Microstructure and compressive properties of chill-cast Mg-Al-Ca alloys
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aa) Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21218 (Received 20 July 2005; accepted 12 September 2005)
Mg82Al8Ca10 was determined to be a pseudo-binary eutectic composition [liquid solidifying into ␣–Mg and (Mg,Al)2Ca in the Mg–Al–Ca ternary system with a eutectic melting temperature of 789 K]. A series of Mgx(Al0.44Ca0.56)100−x alloys, where 75 艋 x 艋 95, were cast into ⌽4 mm rods using copper mold casting. The eutectic alloy exhibits the highest fracture strength, f ⳱ 609 MPa. For 75 艋 x 艋 79, the alloys have hypereutectic microstructures with Mg2Ca as the primary phase, and f is reduced together with diminishing plasticity. For hypoeutectic alloys with 86 艋 x 艋 95, the volume fraction of the primary ␣–Mg dendrites dispersed in the eutectic matrix increases with increasing x, resulting in a gradual decrease of the yield and fracture strengths but improved plastic strain to as large as 9%. The refined microstructures created in bulk samples via chill casting can lead to a good combination of strength and plasticity, with specific strength superior to commercial Mg alloys.
I. INTRODUCTION
Magnesium-based alloys are among the lightest of all structural metals. They have excellent strength (or stiffness)-to-weight ratio and are cost-effective in engineering applications. As such, Mg alloys are gaining growing interest in the automotive industry, as well as in electronics and aerospace applications.1–3 A more recent realization is that microstructural modifications can be achieved by improvements in the techniques used to process the alloy. This has led to extensive studies of property enhancements, particularly elevated strength, induced by tailoring the microstructure. For example, a Mg70Al20Ca10 alloy with a tensile yield strength of 600 MPa2 and a Mg86Al9Ca53 with a compressive fracture strength as high as 727 MPa have been prepared by extrusion consolidation of gas-atomized powders. The high strength was derived from the ultrafine and nanocrystalline grain structures achieved in the consolidated alloys. However, such a powder metallurgy processing route is costly and difficult to use in mass production, and may suffer from inherent flaws, such as porosity, impurities and internal stresses. Fine microstructures can also be induced by increasing the growth velocity (or cooling rate) during solidification a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0074 J. Mater. Res., Vol. 21, No. 3, Mar 2006
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of bulk ingots. In fact, in the extreme case one could attain amorphous Mg alloys at certain multicomponent compositions under chill-casting (copper mold casting) conditions, forming the so-called bulk metallic glasses (BMGs).4–13 The Mg-based BMGs can have a strength higher than all the crystalline Mg-alloys mentioned above. Unfortunately, BMGs plastically deform by the formation of highly localized shear bands that lead to catastrophic failure under unconstrained co
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