Strain Rate And Temperature Dependent Mechanical Properties Study Of Al_0.1CoCrFeNiTi_X High Entropy Alloys

In this research,Al_0.1CoCrFeNiTix（X=0.1,0.3,0.5）high-entropy alloys（HEAs）were prepared through vacuum arc melting furnace,and the effects of the microstructure and the strain rate on the compression behavior of the alloys were investigated by Instron5969 universal testing machine and Split Hopkinson Pressure Bar system（SHPB）.The microstructure and mechanical properties optimization of the Al_0.1CoCrFeNiTi_0.1 HEAs were achieved by thermomechanical treatment.Quasi-static mechanical properties of the annealed Al_0.1CoCrFeNiTi_0.1 HEAs at room temperature（298K）and cryogenic temperature（77K）were tested using the Instron5969 universal testing machine,and the effects of the strain rate on the dynamic room temperature tensile mechanical behavior of the alloy was studied by using the Split Hopkinson Tension Bar（SHTB）.Finally,by analyzing the microstructure evolution of the initial and deformed specimens,the detailed conclusions are obtained:（1）Coarse-crystalline single-phase FCC structure Al_0.1CoCrFeNiTi_0.1,Al_0.1CoCrFeNiTi_0.3and FCC+σphase Al_0.1CoCrFeNiTi_0.5 HEAs were obtained through homogenization annealing.The room temperature compression test results under a wide range of strain rates(10^-4 s^-1～6200 s^-1)indicate that the yield strength,work hardening,and flow stress of the current alloys are significantly improved with the increase of strain rate and Ti content.The remarkable strain rate sensitivity of the alloy at high strain rates is attributed to the severe lattice distortion and viscous effect induced by phonon drag.Through transmission electron microscopy（TEM）analysis,it is concluded that under quasi-static loading,the deformation mode of Al_0.1CoCrFeNiTi_0.1HEAs was controlled by the dislocation slip in the initial deformation process.In the subsequent plastic deformation stage,the dislocation substructure and deformation twinning playing a dominant role,while under dynamic loading conditions,dynamic grain refinement,nanograins and composite structures of nanograins and nanotwins are the main microstructure characteristics of the alloys.（2）The Al_0.1CoCrFeNiTi_0.1HEA with different annealing structures were obtained through different thermomechanical treatment processes.Quasi-static tensile experiments show that the yield strength and the working hardening of the heterostructures alloy annealed at 800℃are significantly improved compared with those annealed at 900℃.The Al_0.1CoCrFeNiTi_0.1HEA with heterostructures exhibit a good combination of tensile strength and engineering fracture strain at room temperature.In addition to the contribution of grain refinement strengthening,the incompletely fine recrystallized grain regions in the heterostructure possess a high density of dislocations,which improve the strength and work hardening ability of the alloy by hindering the movement of dislocations,while the equiaxed regions of the fully recrystallized regions provide plastic deformation capacity.Compared with the tensile strength at room temperature（298K）,the yield strength,ultimate tensile strength and fracture ductility of the alloys annealed at 800°C/2h at low temperature（77K）increased from 381MPa to 563MPa,732MPa to 953MPa,43%increased to 61%,respectively.The EBSD analysis indicates that the grain orientations in the fractured samples are governed by[001]//TD and[111]//TD,and dislocation slip is the main plastic deformation mode at room temperature.When the sample deformed at cryogenic temperature,the grains are prone to the[111]//TD orientation,and a large number of deformation twins and high-density dislocations dominate the plastic deformation process.（3）The dynamic tensile tests of the Al_0.1CoCrFeNiTi_0.1HEA were performed on a split Hopkinson Compression Bar（SHPB）apparatus at the strain rates from 1200 s^-1 to 3000 s^-1,and the results indicate that strength and ductility increase simultaneously with increasing strain rate under dynamic loading.In addition,the yield strength,ultimate tensile strength and the engineering fracture strain are increased to 590MPa,over 1.2GPa and 63%,respectively at a strain rate of 3000/s.Through EBSD analysis,compared with quasi-static tensile tests,increased strength,ductility and working hardening ability under dynamic are due to the strain rate strengthening effect.With the increase of the strain rate,the dislocation density and migration speed inside the alloy gradually increase,which leads to stronger local stress concentration inside the grains,and reaches the critical shear stress of twins,which further induced the nucleation and growth of twinning.