Study On The Phase Stability And Deformation Behavior Of VCoNi Medium-entropy Alloy With FCC Structure

In recent years,medium-and high-entropy alloys(MEAs and HEAs),as a new alloy design concept,have gradually entered the field of vision of material researchers.The alloy is composed of several elements with similar or approximately equal atomic percentage and has a simple solid solution structure.In particular,the MEAs and HEAs of Face Centered Cubic(FCC)structure have potential advantages in structural application due to its superior ductility and cryogenic temperature damage tolerance.The VCoNi MEA with FCC single phase,which possesses an extraordinarily combination of strength and ductility in room temperature(?1GPa yield strength and 30%fracture elongation)but its deformation mechanism at other temperatures is not clear,and strain rate sensitivity(SRS)is rarely studied.In addition,the instability of phase of MEAs and HEAs lead to changes in their mechanical properties,which limit their applications.Therefore,VCoNi MEA with FCC structure was selected as the research object to study the phase stability during heat treatment at different temperatures and the internal mechanism corresponded to its deformation behavior at different temperatures and strain rates.In this study,VCoNi MEA was prepared by vacuum melting and room temperature rolling.VCoNi was aged at 600?,700?,800?,900? and 1000? respectively for 12h,24h,48h,72h and 144h to study the phase stability in the alloy.The results show that the precipitates increase with the increase of aging time and temperature,and the microstructure shows that the precipitates are V-rich ? phase.In the process of isothermal annealing,the dynamic precipitation of ? phase follows the Johnson-Mehl-Avrami-Kolmogorov(JMAK)formula,and the coefficient n?0.025 is independent of temperature.In addition,the temperature dependence of precipitation kinetics can be described by Arrhenius equation,and the apparent activation energy is 116±11 KJ/mol.Finally,the Time-Temperature-Transformation(TTT)diagram for ? phase precipitation in the temperature range from 600? to 1000? of the Medium-entropy alloy in VCoNi was established and drawn by this experiment.The effects of temperature(77K-573K)and strain rate(8.3×10-5s-1-8.3×10-2s-1)on the mechanical properties of the VCoNi MEA were investigated on the basis of phase stability.It is found that the high-density nanoscale dislocation substructure and deformation twins make the alloy have good strength and plastic bonding at 77K deformation.When the temperature is higher than 473K,the plasticity remains stable,although the serrated flow appears on the stress-strain curve.Its underlying formation mechanism is ascribed to the coupling effect of the dynamic strain aging and dynamic precipitation of the V-rich ? phase.At different strain rates,the strength of VCoNi MEA decreases with the increase of strain rate,indicating negative strain rate sensitivity(NSRS).The results show that NSRS is closely related to the precipitation of V-rich ? phase during deformation.The inherent atomic configuration and chemical composition,the generation and migration of dislocation and sufficient time for elements diffusion can contributes to V-rich ? phase precipitation,which is responsible for the precipitation strengthening during deformation at lower displacement rate.