Mechanical Properties and Corrosion Behavior of As-Cast Mg-Zn-Zr-(La) Magnesium Alloys

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Mechanical Properties and Corrosion Behavior of As-Cast Mg-Zn-Zr-(La) Magnesium Alloys Huseyin Zengin, Yunus Turen, Hayrettin Ahlatci, and Yavuz Sun (Submitted August 25, 2017; in revised form October 19, 2017) The effects of La content on the mechanical properties and corrosion behavior of as-cast Mg-Zn-Zr (ZK) magnesium alloys were investigated. The results showed that La addition to ZK60 alloy resulted in a considerable grain refinement and an increase in the volume fraction of second phases by formation of MgZn-La ternary phase. The tensile tests were performed at both room and elevated temperatures. At room temperature, the yield and tensile strengths and elongation-to-fracture significantly increased with 0.2 wt.% La addition, then deteriorated slightly with increasing La additions above 0.2 wt.%. At 200 °C, the yield and tensile strengths continually improved and elongation-to-fracture decreased with increasing La content due to the presence of thermally stable Mg-Zn-La phase. Immersion and electrochemical corrosion tests revealed that the formation of Mg-Zn-La phases led to a higher amount of cathodic sites and preferential corrosion propagation, and thus a decrease in the corrosion resistance of ZK60 alloys. Keywords

corrosion, La modification, mechanical properties, Mg-Zn-Zr magnesium alloys, microstructure

1. Introduction In recent years, researchers have paid more attention to MgZn-based alloys due to the beneficial effects of Zn on mechanical properties and corrosion resistance of pure Mg (Ref 1, 2). Zn addition can improve the strength by combination of several mechanisms such as grain refinement, solid solution strengthening and precipitation strengthening (Ref 1). In addition, Zn contributes to formation of protective film on Mg alloys and improves corrosion resistance (Ref 2, 3). On the other hand, Zr is known as the most potent grain refiner in Mg by generating nucleation sites in liquid phase during casting (Ref 4). However, the grain refining ability caused by Zr can be poisoned by Al, Mn and Si. Therefore, Mg-Zn-Zr alloy, also denoted as ZK, system has risen to prominence since they possess good combination of strength and ductility at room and elevated temperatures. However, these properties are still inferior to other structural metals such as aluminum and steel. In order to further improve the mechanical properties of ZK series magnesium alloys, rare earth elements (RE) and different processing conditions such as solution heat treatment, aging treatment and hot forming processes, have been widely employed thus far (Ref 5-8). It was reported that about 1 wt.% Y addition improved mechanical properties of as-cast Mg-Zn-Zr alloy (Ref 6). Lan and Huo (Ref 7) investigated the effects of 1 wt.% Nd partial substitution for Y on mechanical properties of Mg-Zn-Y-Zr alloys and showed that the Nd substitution had a detrimental effect on the as-cast mechanical

Huseyin Zengin, Yunus Turen, Hayrettin Ahlatci, and Yavuz Sun, Department of Metallurgical and Materials Engineeri

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