| In recent years,high-entropy alloys(HEAs)have attracted the wide attention of researchers because of its excellent mechanical properties and physical and chemical properties.The HEAs HEAs were originally defined as a blend of 5 or more elements with concentrations between 5 and 35 at.%.Among the reported high-entropy alloys systems,dual-phase HEAs with transformation-induced plasticity show excellent properties beyond most existing dual-phase steels.However,the related studies have mainly focused on the microstructure and properties under conventional annealing,but no observations have been reported on the rapid annealing of HEAs.Rapid annealing can delay recrystallization behavior,thus inhibiting grain growth and achieving the effect of grain refinement,which makes the mechanical properties of the alloy better than that of conventional annealing.In this paper,the microstructure evolution and mechanical properties of the as-cast,cold-rolled,and high-pressure torsioned Fe50Mn30Co10Cr10 dual-phase HEA were studied by rapid annealing.Firstly,the relationship between microstructures and mechanical properties of as-cast Fe50Mn30Co10Cr10 dual-phase HEA under rapid annealing(UHDCT,flash lamp heating)was studied.It is found that the sample after the UHDCT in a short time exhibits mechanical properties comparable to those obtained by thermo-mechanical processing.The as-cast samples have the dual-phase HCP+FCC structure.The content of the coarse crisscrossing HCP martensites is as high as～80%,leading to low overall strength and plasticity.After the UHDCT process,the overall strength and plasticity are greatly improved.It is found that the content of the austenite increases sharply to about 70%after rapid annealing,and the grains are refined.At the same time,a nano-gradient structure is formed on the sample surface,which shows higher hardness than that of the inferior.Through observing the deformation process,it is found that the main deformation mechanism of the alloy is transformation-induced plasticity and dislocation proliferation except for the surface layered structure.The nano-gradient surface structure strongly impedes the occurrence of martensitic transformation and then promotes the mechanical properties of the alloy.Secondly,the effect of the rapid annealing process(joule heating)on the microstructure and mechanical properties of the cold-rolled dual-phase HEA was studied.Ultra-fast annealing under a heating rate of 100℃/s was carried out by a thermal simulator,and then the microstructure and mechanical properties of annealed samples at low,medium,and high temperatures were characterized.It is found that during rapid annealing at low temperatures(400℃-600℃),the mechanical properties of the rapidly annealed sample are not superior to and even worse than those of the conventionally annealed sample,which is mainly due to the insufficient recovery recrystallization and austenite recovery.As a result,the structure is mainly HCP martensite,leading to obvious brittleness.During rapid annealing between 700℃ and 1200℃.the samples show higher yield strength and ultimate tensile strength since the rapid annealing avoids excessive grain growth and simultaneously a large number of fine-grained structures are generated around shear bands.Moreover.some coarse-grained regions in the samples obtained under conventional annealing ensure certain plasticity but decrease strength.In addition.it is also found that the grains are easy to be coarsened at high temperatures(e.g..1200℃),which deteriorates the mechanical properties(even at an extremely fast heating rate of 100℃/s with an extremely short holding time of 30 s).Thirdly,the microstructure evolution and mechanical properties of the high-pressure torsioned HEAs under rapid conditions(flash lamp heating)were studied.Since high-pressure torsion can introduce a large number of dislocations and slip bands.the high-pressure torsioned samples show extremely high hardness,far exceeding those of the reported HEAs.At the same time,the recovery from martensite to austenite becomes difficult due to severe deformation.Since the high-pressure torsioned HEAs need more energy to eliminate a large number of dislocations and slip bands,a higher annealing temperature is often required to achieve complete recrystallization.i.e.,full austenitic structures. |
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