Electrospark deposition of AlCoCrFeNi high entropy alloy on AISI 1045 carbon steel

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Electrospark deposition of AlCoCrFeNi high entropy alloy on AISI 1045 carbon steel


Author: Li, Qiaohui
Title: Electrospark deposition of AlCoCrFeNi high entropy alloy on AISI 1045 carbon steel
Degree: M.Phil.
Year: 2012
Subject: Carbon steel -- Corrosion.
Metals -- Surfaces.
Corrosion and anti-corrosives.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Industrial and Systems Engineering
Pages: xiv, 153 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2522687
URI: http://theses.lib.polyu.edu.hk/handle/200/6586
Abstract: Medium-carbon steel is used extensively for a wide range of engineering applications because it is relatively inexpensive and possesses a good balance of mechanical properties. Unfortunately, it has poor corrosion resistance that limits its applications in most aggressive corrosive environments. As corrosion is a surface or near-surface phenomenon, it has long been realised that corrosion resistance of a material can be improved by different surface coating techniques. Among the many surface coating methods, electro-spark deposition (ESD) is considered to be an ideal process for improving corrosion resistance, while retaining part dimensions, with little effect on the substrate microstructure. In this research study, ESD has been successfully employed to clad the AlCoCrFeNi high entropy alloy (HEA) on AISI 1045 carbon steel. The aim of the research being to improve the corrosion resistance of the steel has been achieved. The microstructure and solidification behaviour, corrosion properties, and wear properties of the HEA coatings were studied and, furthermore, they were compared with those of a copper mould cast material. Indeed, this study was the first attempt of utilising ESD to deposit HEA coatings on steel and to study, in detail, the relationship of the microstructure-properties relationship of the coatings.
The research study found that AlCoCrFeNi HEA coatings, with only a small amount of porosity, can be fabricated onto the carbon steel by ESD. More importantly, a metallurgically bonded interface which was crack free was obtained between the steel substrate and the coating. When the number of deposition passes was beyond a certain level, in this study fifty, only a narrow dilution zone formed at the substrate-coating interface, and the remainder of the coating was undiluted. Considering microstructural analysis, essentially, only simple BCC phases were found in the copper mould cast material and the electrospark deposited coatings. However, there are two major microstructural differences that exist between the cast HEA material and the HEA coatings. First, the cast material comprises both columnar and equiaxed crystals and there is a columnar-to-equiaxed transition (CET), whereas the HEA coatings consist of an entire columnar crystal structure. Second, unlike the cast HEA material, there was no appreciable interdendritic phase and second phase precipitates found in the HEA coatings. This second difference was found to have a major effect on the corrosion properties. The conditions for CET for the copper mould casting and the ESD of AlCoCrFeNi were analysed based on Hunt’s criteria. The results agreed with the findings of the microstructural examination. With regard to corrosion properties, the corrosion rate of the HEA coated specimen was seven times and three orders of magnitude lower than that of the cast HEA and 1045 steel, respectively. The superior corrosion resistance of the HEA coating was confirmed by the results of the electrochemical impedance spectroscopy test. This demonstrates that ESD of AlCoCrFeNi coatings on 1045 carbon steel can significantly improve the corrosion resistance of the substrate material. The excellent corrosion resistance of the electrospark deposited HEA was largely due to the absence of interdendritic segregation and the absence of precipitates within the dendrites. With regard to hardness, the HEA coating has a hardness value of twice that of the steel substrate, and was even higher than that of the cast HEA. However, the wear resistance of the HEA coating did not correspond to its high hardness value when compared to the relatively soft 1045 steel. This was attributed to the HEA coating having a single hard BCC structure which suffered from brittle abrasive wear.

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