Abstract
The thesis studies the deformation annealing behaviors and mechanical properties of FCC-type Al0.5CoCrCuFeNi high-entropy alloy, and finds the difference from traditional alloys and the possible method to produce nanocrystalline bulk alloys. In the first part, cast Al0.5CoCrCuFeNi was homogenized in vacuum at 1100 oC for 24 h with subsequent furnace cooling. The as-homogenizes samples were rolled with different thickness reductions at ambient temperature. The 50%-rolled sample was also annealed at 900 oC for different time. All the microstructural evolutions and mechanical properties were investigated. The results show that the alloy displayed significant work hardening and thus low dynamic recovery even during 900 oC forging. It is attributable to the multi-principal-element effect. The matrix with concentrated solute atoms had solution hardening to resist dislocation movement. In addition, dislocation cross-slip was difficult to operate because stacking fault energy and vacancy diffusion were both largely reduced. The initial deformation of Al0.5CoCrCuFeNi is accompanied by a lot of nano-twinning. This is attributable to the nano-precipitates in the matrix, which increases the stress for slip, and the low stacking fault energy which decreases the stress for twinning, respectively. Upon further deformation, the nanotwins intersected each other, forming nanograins. This is unique suggesting that a bulk nanocrystalline alloy can be obtained by simple rolling. In the annealing experiments, fully-annealed state was achieved after annealing for 5 h at 900 oC, suggesting large resistance to recrystallization. This is attributed to the low twin boundary energy and grain boundary energy which give a low driving force for recrystallization. In addition, sluggish diffusion effect also slows down the movement of grain boundary and dislocations. In the second part, cast Al0.5CoCrCuFeNi was homogenized in air at 1100 oC for 24 h with subsequent water quenching. The homogenized samples were then cold-rolled with 80 % thickness reduction. Some samples were further annealed at different temperatures. The results show that the as-rolled sample had high yield strength (1284 MPa) and moderate elongation to failure (7.6 %). After 900 oC for 10 min, the strength and elongation combination is optimized: elongation doubled to 15.2 % and yield strength (1021 MPa) decreased by 20 %. Longer annealing at 900 oC significantly decreased the strength due to further recovery and recrystallization. Lower temperature annealing below 800 oC increased the strengths but reduced the ductility due to the precipitation of BCC phase. From tensile testing, the elongation was quite low between 300 and 600 oC revealing intermediate-temperature embrittlement as seen in stainless steels. This phenomenon is attributed to the formation of BCC phase. The elongation gradually increases when the testing temperatures is higher than 700 oC presumably due to the activation of grain-boundary sliding. The above results suggest that the present alloys should avoid the heat treatment or applications between 300 and 600 oC.