Effects Of Cold Rolling And Heat Treatment On Microstructure And Properties Of Al_0.5CoCr_0.8FeNi_2.5V_0.2 High Entropy Alloy

The traditional strategy based on the concept of alloy design of one or two primary elements is difficult to meet the demand of higher performance materials.In recent years,the design concept of high-entropy alloys(HEAs)has opened up a wider space for composition design and provided more possibilities for exploring new materials with higher performance.By controlling the crystal structure and microstructure of HEAs utilizing composition design and thermo-mechanical treatment,or developing HEAs with heterogeneous microstructure and nanometer precipitation phase,the strength-ductility trade-off can be effectively overcome to obtain excellent comprehensive mechanical properties.In this thesis,the Al0.5CoCr0.8FeNi2.5V0.2 HEA was taken as the research object.The microstructure and mechanical properties of the alloy were regulated by cold rolling and heat treatment,and the deformation behavior and internal mechanism of Al0.5CoCr0.8FeNi2.5V0.2 HEA were systematically clarified.Firstly,the changes in microstructure and mechanical properties of homogenized Al0.5CoCr0.8FeNi2.5V0.2 HEA with cold rolling deformation were studied.The phase structure of homogenized alloy was composed of disordered FCC matrix and coherent ordered L12 precipitates,and the phase composition remained during cold rolling(CR)proceess.The microstructure of the homogenized sample presents a typical coarse equiaxed grain structure.With the increase of CR reduction,the grains were gradually stretched and deformed along the rolling direction.There are a lot of dislocations,slip bands and broken grain in the grains.Under 50%CR reduction,the deformation textures started to become evident and were governed by typical {111}<112>F,{110}<100>Goss and {112}<111>Cu texture components.When the CR reduction approached 90%,the deformation textures mainly contained the {110}<111>A,{114}<110>X,and {112}<111>Cu texture components.The yield strength of the homogenized sample at room temperature is 458±10 MPa,and the ductility is 45.3 ±0.5%.After cold rolling,it is a large number of pre-existing dislocations,slip bands,broken grain,cold rolled texture and grain boundaries or grain boundary-like structures appearing in the process of cold rolling that seriously hindered the movement and expansion of new dislocations.Meanwhile,the texture induced anisotropy and work hardening behavior,which restricted the easy-to-start matrix slip system and made it difficult to slip and expand.With the increase of rolling reduction,the strength of the alloy obviously increased and the ductility gradually decreased.Among them,the yield strength of the CR90%sample is about 1499 MPa,which is 3.27 times higher than that as-homogenized sample.Secondly,the evolution of microstructure and mechanical properties of cold rolled Al0.5CoCr0.8FeNi2.5V0.2 HEA with annealing temperature was investigated.The CR90%alloys were annealed at 873K,973K,1023K,1073K,and 1173K for 1h.After annealing,the L12 precipitates on FCC matrix,and the content of the L12 precipitates increase with the increase of annealing temperature.When the annealing temperatures reaches 1173K,the microstructure was completely recrystallized and consisted of equiaxed crystals and annealed twins.When the annealing temperatures were under 1173K,the samples were incompletely recrystallized to form heterogeneous microstructure.The mechanical properties of CR HEA are improved after annealing because of recrystallization grain refinement and precipitation strengthening of L12 phase.The sample at 873K has extremely high yield strength(～1582 MPa),and the sample at 1173K has excellent ductility(～44.1%),Among them,873K annealed sample obtained a high yield strength(-1582 MPa)while maintaining a certain ductility(～8.4%).For the 1023K sample,the yield strength of the sample reached 1434±5 MPa and the ductility remained at 15.6 ± 0.5%.which well realized the coordination between the strength and plasticity of the alloy.After annealing at 1023K for 1h,the CR samples not only significantly improved the yield strength of the material,but also maintained good plasticity and stable work hardening ability.Finally,the effect of annealing time on the microstructure and properties of CR Al0.5CoCr0.8FeNi2.5V0.2 high entropy alloy was studied.Based on the previous research,the CR90%alloys were annealed at 873K for 1h,6h,24h,48h,and 50h.The results show that the matrix of different times annealed sample keeps FCC structure and precipitates L12 phase on the substrate.With the extension of annealing time,the alloy structure gradually evolved into a unique layered heterogeneous microstructure.The heterogeneous microstructure leads to a large number of geometric necessary dislocation(GND)in the deformation process of the alloy,and the strain hardening ability increases with the increase of yield strength.This additional strengthening mechanism by back-stress leads to good combination of strength and ductility.The strength and ductility of the alloy have very complex changes under the comprehensive action of various mechanisms,and as a whole,it decreases first,then increases,and then decreases with the increase of annealing time.The yield strength and tensile strength of the samples at 873K for 48h reached a very high level,which was 1623±5 MPa and 1698±5 MPa,respectively,while the ductility remained at 16.3±0.5%,which realized a balance combination of high strength and ductility.