Effect of Sr Additions on the Microstructure and Strength of a Mg-Al-Ca Ternary Alloy
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I.
INTRODUCTION
THE development of cost-affordable magnesium alloys is an important issue in the automotive industry as part of an effort to achieve higher fuel efficiency through vehicle weight reduction.[1] In the last decade, there has been particular interest in creep-resistant magnesium die-cast alloys that are applicable to the powertrain components, such as engine blocks and transmission cases, which are to be exposed to the temperature range of 150 C ~200 C during service.[2] The recently developed creep-resistant die-cast alloys are mainly based on the Mg-Al alloy system with Al contents of 2 to 6 wt pct and up to 4 wt pct of alloying elements including Ca, Sr, Si, or RE (RE: rare earth mishmetal, which usually consists of Ce, La, Nd, and Pr).[3–8] The alloy design approach for attaining high-temperature creep resistance typically focuses on improving solid-solution strengthening of the a-Mg phase and increasing grain boundary strengthening by formation of intermetallic compounds that suppress the local deformation or sliding in the vicinity of grain boundaries. The contribution of the solid-solution strength to the critical resolved shear stress Ds of magnesium alloys can be generally described as follows: 4=3
Ds / eL C 2=3 where C is the solute concentration and eL is a parameter described as (g2 + a2d2)1/2 using the size misfit parameter d, the relative change of the shear A. SUZUKI, Research Fellow, N.D. SADDOCK, Graduate Student, J.W. JONES and T.M. POLLOCK, Professors, are with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA. Contact e-mail: [email protected] L. RIESTER, Senior Technical Advisor, and E. LARA-CURZIO, Distinguished Research Staff Member, Leader, are with the Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. Manuscript submitted June 12, 2006. 420—VOLUME 38A, FEBRUARY 2007
modulus at alloying g, and a numerical constant a [9,10] Among the elements listed previously, Al is the most efficient solid-solution strengthener for Mg. The atomic size misfit between Al and Mg is –11 pct,[11] and the maximum solubility of Al in Mg is 12.6 wt pct.[12] Other elements have larger atomic misfit than Al, but their solubility is almost zero, except for Ca, with 0.7 wt pct maximum solubility.[13,14] In Mg-Al-X ternary alloys where X is an element with low solubility in the a-Mg phase, the Al solute content in the primary a-Mg phase depends on the tie-line between the primary a-Mg and liquid (L) phases during solidification. At a temperature just below the liquidus temperature, the a + L tie-line lies between the primary a-Mg phase composition and the alloy composition. With decreasing temperature, the composition of the a phase shifts toward higher Al concentration according to the Mg-Al binary diagram, [12] and the composition of the liquid phase shifts toward higher ternary element concentration. When the composition of the liquid phase reaches that of the eutectic reaction, the liquid phase starts to decompos
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