IP: 5.189.200.22 On: Tue, 18 Dec 2018 11:34:12 Copyright: American Scientific Publishers Delivered by Ingenta Copyright © 2018 American Scientific Publishers All rights reserved Printed in the United States of America Article Journal of Nanoscience and Nanotechnology Vol. 18, 6081–6089, 2018 www.aspbs.com/jnn Microstructural Evolution and Electrochemical Properties of HRDSR AZ61-X (X = Ca, Ti) Alloys Min Gyu Kim 1 , Woo Jin Kim 2 , Gyeung-ho Kim 3 , Kwon-Koo Cho 4 , Jun Hyun Han 5 , and Hye Sung Kim 1 ∗ 1 Departmemt of Nanofusion Technology, Pusan National University, 50 Cheonghak-ri, Samnangjin-eup, Miryang-si, Kyongnam 627-706, Korea 2 Department of Materials Science and Engineering, Hongik University, 72-1 Mapo-gu, Sangsu-dong, Seoul 121-791, Republic of Korea 3 Advanced Analysis Center, Korea Institute of Science and Technology, Cheongryang, P.O. Box 131, Seoul, Korea 4 Department of Metallurgical and Materials Engineering, Gyeongsang National University, 501 Jinju-daero, Jinju 52828, Gyeongsangnam-do, Republic of Korea 5 Department of Nanomaterials Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea The microstructure and corrosion properties of as-cast AZ61 (Mg–6%Al–1%Zn) and AZ61 alloys doped with titanium and calcium and subjected to high ratio differential speed rolling were inves- tigated. Addition of the alloying elements to the AZ61 alloy resulted in remarkable modification of the morphology and the amount of continuous  (Mg 17 Al 12 -phase. Addition of Ti to the as-cast AZ61 alloy causes a decrease in the volume fraction (or discontinuity of the -phase), leading to strong anodic dissolution. In contrast, addition of Ca to the as-cast AZ61 alloy is rather effective for preventing pitting corrosion. This is attributed to the formation of a semi-continuous network -structure. The (Mg, Al) 4 Ca phases dispersed between the  (Mg 17 Al 12 -phases led to continuity in the AZ61 alloy with Ca. The AZ61 and AZ61-X(Ca, Ti) alloys subjected to severe plastic defor- mation via high-ratio differential speed rolling possessed a nano-composite-like microstructure in which the -Mg matrix with an ultra-fine grain was surrounded by a large number of fine  particles. These particles were either dynamically precipitated or broken at the grain boundaries, as well as in the grain interiors, by the high ratio differential speed rolling process. The corrosion resistance of the AZ61 and AZ61-X (X = Ca, Ti) alloys subjected to high ratio differential speed rolling was largely improved by the microstructural modification. The high ratio differential speed rolling process greatly influenced the texture of the Mg alloys, which significantly affected their corrosion behavior. Keywords: AZ61-X(Ti, Ca) Alloys, Ultra-Fine Grain, Microstructural Modification, Potentiodynamic Polarization, High-Ratio Differential Speed Rolling. 1. INTRODUCTION Magnesium and magnesium alloys have been studied for applications in various fields, including the automotive, aerospace, microelectronics, and telecommunication indus- tries. Despite the various advantages of Mg alloys, their application as engineering materials is still limited due to their susceptibility to corrosion in aqueous environment as well as their relatively low strength. Because of their high Hall-Petch strengthening coeffi- cient, grain refinement has been proposed as one method of improving the strength of Mg alloys. Equal channel ∗ Author to whom correspondence should be addressed. angular pressing (ECAP) or high ratio differential speed rolling (HRDSR) processing have also been proposed as effective processing methods for decreasing the grain size of Mg alloys. 1 Another approach for obtaining a fine grain size is the addition of alloying elements. Mg–Al–Zn (AZ) alloys containing 6–9% aluminum are the most common commercial alloys that the added aluminum significantly improves the strength and corrosion resistance of magne- sium due to preferential precipitation of the more inert Mg 17 Al 12 (-phase in chloride solution rather than the -Mg matrix during solidification. 2–6 For pure Mg and Mg alloys, spontaneous ignition at low temperature (less than 400  C) in air becomes a serious problem. Ca has J. Nanosci. Nanotechnol. 2018, Vol. 18, No. 9 1533-4880/2018/18/6081/009 doi:10.1166/jnn.2018.15609 6081