibria occur in these systems, and these alloys should not be simplified as ''binary'' Mg-(Ca + Sr) or Al-(Ca + Sr) alloys. Experimental work was limited, and, thus, reported results are affected by a relatively high level of uncertainty. 1. Introduction The global goal of this study is to generate a thermodynamic description of phase equilibria in the Mg-Al-Ca-SrMn alloy system, validated with an emphasis to Mg-rich alloys. These alloys are important for an extension of the portfolio of lightweight magnesium alloys, where the alloying elements are usually denoted by the ASTM letter code for Al (A), Ca (X), Sr (J), Mn (M). This covers the area of advanced creep-resistant alloys of the AJ and AJX series, but also to the modification of AM using Ca and Sr for improved properties. Combined with a major database including the important component Zn (Z),[1] it will also cover such modifications involving the popular Mg alloy series AZ. Thus, thermochemical calculations will be enabled in the (Mg)-AZMXJ alloy systems, which are an important tool for focused alloy development and process optimization.[2] The main focus is presently on the Mg-Al-Ca-Sr phase equilibria, where the investigation of two of the ternary subsystems were recently finished, namely Mg-Al-Ca[3] and Mg-Al-Sr.[4] The remaining ternary systems Mg-Ca-Sr and Al-Ca-Sr involve two chemically rather similar alloying elements, Ca and Sr. Even so, it will be shown that nontrivial ternary phase equilibria occur in these systems. This highlights the need to study the ternary interactions in detail rather than summing up the Ca and Sr contents and treating these alloys as simplified binary Mg-(Ca + Sr) and Al-(Ca + Sr) alloys. The purpose of part I of this work is to generate a consistent thermodynamic description of the phase equilibria in both ternary systems by combining experimental work with thermodynamic modeling. 2. Experimental Data and Thermodynamic Descriptions in the Literature Experimental information on ternary phase equilibria or thermodynamics for these two systems is scarce. No data could be found in the literature for the ternary Mg-Ca-Sr system, and only one study relates to the Al-Ca-Sr system. Zhang et al.[5] investigated six alloys in the section Sr1 xCaxAl2 (0 x 1) with x-ray diffraction (XRD). Their main interest was in the hydrogenation behavior of these alloys, but they also characterized the alloys before exposure to hydrogen. Based on the variation of lattice parameters of the terminal compounds, which crystallize in significantly different structures, they determined a small solubility of Ca in the orthorhombic Zintl phase Al2Sr (space group Imma, Pearson symbol oI12) and a very large Sr solubility in the cubic Laves phase C15-Al2Ca (space group Fd 3m, Pearson symbol cF24). A narrow two-phase region was found to exist between these phases. The six samples were analyzed in as-cast condition after four times remelting in an arc furnace. Ternary thermodynamic calculations reported in the literature are all based on extrapolations of Calphad-type assessments of the binary edge systems. For the Mg-Ca-Sr system Zhong et al.[6] performed such calculations, using the assumption of ideal mixing between the C14 Laves phases Mg2Ca and Mg2Sr and employing a reassessed binary Ca-Sr dataset. Aljarrah and Medraj[7] reassessed the three binaries using the modified quasi-chemical model. With these datasets and Kohler-based extrapolation for the ternary liquid phase they calculated some Mg-Ca-Sr liquidus projections assuming three different scenarios: (a) zero solubility between Mg2Ca and Mg2Sr, (b) a random solution model for the liquid phase, and (c) complete solid solubility between Mg2Ca and Mg2Sr. These authors[8] applied the same procedures also to the Al-Ca-Sr system. They converted existing thermodynamic descriptions to the modified quasi-chemical model in a reoptimization of the binary subsystems. No ternary solubilities of the binary phases were considered in that work; specifically, the only experimental work in that system, revealing extended solutions on the Al2Ca-Al2Sr section,[5] was not considered. 3. Experimental Study 3.1 Sample Preparation In the present study we have used such ternary thermod (...truncated)


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A. Janz, R. Schmid-Fetzer. Thermodynamics and Constitution of Mg-Al-Ca-Sr-Mn Alloys: Part I. Experimental Investigation and Thermodynamic Modeling of Subsystems Mg-Ca-Sr and Al-Ca-Sr, Journal of Phase Equilibria and Diffusion, 2009, pp. 146-156, Volume 30, Issue 2, DOI: 10.1007/s11669-009-9467-4