Abstract
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 α-Mg grains with the size of 52.8 ± 1.9 μm surrounded by eutectic precipitations enriched in rare-earth elements (Y, Nd). The thixo-cast microstructure contained α-Mg globular grains with the size of 65.5 ± 2.1 μm 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.
Highlights
Magnesium belongs to the group of active metals with relatively low electrochemical potential (E° = À 2.4 V vs. SHE), indicating a very high tendency to undergo oxidation (Ref [1,2,3,4,5])
The grains of a-Mg surrounded by the eutectic can be seen giving a lightgray diffusive contrast and a strong bright contrast (3.1 vol %) coming from the phases which solidified as the last ones
The proposed conditions of the SSM process and T6 heat treatment conducted on the WE43 magnesium alloy allowed material to be obtained with improved mechanical parameters and corrosion resistance in a chloride environment (0.1 M NaCl)
Summary
A new alternative to the abovementioned forming methods is semisolid metal processing (SSM), which utilizes the thixotropic behavior of metal alloys containing 15-80% of liquid fraction This is possible when the microstructure in the solidus–liquidus range consists of globular grains surrounded by a uniformly distributed liquid (Ref [17, 18]). The scientific goal presented in this paper was to analyze how the SSM process (thixo-casting, thixoforming) and the T6 heat treatment influence the microstructure, mechanical properties and corrosion behavior of the WE43 magnesium alloy. This has been carried out due to a lack of knowledge about the possible use of the WE43 alloy as a biomedical material, where. Volume 29(9) September 2020—6181 its properties (like mechanical strength and corrosion resistance) after SSM should be explained in more detail
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