Effect Of Annealing And Deformation On The Properties And Microstructure Of Al_0.3CoCrFeNi High-entropy Alloy

As a new alloy material design and preparation direction,high-entropy alloy has attracted the attention of researchers as soon as its concept was proposed.Research has found that high-entropy alloys have many unique properties due to their unique crystal structure,such as high strength,good plastic toughness,and excellent corrosion resistance,oxidation resistance,wear resistance,and thermal stability.These excellent properties also make high-entropy alloys have high potential application in the fields of aerospace and nuclear energy.Due to the wide design range of high-entropy alloys,many more mature alloy systems have been successfully designed and prepared.Among them,Al_xCoCrFeNi high entropy alloys have different microstructures under different Al element contents,and their performance characteristics also have a large adjustable range,which is an alloy system that has been studied more recently.In this paper,Al_0.3CoCrFeNi high entropy alloy was chosen as the research object.The deformation and annealed microstructures were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),and transmission electron microscopy(TEM).Mechanical properties of the alloy were tested by stretching machine.Al_0.3CoCrFeNi high-entropy alloy ingot was prepared by arc melting.Then,the as-cast alloy was annealed under different conditions.It was found that the as-cast Al_0.3CoCrFeNi high-entropy alloy is a face-centered cubic single-phase solid solution with a coarse grain size of about 340?m and the as-cast alloy has excellent plasticity but not high strength.The annealed samples were found to have an ordered L12 phase in the grains in the temperature range of 500°C to 700°C,and the size of the L12phase grew from 1nm at 500°C to 50nm at 700°C as the annealing temperature increased.After annealing at 800°C,an ordered B2 phase with a body-centered cubic structure is precipitated in the grains and grain boundaries of the alloy.This phase particle has a Kurdjumov-Sachs(K-S)relationship with the matrix.As the annealing temperature increases,the yield strength of the alloy also increases in the temperature range of 500?to 660?.This may be due to the pinning effect of L12 phase to the relative dislocation.The strength of the annealed sample at 700?was lower than that of the sample at 660?,which may be due to the coarsening of the precipitated L12phase and the decrease of the distribution density.After annealing at 800?,the strength of the alloy is significantly improved due to the precipitation strengthening effect of the B2 phase in the grains.However,the B2 phase precipitated at the grain boundary makes the crack easily growth at the grain boundary during the deformation process,making the alloy poor in plasticity,and the elongation of the tensile sample is only34%.After cold rolling deformation of Al_0.3CoCrFeNi high-entropy alloy with 90%deformation,the alloy grains are broken and deformed,and dislocation lines in the grains are entangled and plugged,which has a significant work hardening effect.The strength of the alloy is increased to 1260MPa and the elongation dropped to 2.5%.Subsequently,the as-rolled samples were annealed at different temperatures.As the annealing temperature increased,the recrystallization degree of the alloy continued to increase,and all regions were recrystallized after annealing at 800°C.After annealing at 660°C,the as-rolled region and the recrystallized region of the alloy show a band-like phase distribution,and precipitated phase particles begin to precipitate inside the recrystallized grains.The yield strength of the annealed sample at 660?reached1010 MPa,and the elongation was 22%.It has excellent comprehensive mechanical properties.After annealing at 660?and higher,there are many annealing twins in the recrystallized grains of the sample.After rolling and annealing,bcc phase,?phase and B2 phase particles were precipitated in the alloy.The bcc phase precipitates at grain boundaries at lower annealing temperatures of 500°C,540°C,and 590°C.It is a body-centered cubic structure phase with a Cr element content of 80wt%.The?phase appears at all annealing temperatures and belongs to the 136-P42/mnm space group structure.Generally,the B2 phase is accompanied by precipitation near the?phase.The B2 phase is the most common phase particle at all annealing temperatures,and is also the largest number of phase particles.It is a body-centered cubic structure phase.The precipitated phases in the alloy grains are all B2 phases.Compared with the B2phases precipitated at the boundaries,the phase precipitated grains have lower Al and Ni element content.