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Development of high strength lightweight high entropy alloys (LWHEAs)

Polat, Gökhan.
In this thesis, the HEA formation ability of low-density elements was carefully studied in detail. In this respect, lightweight or relatively lightweight elements such as B, Mg, Al, Si, Ti, V, Cr, Mn and combinations of these elements with high-density elements such as Cu, Ni were used to produce novel lightweight high entropy alloys (LWHEAs) relative to the density of steel parts (~7.86 g/cm3). This thesis involves the design, production, and characterization of LWHEAs. The design and selection of the proper elements to be used in such alloys were calculated based on thermophysical calculations, Thermo-Calc HEA database and first-principle calculation with Vienna Ab Initio Simulation Package (VASP). Firstly, the HEA formation ability of selected elements and their compositions were determined by these computational methods. Then, the selected compositions were produced by copper heart arc-melting set-up adapted with suction casting technique. The LWHEAs are investigated in detail by using electron microscopy (SEM, TEM), X-ray diffraction (XRD) and thermal analysis techniques. The specimens were analyzed to determine the corresponding mechanical properties under compression test. The most promising alloy was found to be Cu29Ni29Al14Ti14Cr14 with its relatively low density (6.76 g/cm3) among the investigated LWHEAs. The suction cast LWHEA is annealed at 900 °C for 2 h. based on the phase diagram constructed by Thermo-Calc software. The annealed Cu29Ni29Al14Ti14Cr14 LWHEA shows optimal mechanical behavior with 855 MPa compressive strength, 22.6 % ductility and 338 HV hardness. Also, Cu29Ni29Al14Ti14Cr14 LWHEA was produced from industrial grade raw materials at scale-up using the induction casting technique. The 5 mm cylindrical samples annealed at 900 °C for 2 h. show 902 MPa yield strength, 1332 MPa fracture strength and 12.7 % ductility.