Dynamic Mechanical Properties Of CoCrFeNiAl_x High Entropy Alloys

High-entropy alloys?HEAs?are generally composed of at least five alloying elements,where each element is mixed in equal molar ratio or near equal molar ratio that results in a simple solid solution structure.Recent research results show that compared with traditional alloys,the HEAs exhibit a series of adjustable and excellent properties,such as high strength and hardness,excellent toughness and plasticity,potential high-and low-temperature properties,and good oxidation resistance,wear resistance and corrosion resistance,etc.These features lead to the HEAs as good candidates for the future engineering materials.At present,most investigations on the mechanical behavior of HEAs are still based on the quasi-static loading,and extreme conditions such as high-speed dynamic loading and related deformation mechanisms are scarcely reported.Therefore,it is very necessary to do this research.In this paper,three classic HEAs were prepared by high vacuum arc melting method,and the dynamic compression tests were carried out by using Split Hopkinson Pressure Bar?SHPB?,accompanied by the exploration of the deformation mechanisms.The specific contents and conclusions of this paper are as follows:?1?At room temperature,CoCrFeNiAl_x?x=0,0.6,1?HEAs all belong to the simple solid solution structure.CoCrFeNi is a single FCC phase,CoCrFeNiAl_0.6.6 is FCC+BCC dual phase structure,and CoCrFeNiAl is a single BCC phase.With the increase of Al content,the content of BCC phase increases and the proportion of FCC phase decreases,indicating that Al is the BCC phase stabilized element and promotes the formation of BCC phase.?2?In the range of 10^-44000 s^-11 strain rates,the Co CrFeNiAl_x?x=0,0.6,1?HEAs show obvious work hardening and strain-rate hardening effects.The yield strength and flow stress evidently increase with the increase of strain rate.Compared with quasi-static loading,the present HEAs exhibit the higher strain rate sensitivities?SRS?under dynamic compression.With the increase of Al content,it presents an increasing trend for both strength and SRS.?3?Microstructure observations were performed by means of transmitted electron microscopy?TEM?and high resolution TEM?HRTEM?for both quasi-static and dynamic loading.The deformation mode and strengthening mechanism of the current alloys are thus extracted from the data analysis.A number of dislocations and deformation twins are visible in CoCrFe Ni HEAs after quasi-static and dynamic compression,while deformation twins are only detected in the FCC phase of CoCrFeNiAl_0.6.6 HEAs by dynamic loading,and the deformation mode of CoCrFeNiAl HEAs is dislocation-slip dominated for both loading conditions.?4?Based on the modified Johnson-Cook?J-C?model,the constitutive relations of the present HEAs upon dynamic loading are established,and the corresponding plastic flow behavior at different strain rates can be predicted effectively.