Microalloying Effect on the Precipitation Processes of Mg-Ca Alloys
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I.
INTRODUCTION
BEING the lightest of all structural metals, magnesium alloys have the potential to replace conventional structural materials in certain applications where weight reduction is critical. However, only cast alloys are currently used in limited applications such as engine blocks, and no wrought products are used as structural components in transportation vehicles because magnesium alloys lack sufficient formability, strength, ductility, and corrosion resistance. When energy efficiency becomes more important than materials’ cost in transportation vehicles in the future, it might be possible to use wrought magnesium alloys as substitutes for some existing structural materials. For wider applications of wrought magnesium alloys, it is essential to develop higher strength wrought alloys. One way to achieve this is to develop agehardenable alloys that can be solution treated before wrought processes, and then the strength of final products can be enhanced by short heat treatments.[1] In our previous studies,[2–4] we reported that minor additions of Ag + Ca substantially enhance the agehardening response of the Mg-Zn alloy, which is attrib uted to the refinement of MgZn2 b01 precipitates.[2] The wrought alloys developed based on this system, Mg-2.4 Zn-0.1Ag-0.1Ca-0.16Zr (ZQXK), show a yield strength of 325 MPa with an elongation of 14 pct after a T6 heat treatment.[3,4] The fine precipitates that form dynamically during the extrusion process pin the grain boundaries to refine the recrystallized grain size after the wrought process, and the T6 heat treatment induces the dispersion CHAMINI L. MENDIS, Post Doctoral Researcher, and KAZUHIRO HONO, Manager of Magnetic Materials Unit, are with the National Institute for Materials Science, Tsukuba 305-0047, Japan. Contact e-mail: [email protected] KEIICHIRO OH-ISHI, formerly with the National Institute for Materials Science, is now Adjunct Associate Professor, with Nagaoka University of Technology, Nagaoka 940-2188, Japan. Manuscript submitted June 27, 2011. Article published online January 20, 2012 3978—VOLUME 43A, NOVEMBER 2012
of nanosized precipitates within the grains, which increases the yield strength to more than 300 MPa. A similar microstructure development has also been reported in Mg-Sn alloy microalloyed with Zn and Al.[5] These investigations suggest that the search for microalloying elements to base binary two-phase alloy is an effective way to develop new age-hardenable wrought alloys. The equilibrium phase diagram of the Mg-Ca system indicates that an a + Mg2Ca two-phase region exists.[6] Because the melting point of the Mg2Ca phase is ~711 C,[6] the alloys containing Mg2Ca may be a potential creep-resistant alloy.[7] Thus, in this article, we focus on the precipitation process of Mg-Ca based alloys. The age-hardening response of binary Mg-Ca alloys is almost nil because of a sluggish precipitation of coarse equilibrium Mg2Ca phase.[8] However, microalloying with Al, Zn, and In causes the precipitation of metastable plate-like precipitates with a h
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