Influence of Heat Treatment on the Microstructure and Corrosion Behavior of Thixo-cast Mg-Y-Nd-Zr
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JMEPEG https://doi.org/10.1007/s11665-020-05085-1
Influence of Heat Treatment on the Microstructure and Corrosion Behavior of Thixo-cast Mg-Y-Nd-Zr Z. Szklarz and Ł. Rogal (Submitted May 31, 2019; in revised form July 26, 2020) The influence of semisolid metal processing (SSM, also called thixoforming) and T6 heat treatment (HT) on the microstructure and corrosion behavior in chlorides of Mg-Y-Nd-Zr (WE43B) magnesium alloy was investigated. The as-cast microstructure is composed of a-Mg grains with the size of 52.8 ± 1.9 lm surrounded by eutectic precipitations enriched in rare-earth elements (Y, Nd). The thixo-cast microstructure contained a-Mg globular grains with the size of 65.5 ± 2.1 lm surrounded by a fine eutectic mixture in the volume of 26.6%. The T6 HT (heat treatment and saturation at 525°C/5 h, cooling in H2O and aging at 190°C/48 h) caused an increase of yield strength to 180 MPa and tensile strength to 280 MPa at the hardness 105 ± 4 HV5. Next, the electrochemical response was investigated in 0.1 M NaCl using the global and local LSV (linear sweep voltammetry) and EIS (electrochemical impedance spectroscopy) methods. The EIS method suggests the same mechanism for the processes occurring at the electrode/electrolyte interface and shows higher values of the polarization resistances of treated samples after 24-h immersion tests. In particular, better corrosion resistance in chlorides is observed in the alloy after SSM compared to the SSM/ HT specimen, which has also been confirmed by the LSV tests performed after 24-h immersion. By using a local technique, a higher susceptibility of the matrix of SSM and SSM/HT samples to pitting corrosion has been revealed. Keywords
corrosion behavior, electrochemical impedance spectroscopy, magnesium alloys, micro-capillary, microstructure, thixoforming
1. Introduction Magnesium belongs to the group of active metals with relatively low electrochemical potential (E = 2.4 V vs. SHE), indicating a very high tendency to undergo oxidation (Ref 1-5). This feature of pure Mg and its alloys results in electrochemical activity, which is the reason for susceptibility to corrosion in highly conductive environments, like water solutions. On the other hand, magnesium alloys are promising as structural materials due to their low weight and environmental friendliness (Ref 6, 7). Nevertheless, their application is still relatively limited because of their low ductility and strength. At present, studies are focused on developing new chemical compositions and their further optimization as well as on new forming technologies (Ref 8, 9). Rokhlin et al. (Ref 10) indicated that the addition of rare-earth elements (RE), such as Gd, Y, Nd and mischmetal, leads to significant improvement in specific strength at both room and elevated temperatures, as well as creep resistance. There are many types of conventional magnesium-forming technologies (e.g., casting or plastic deformation, like rolling or Z. Szklarz, Department of Chemistry and Corrosion of Metals, Faculty of Foundry Engineering, AGH-Un
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