Effects of Calcium on Strength and Microstructural Evolution of Extruded Alloys Based on Mg-3Al-1Zn-0.3Mn
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UCTION
MAGNESIUM (Mg) is the lightest structural metals and is now finding increased use in the transportation industry.[1–3] Currently, the most widely used Mg-alloys for engineering applications are cast alloys, and the adoption of Mg extrusion alloys has in contrast been quite limited.[4–6] Compared with aluminum extrusion alloys, the Mg extrusion alloys typically have lower extrudability, poorer corrosion resistance, and lower strength. Thus, improving the strength of Mg extrusion alloys would overcome a significant barrier for the wider
Z.R. ZENG, Y.M. ZHU, and J.F. NIE are with the Department of Materials Science and Engineering, Monash University, Clayton, VIC 3800, Australia. Contact e-mail: [email protected] S.W. XU is with the Research Institute (R & D Center), Baoshan Iron & Steel Co., Ltd, Shanghai 201900, China. Contact e-mail: [email protected] C.H.J. DAVIES is with the Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC 3800, Australia. N. BIRBILIS is with the Department of Materials Science and Engineering, Monash University and also with the College of Engineering and Computer Science, Australian National University, Canberra, ACT 2601, Australia. Manuscript submitted March 4, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
adoption of Mg extrusion alloys. In the last two decades, considerable efforts have been made to fabricate high-strength Mg extrusion alloys. Essentially, even without designing a new alloy, the commodity alloy Mg-3Al-1Zn-0.3Mn (AZ31) extrusion can be significantly strengthened by tailoring the grain size to the submicron scale, via low-temperature extrusion at 175 C.[7] However, low-temperature extrusion leads to tool wear in an industrial context. If extrusion temperature is increased, then grain growth tends to increase significantly, leading to coarse-grained structures and low strength. Therefore, there still remain optimization possibilities for alloy composition to produce highstrength Mg extrusion alloys. With respect to alloy design, the general approach adopted to date is to add a high concentration of rare-earth (RE) elements, such as Gd, Nd, Y, etc. For example, when produced by rapid solidification and hot extrusion, Mg-6.7Y-2.5Zn alloy has an exceptionally high strength of ~ 610 MPa, which is comparable to the high-strength Al-alloy AA7075-T6.[8] In addition, Mg-RE extrusions produced from bulk cast alloys can also have high strength. The strength of Mg-10Gd-5.7Y-1.6Zn-0.7Zr alloy is as high as 473 MPa.[9] Combined with other thermomechanical processing methods, such as rolling, the strength of
Mg-RE-based alloys can be further increased.[10,11] The high strength in the Mg-RE-based alloy is caused by the combined effect of fine grains and effective hardening from different types nanoprecipitates.[9,12,13] Although the strength of Mg extrusions can be effectively improved by the major addition of rare-earth elements, such alloying addition will—rather significantly— increase the alloy cost and the alloy density. Several RE-free hi
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