Mechanical Response Of CoCrNiAl_0.1Si_0.1 Medium Entropy Alloys Upon Cryogenic Temperatures And Dynamic Loading

High entropy alloys(HEAs)and medium entropy alloys(MEAs)are a class of novel multi-principal component metal materials,which have attracted extensive attentions and becomes a new research highlight recently owing to their unique structure and extraordinary mechanical properties.In this paper,the as-cast medium entropy alloys CoCrNiAl_0.1Si_0.1 were prepared by arc-melting a mixture of 99.9 wt.%pure element metals in a Ti-getter high-purity argon environment.The samples were obtained by processing technic,including cold rolling,annealing,and EDM cutting machine to carry out the quasi-static tensile test and the dynamic mechanical test.The mechanical tensile testing machine was applied to carry out the quasi-static tensile test at room temperature(298 K)and low temperature(77 K),and the Hopkinson bars were employed at room temperature for the dynamic mechanical test.The structural characteristics of the specimens before and after deformation were investigated by X-ray diffraction(XRD),electron backscattered diffraction(EBSD),and transmission electron microscopy(TEM).The yield strength and strain hardening related to the temperature and strain rate were investigated by constitutive modeling.The corresponding conclusions are as follows:(1)CoCrNiAl_0.1Si_0.1 MEAs have significant strength and considerable ductility upon quasi-static tensile loading at low-temperature of 77 K.Compared with the results at room temperature,the yield strength increases from 480 MPa to 700 MPa,the tensile strength increases from 950 MPa to 1250 MPa,and the elongation increases from 58%to 72%.Under dynamic loading,the yield strength of CoCrNiAl_0.1Si_0.1 increases from 476 MPa to 590 MPa with the increase of strain rate from 10^-3 s^-1 to 3000 s^-1,which exhibits the positive strain rate sensitivity(SRS).And the elongation also exhibits a positive SRS and more than 70%under dynamic tensile loading at room temperature,which is measured on the specimen after fractured.(2)The friction stress of Peierls lattice induced by severe lattice distortion of CoCrNiAl_0.1Si_0.1 MEAs is related to thermal vibration,which increases with the decrease of the temperature results in the decrease of the effective width of dislocation cores.Meanwhile,the existence of co-clustering and/or short-range order(SRO)is also the thermal barrier to dislocation motion,resulting in the temperature dependence of yield strength.(3)The strong SRS of CoCrNiAl_0.1Si_0.1 MAEs are related to the thermal activation mechanism and dislocation damping mechanism.Thermal activation analysis reveals that the activation volume of H/MEAs is much smaller than that of traditional FCC metals,which is considered to be related to severe lattice distortion of the alloys and the presence of short-range ordered or co-clusters.(4)At room temperature,the deformation mechanism of CoCrNiAl_0.1Si_0.1 MEAs is dominated by dislocation cell,deformation twinning,and the interaction between twinning and dislocation.At 77K,high-density dislocation,nano-twinning,secondary nano-twinning,and microbands,together with other mechanisms promote stable strain hardening and significant plasticity of the MEAs.Deformation twins exist in the MEAs under dynamic tensile loading,which can significantly improve the strain hardening ability and delay the occurrence of plastic instability,resulting in the enhancement of the plasticity of the MEAs under dynamic loading.In addition,a constitutive model based on dislocation density and twinning volume fraction evolution is established to predict the plastic flow behavior of the MEAs at room temperature and low temperature,and the parameters are identified.The phenomenological Johnson-Cook constitutive model related to viscoplasticity is employed to predict the plastic flow of the alloy under dynamic loading.The fitting results of the two models are in good agreement with the experimental results.