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Physical and Corrosion Properties of CoCrNi Medium Entropy Alloy Thin Films
Mohamed, Omer Fathalrahman
Mohamed, Omer Fathalrahman
Date
2021-11
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Thesis
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Description
A Master of Science thesis in Mechanical Engineering by Omer Fathalrahman Mohamed entitled, “Physical and Corrosion Properties of CoCrNi Medium Entropy Alloy Thin Films”, submitted in November 2021. Thesis advisor is Dr. Wael Abuzaid and thesis co-advisor is Dr. Mehmet Egilmez. Soft copy is available (Thesis, Completion Certificate, Approval Signatures, and AUS Archives Consent Form).
Abstract
Multi-principal element alloys (MPEA) have garnered significant research interest in the last decade due to their desirable physical and mechanical properties which defy well-known trends. This thesis considers the equiatomic CoCrNi alloy, a typical multi-principal element alloy, which in bulk form, has shown superior mechanical properties in addition to excellent oxidation and corrosion resistance. Such a combination of desirable characteristics motivates further research into the potential use of CoCrNi in thin-film form for various applications. This work aims to grow and optimize the deposition parameters of CoCrNi thin films and investigate their influence on the physical properties. In addition, the corrosion properties of the films as a function of thickness will be studied. The CoCrNi medium entropy alloy thin films were deposited onto Si/SiO₂ substrates by direct current (DC) magnetron sputtering at different substrate temperatures and working gas pressures using a pre-alloyed target. The equiatomic polycrystalline composition was obtained at optimized pressure (2 mTorr) and temperature (400 °C). The electrical and magnetic properties were measured and contrasted with the physical properties of the bulk CoCrNi. The obtained magnetic properties indicate stabilization of the ferromagnetic phase in CoCrNi thin films. Anomalous Hall effect has been observed for all studied films. To study the corrosion behavior, the polycrystalline thin films, 225 nm - 600 nm thickness range, were deposited on mild steel substrates and the corrosion characteristics were investigated in 3.5 wt. % NaCl solution. The corrosion rates were determined for CoCrNi in both, bulk and thin-film form. At relatively small thicknesses, the CoCrNi films result in significant improvement in corrosion resistance, and the highest efficiency achieved relative to the mild steel was 94.79%. In summary, this thesis shows that deposition conditions (e.g., substrate temperature and working pressure) can be utilized to control the microstructure and tailor the physical properties of MPEA thin films for desired applications. For example, and as shown in this work, through proper optimization of CoCrNi thin film composition and thickness, superior corrosion properties can be attained which allow for a considerable enhancement in the performance of widely utilized mild steels.