On the observation of a new ternary MgSiCa phase in Mg-Si alloys
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Research Associate, formerly with the Department of Mining and Metallurgy, Universite´ Laval, is with InTerMag Technologies, Inc., Sainte-Foy, PQ, Canada, G1P 4N7. A. COUTURE, Assistant Professor, A. VAN NESTE, Adjunct Professor, and R. TREMBLAY, Professor, are with the Department of Mining and Metallurgy, Universite´ Laval, Sainte-Foy, PQ, Canada G1K 7P4. Manuscript submitted November 12, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—Equilibrium phase diagram of the Mg-Si binary system.[17]
This thermal stability, being connected to diffusion phenomena, is proportional to the melting temperature of the precipitates. In this regard, Mg-Si alloys present a high potential for creep resistance due to the high melting temperature of the intermetallic, Mg2Si. The closest high melting point intermetallic containing Mg is found in alloy WE54 (Mg2Y at 780 7C).[17] Recent work by Carbonneau et al.[29] on the Ca-modified Mg-Si system has led to the observation of a new ternary phase identified as Mg-Si-Ca. This ternary phase is present in Mg-2 pct Si and Mg-4 pct Si alloys whenever the Ca weight fraction is greater than 0.8 pct. To our knowledge, the occurrence of this ternary phase has never before been reported in Mg alloys. Chemical analysis of the cast experimental alloys was performed by inductively coupled plasma from small chilled coupons (12-mm thick by 42 mm in diameter). The chemical composition of the cast alloys appears in Table I. The experimental melting, alloying, and casting procedures are described elsewhere.[29,30] Metallographic observations and quantitative metallography were performed using a Nikon Epiphot metallograph equipped with a Clemex Vision image analysis software (version 2.2.045). Volume percentages of the intermetallics, Mg2Si and MgSiCa, present in the alloys are given in Table II. As can be observed, increasing the Ca content from 0.8 to 2.6 pct in Mg-2 pct Si alloys (alloys A, B, and C) increases the volume fraction of the intermetallic MgSiCa from 1.6 to 5.5 pct, while the intermetallic Mg2Si is only present in alloy A (2.5 pct) at the 0.8 pct Ca level. The eutectic phase (Mg 1 Mg2Si) seems to be less affected by the presence of MgSiCa than the primary phase Mg2Si. These observations are confirmed by the micrographs presented in Figures 2(a) through (c). In these figures, the intermetallic Mg2Si has a rounded morphology (Figure 2(a)) and the ternary phase MgSiCa has a more needlelike shape (Figures 2(b) and (c)). The addition of Ca to the Mg-4 pct Si alloys shows a somewhat similar behavior. Indeed, as the Ca content increases from 0.8 to 1.6 pct in Mg-4 pct Si alloys (Figures 2(d) and (e)), the volume fraction of the MgSiCa ternary phase increases from 1.1 to 3.0 pct, while the volume fraction of the intermetallic Mg2Si decreases from 10.4 to 8.0 pct. As explained by Carbonneau et al.,[29] increasing the Ca content to values higher than the optimum value (0.4 VOLUME 29A, JUNE 1998—1759
Table I. Alloy A B C D E
Chemical Composition of Experimental Mg-2 Pct Si and Mg-4 Pct Si
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