Microstructure And Mechanical Properties Of AlCoCrTiFeNi-Cux High-entropy Alloy

Different from traditional alloys,which consist of only one or two principal components,high-entropy alloys composed of five or more principal components have excellent mechanical properties and show great application potential as structural materials.Up to now,a lot of researches have focused on high entropy alloys with face-centered cubic structure because of their good ductility.In general,the strength of single-phase high-entropy face-centered cubic alloys is not enough,which limits the application of these alloys as structural materials.In order to overcome this limitation,a series of new high-entropy alloys containing Cu were designed,and the effect of Cu content on the microstructure and mechanical properties of as-cast high-entropy alloys was investigated.At the same time,combined with the means of deformation heat treatment,the strategy of optimizing the microstructure and properties of a new type of high entropy alloy containing Cu is put forward.The microstructure evolution and precipitation behavior of a new type of Cu-containing high-entropy alloy during deformation heat treatment were also investigated.In this thesis,a series of non-isoatomic high entropy alloys with AlCoCrTiFeNi-Cux ratio are designed,the microstructures and mechanical properties of as-cast high entropy alloy were studied by microhardness tester,tensile tester,optical microscope,transmission electron microscope and atomic probe tomography.In this thesis,the tensile mechanical properties of the designed high entropy alloy were improved by cold rolling and annealing,and the microstructure evolution,nano-phase precipitation mechanism and strengthening behavior during annealing were studied,the main findings are as follows:（1）In as-cast AlCoCrTiFeNi-Cux high entropy alloy,the increase of Cu content coarsened the dendrite structure,promoted the precipitation of L1₂ nano-phase,and decreased the tensile strength of the as-cast alloy.（2）Cold-rolled AlCoCrTiFeNi-Cux columnar crystal high entropy alloy can obtain the microstructure with uneven deformation.The multi-scale grains can be formed stably by annealing the alloy with non-uniform deformation.Large deformation cold rolling combined with annealing at 700^oC for 4h can change the size and distribution of grain and lamellar conformal nano precipitates,the balance of strength（about 1.68 GPa）and elongation（about13.5%）is achieved in T700 high entropy alloy,and the comprehensive mechanical properties are better than those reported previously.The theoretical results are in good agreement with the experimental results,which provides a new strategy for the development of high entropy alloys and other similar alloys.（3）The AlCoCrTiFeNi-Cu2.5 high entropy alloy was annealed at 550^oC after cold rolling.After annealing for 1h,L1₂ precipitates nucleated and grew up,and the hardness of the Cu2.5alloy increased.The L1₂ phase continued to precipitate after annealing for 4h,and the hardness remained stable.After annealing for 32h,the density of precipitates decreased,the size of precipitates increased,and the hardness of Cu2.5 alloy decreased again.After annealing for 64h,the morphology of precipitates changed from spherical to lamellar,and the second peak of hardness appeared.After annealing for 256h,the lamellar coarsened,the strengthening effect weakened and the hardness decreased again.（4）The AlCoCrTiFeNi-Cu2.5 high entropy alloy was annealed at 550^oC after cold rolling.The chemical composition of L1₂ precipitates was not stable after annealing for 4h.After annealing for 32 h,the concentration of Ni,Cu,Al and Ti in the matrix decreased,while the concentration of Co,Cr and Fe in the matrix increased.After annealing for 64 h,the chemical composition of precipitates was stable.After annealing for 256h,the concentration of each element no longer changed obviously,and L1₂precipitates reached the ideal stoichiometry of（Ni,Cu）₃（Al,Ti）.