Production and characterization of titanium (Ti) doped AlCoCuNi high entropy alloys by mechanical alloying

In this thesis, 4-8% (at.) Ti doped AlCoCuNiTi high entropy alloys were produced as nanostructured powder by mechanical alloying at different milling times. The microstructural characterization of the produced alloys was performed by X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray (SEM/EDX) analyzes, and also thermal characterization was performed by differential thermal analysis (DTA). The surface morphologies of the samples formed as a result of powder metallurgy processes were examined with an optical microscope (OM) and microhardness analysis was also performed. XRD results showed the formation of intermetallic and solid solution phases at 80-120 h milling, and also showed that as the milling time increased, the crystallite size decreased while the lattice stress and dislocation density increased. By using the maximum peak values obtained in XRD scanning in the Scherrer equation, the minimum crystallite size value was found to be approximately 3 nm for the 8% Ti doped AlCoCuNiTi high entropy alloy. SEM/EDX analyzes showed that as the milling time increased, fragmentation, breakage, aggregation and welds occurred, the particle size decreased and a more homogeneous structure was formed. DTA analyzes showed an exothermic peak indicating the crystallization of the AlNi3 phase at a temperature of about 565 oC at low milling times, while it did not show any peak at high milling times (30-120 hours). OM analyzes showed that as the milling time increased from 0 to 120 hours, there was a decrease in particle size, a more homogeneous structure and a decrease in the amount of voids. It was observed that the microhardness of the pressed and sintered AlCoCuNiTi high entropy alloy samples increased as the milling time increased as a result of the formation of new intermetallic or solid solution phases. The maximum microhardness value in the produced alloys was found to be 882 ± 20 HV1 in the 8% Ti added AlCoCuNiTi alloy sample produced by milling for 120 h and sintered at 815 oC. According to the results obtained, it was understood that the mechanical properties of the 8% Ti doped AlCoCuNiTi alloy produced by milling for 120 h were superior to the mechanical properties of all other alloys produced in this study.