Deformation Mechanism Of A Dual-Phase High-Entropy Alloy CrFeCoNiAl_0.7 At Room And High Temperatures

The development of traditional solid-solution alloy systems is gradually approaching the bottleneck,and the concept of high-entropy alloys came into being.High-entropy alloys（HEAs）could be divided into single-phase,dual-phase and multi-phase HEAs according to their microstructures.Generally,single-phase HEAs exhibit unilateral performance advantages.For example,HEAs with a face-centered cubic（FCC）structure have high ductility but with low strength,while HEAs with a body-centered cubic（BCC）structure have high strength but with low ductility.Dual-phase HEAs can combine the advantages of each phase structure.For example,HEAs with an FCC+BCC dual-phase structure has much better comprehensive mechanical properties than those HEAs with a single BCC phase or a single FCC phase in terms of ductility and strength.The multi-phase HEAs contains more types of phases,and the phase structure is more complex,which may necessarily bring performance advantages.On the whole,dual-phase HEAs have a great application potential for further research.In the dual-phase high-entropy alloy system,based on the CrFeCoNi high-entropy alloy with excellent toughness,the FCC+BCC dual-phase structure CrFeCoNiAl_x（0.5<X<0.9）dual-phase high-entropy alloy system is formed by adding Al element.The excellent performance has attracted the attention of researchers.The ratio of FCC phase to BCC in the dual-phase HEA system,the morphology and the properties of the alloy all change continuously with the addition of Al.Among them,the CrFeCoNiAl_0.7 HEA has a lamellar FCC phase and BCC phase in the same proportion,showing excellent comprehensive properties with strength and toughness,which may benefit from its special dual-phase structure.However,there is still a lack of relevant research on the substructure evolution of FCC phase and BCC phase during deformation.In order to explore the mechanism of the influence of the phase structure on the mechanical properties and to further guide the composition optimization and process improvement of the dual-phase HEA,this study prepared the CrFeCoNiAl_0.7 dual-phase HEA by vacuum induction melting,and studied the deformation mechanisms during deformation at room and high temperatures.The CrFeCoNiAl_0.7 dual-phase HEA contains 77.3%FCC phase and 22.7%B2 phase,and there is about 4.7%lattice mismatch between the two phases.The yield strength and tensile strength at room temperature are as high as 876 MPa and 1198 MPa,respectively,and the elongation is about 9%.Dislocations gliding in the FCC phase govern the plastic deformation at the early stage,and disordered dislocations form dislocation walls as the deformation proceeds.With further increase in strain to a high level,the stacking dislocation caused by geometrically necessary dislocation will be the potential heterogeneous nucleation point of deformation twins.The stress-strain curves obtained at high temperature show a typical softening effect dominated by dynamic recrystallization（DRX）.A strong temperature and strain rate dependence of DRX was observed.The constitutive equation describing the flow stress as function of strain rate and deformation temperature was established and the activation energy was calculated to be 380 k J/mol.Continuous DRX governs the flow softening of the present dual-phase HEA,which is preferred in FCC phase.With the proceeding of thermo-compression,the substructure within the deformed microstructure evolves in a sequence from the disordered dislocations,cellular structure,sugrain to DRX grain.This study explored the relationship between the two phases of the CrFeCoNiAl_0.7dual-phase HEA and the deformation mechanism at room temperature and high temperature.There is a slight lattice mismatch between the two phases and has a positive effect on the improvement of the alloy performance.The FCC phase is the main deformation phase during room temperature and thermal deformation,and the B2 phase is the strengthening phase.The research results can provide theoretical reference for further optimizing the composition and thermal processing parameters of dual-phase HEAs.