| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 107 === High entropy alloys (HEAs) with a face-centered cubic structure have recently received significant attentions due to their outstanding ductility and fracture toughness. In this study, the effects of Ti addition on the microstructures and mechanical properties of CoCrFeMnNi were investigated systematically. Three CoCrFeMnNiTix (x=0, 0.2 and 0.5 in molar ratio) HEAs were prepared by arc melting and studied in as-cast and annealed condition, respectively. It was found that Ti additions in HEAs led to the formation of intermetallic phases and increased the hardness. While Ti0.5 was embrittled due to the presence of hard intermetallic phases, the Ti0 and Ti0.2 ingots were then reduced in thickness of 80% by cold rolling and annealed for 1 h at two annealing temperatures, 1073 and 1173 K. The microstructure evolution, crystallographic texture, hardness and tensile properties were studied. The Ti0 alloy exhibited a single-phase FCC solid solution, while (Cr, Fe)-rich sigma phase and (Co, Ni)3Ti Laves phase precipitated in the Ti0.2 alloy after annealing. With precipitation of the hard yet brittle sigma phase and Laves phase, the yield strength of Ti0.2 alloy increased significantly but with the sacrifice in the tensile elongation. Both grain boundary strengthening and precipitation strengthening were the main strengthening mechanisms of the alloy doped with the Ti element.
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