Microstructure Control And Dynamic Tension Deformation Behavior Of CrFeNi Medium Entropy Alloys

Compared with conventional dilute alloys,high entropy alloys（HEAs）generally process more metastable states and,therefore,can achieve a wider range of tailorable mechanical properties.HEAs have excellent elevated and cryogenic temperature properties,super welding properties,good fatigue and fracture resistance,and good hydrogen embrittlement.These excellent properties give them the potential to be the next generation of structural materials.In order to promote the application of HEAs/MEAs（medium-entropy alloys）in extreme conditions,it is necessary to reveal their dynamic/cryogenic mechanical behavior and microscopic-deformation mechanisms.However,there are only several studies on the mechanical response of HEAs/MEAs at cryogenic temperature and high strain-rate tension,and many academic issues such as deformation mechanism and strain rate effect of HEAs/MEAs at cryogenic temperature and dynamic tension are lack of in-depth discussion.Therefore,it is of great significance to study the dynamic tensile mechanical behavior of HEAs/MEAs at room and cryogenic temperature.In this study,CrFeNi MEA is selected as the subject of our study.The effects of the thermomechanical processing on the microstructure and mechanical properties are investigated first,and then the effect of temperatures and strain rates during tension are studied based on samples with excellent room temperature and quasi-static tension properties.（1）CrFeNi MEA is single-phase FCC structure at high temperatures.Compared with Fe_100-x-yCr_xNi_y（x+y<50）alloys,homogenized CrFeNi MEA display a simultaneous enhancement of strength and ductility.Cr-rich BCC phase can be introduced into FCC matrix by low temperature annealing,the microstructure is heterogeneous,with FCC+BCC duplex structure and heterogeneous grain-size distribution.The yield strength and uniform elongation of this sample are 490 MPa and 25%,respectively.（2）Under dynamic loading,a traditional strength-ductility trade-off come into being in the current CrFeNi MEA.The yield strength was significantly increased,since the dislocation motion becomes popular under high strain-rate loading.High-density dislocations together with brittle BCC phases within the FCC matrix cause the reduction of uniform elongations upon dynamic loading.The strain-rate sensitivity of the CrFeNi MEA（0.0371under quasi-static loading and 0.340 under dynamic loading）is higher than that of pure FCC metals and complex solid-solution alloys,such as stainless steels,HEAs,and MEAs.The strong strain-rate dependence of the yield strength is closely related to the presence of short-range dislocation obstacles,BCC phases,and ultra-fine grains.（3）With the temperature decreases from 298 K to 77 K,both the strength and uniform elongation are significantly improved upon quasi-static and dynamic loadings,which is attributed to the activation of twins,causing a dynamic Hall-Petch effect.A strain rate-dependent and temperature-dependent constitutive model was well established to describe the deformation behaviors of the CrFeNi MEA over a wide range of strain rates and temperature,the predicted results are in good accord with the experimental data.