Heat Treatment, Microstructure And Properties Of Some Selected Multi-principal Elements High-entropy Alloys

Multi-principal elements high-entropy alloy is a kind of new alloy design idea. In this thesis, Al0.5CoCrCuFeNi and AlCoCrFeNiTio.5 high-entropy alloy systems with potential application were studied verifiably as the first step, and some new properties were studied complementally. In order to explore the effective way to reduce alloy cost, two high iron content high-entropy alloy systems, i.e., AlCoCrFe1.5Ni and Al0.5CoCrFe2Ni were designed. The high-entropy alloys were prepared by water cooled copper crucible vacuum induction levitation melting, and the effects of heat treatment on microstructure, mechanical properties, corrosion resistance and oxidation resistance were studied systematically. The effects of hot rolling on microstructure and mechanical properties of Al0.5CoCrCuFeNi and Al0.5CoCrFe2Ni high-entropy alloys were also studied in this work. The main results were obtained as follows:The phase structures of both Al0.5CoCrCuFeNi and AlCoCrFeNiTi0.5 high-entropy alloys are simple, which is composed of two FCC phases and two BCC phases, respectively. Both alloys are dentrite morphology with dendrite segregation. The hardness of the two alloys is HV255.52 and HV597.06 for as-prepared samples, respectively, higher than the alloys prepared by other methods. Both of the hardness increase after heat treatment. Furnace cooling is better than air cooling and water cooling in increasing the hardness of the two alloys. Both alloys show good thermal stability, since their microstructure and hardness have no considerable changes even after annealed at 800 ℃ for 480h. Their corrosion resistance in NaCl solution (1 mol/L) is better than that of 304 stainless steel. After hot rolling, Al0.5CoCrCuFeNi high-entropy alloy become brittle, since its tensile strength and elongation were decreased. As to oxidation resistance at high temperature, Al0.5CoCrCuFeNi high-entropy alloy is antioxidant level at 800℃ and 900℃, and inferior antioxidant level at 1000℃ and 1100℃, while AlCoCrFeNiTio.5 high-entropy alloy is antioxidant level at 800 ℃,900 ℃ and 1100℃, but completely antioxidant level at 1000℃.For the high iron content high-entropy alloy systems AlCoCrFe1.5Ni and Al0.5CoCrFe2Ni designed in this paper, their phase structures are also simple. AlCoCrFe1.5Ni high-entropy alloy is composed of single BCC phase in equiaxed grain morphology, while Al0.5CoCrFe2Ni high-entropy alloy is composed of single FCC phase in dentrite.morphology. The compositions of the two alloys are uniformity, and no obvious segregation was found. The hardness of the two alloys in as-prepared state is HV510.06 and HV148.8 respectively. And it depends severely on annealing temperature. When increasing annealing temperature, the hardness of AlCoCrFe1.5Ni high-entropy alloy decrease, and reaches its minimum HV335.04 at 1000℃, while the hardness of Al0.5CoCrFe2Ni high-entropy alloy decreases in linear before 700 ℃, and reaches its minimum HV153.16 at 700℃, but suddenly increase to HV239.66 at 800℃, and then reaches its maximum HV295.94 at 900℃. The corrosion resistance of both alloys in NaCl solution (1 mol/L) is better than that of 304 stainless steel. The tensile strength and elongation of as-prepared AlCoCrFe1.5Ni and Al0.5CoCrFe2Ni high-entropy alloys are 822MPa and 11.17%,440MPa and 22.68%, respectively. After 30% hot rolling, the hardness of Al0.5CoCrFe2Ni high-entropy alloy increases about 103%, but with the further increase of rolling amount, the hardness of the alloy increases only a little. The tensile property of the alloy is improved after hot rolling, for its tensile strength and elongation are 796MPa and 25.50%, respectively.