Effect Of Grain Refinement And Precipitation Strengthening On Mechanical Properties Of Al_0.1CoCrFeNi High Entropy Alloy

The high-entropy alloy?HEA?is a new class of alloy system different from traditional alloys.The traditional alloy-design strategy is based on one or two major element,by adding a small amount of other elements to optimize the properties.The ability to optimize the structures and properties of the alloy is limited.However,the HEA is synthesized from five or more different element in equimolar ratio or near equimolar ratio.Due to the unique high mixing entropy,the multicomponent alloys easily obtain simple solid solutions instead of complex compounds.This kind of alloy-design concept breaks through the limitation of traditional strategy and opens up a broad prospect for the alloy development.Generally,the face-centered cubic?FCC?solid-solution HEAs exhibitexcellentplasticity,loweryieldstrength,andgood thermodynamic stability.Therefore,they have great potential as future engineering materials.In this thesis,the Al_0.1CoCrFeNi HEA is chosen as the base alloy.Hall-Petch relationship,grain-size effect and deformation mechanism of the current alloys are systematically studied.Moreover,the property is further improved by means of processing adjustment and element addition,and the underlying strengthening mechanism is quantitatively investigated.The main research contents are presented as follows:?1?Al_0.1CoCrFeNi HEAs with different grain sizes are obtained through cold-rolling and annealing at different temperatures.Based on the quasi-static tensile tests,the Hall-Petch relationship is obtained.Through electron back-scattered diffraction?EBSD?and transmission electron microscopy?TEM?experiments,the deformation mechanism of coarse-grained?CG?and fine-grained?FG?alloys is analyzed and the twinning behavior of the alloys is also studied.The role of twins on the work-hardening process is uncovered,and the reasons of FG alloys having higher strength and lower plasticity than CG alloys are also explained.?2?Further strengthening and toughening of the Al_0.1CoCrFeNi HEA are attained by combined effects of high leveled rolling and annealing.The microstructures of the alloys are analyzed by scanning electron microscopy?SEM?and TEM,as well as the room-temperature mechanical properties by quasi-static tensile tests.On the basis of four strengtheningmechanisms,suchassolutionstrengthening,grain-boundary strengthening,dislocation strengthening and precipitation strengthening,the strengthening mechanism of the alloys is quantitatively analyzed,and the role of each strengthening on the overall strengthening is also discussed.?3?A trace amount of boron is added to the original composition to precipitate the second phase in the alloy.The microstructures of alloys with different boron content are analyzed by optical electron microscopy?OM?and SEM.The effects of boron addition on the tensile and compressive properties of Al_0.1CoCrFeNi high entropy alloy are investigated.