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
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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)