Research On Heat Treatment Microstructure And Plastic Microforming Mechanism Of FeCoNiCrTi_0.2 Alloy Sheet

The large demand for electronic devices and fine instruments has driven the trend of product miniaturization,so the application of metal sheet in the field of microforming process is becoming more and more extensive.However,the size effect of materials at mesoscopic scale seriously affects the formability and serviceability of parts and restricts the rapid development of various fields to a certain extent.At the same time,the emergence of high entropy alloy with excellent properties provides a new thinking direction for reducing the effect of size effect phenomenon,and the balance of ultra-high strength and good ductility is expected to break through the material limitation at mesoscopic scale.However,the current research on high entropy alloy is mainly focused on composition design,regulation and performance optimization,etc.Therefore,it is of great significance to clarify the deformation behavior and deformation mechanism of high entropy alloy sheet through microforming test.In this paper,the as-cast FeCoNiCrTi_0.2 alloy was prepared and the thickness of the alloy was reduced to about 0.2mm by rolling process.Secondly,the process of solution and aging treatment was determined to control the microstructure of the rolled alloy sheet.The thin sheets samples with different characteristic sizes were obtained by holding at 1150?for different times.The morphology,structure and distribution of nanometer precipitated(Ni,Co)₃Ti were determined at 800?as aging temperature.The influence of grain size and precipitation on mechanical properties of FeCoNiCrTi_0.2 alloy sheet was analyzed by uniaxial microtensile test.It is found that when the grain size of solid solution sample is 57.8?m,the alloy exhibits obvious flow stress size effect.This is because the surface grain as a free surface can not effectively hinder the dislocation movement.As the grain size increases,the surface layer proportion is too high and the flow stress decreases rapidly.The aging precipitates can significantly improve the strength of the alloy,but the aging samples do not show obvious size effect due to the predominance of precipitation strengthening.Combined with EBSD analysis and TEM analysis,the deformation mechanism of FeCoNiCrTi_0.2 alloy at mesoscopic scale was identified,and it was found that the dislocation slip and the generation of mesh stacking stacking were the main deformation modes.The deformation degree of the surface layer is obviously weaker than that of the core grain,and the difference between them becomes smaller with the increase of strain.Based on the surface layer model,the flow stress constitutive equation of the alloy sheet is established,which is in good agreement with the experimental results,and can accurately predict and evaluate the stress variation state of the alloy.The micro-bending test of FeCoNiCrTi_0.2 alloy sheet was carried out,and the variation process of micro-bending load and springback was discussed.It was found that the bending load and springback of the sheet increased with the decrease of grain size,and the springback angle also produced size effect phenomenon.Based on the tension flow stress constitutive equation,the springback of thin sheets is theoretically analyzed and verified.