Study On Rate-temperature Related Mechanical Properties And Deformation Mechanisms In The CoCrNiSi_0.3C_x Medium-entropy Alloys

Compared with traditional alloys,Medium/High entropy alloys（M/HEAs）exhibit excellent comprehensive mechanical properties due to their unique design concept and microstructure,such as high strength,high wear resistance,high oxidation resistance,high-temperature resistance,corrosion resistance,impact resistance,etc.Therefore,M/HEAs are expected to act as competitive candidates for structural materials in industrial,aerospace,and biomedical applications.Numerous studies have demonstrated that interstitial C elements have a significant strengthening effect in M/HEAs,which make that interstitial M/HEAs（i M/HEAs）can be expected to act as the carrier and coordinator of various strengthening mechanisms upon deformation to further improve the comprehensive properties.However,the microstructure and mechanical properties of i M/HEAs have not been comprehensively studied,especially at the high strain rate and high-temperature environment,and further exploration is needed.In this paper,an interstitial carbon-doped CoCrNiSi_0.3C_x MEA（x=0.016,0.048）was designed based on CoCrNiSi_0.3 MEA.The mechanical properties of CoCrNiSi_0.3C_0.048MEA were further tuned and optimized via traditional heat treatment processes such as cold rolling and annealing.In addition,the compressive mechanical properties of two CoCrNiSi_0.3C_0.048MEAs with different microstructures at wide temperatures and strain rate ranges were conducted by an electronic testing machine and split Hopkinson pressure bar with synchronous assembly system,and the strain rate and temperature effects were analyzed.The mechanism of microstructure evolution of two alloys in a rate-temperature coupled environment was studied by microscopic characterization.Finally,based on the Taylor impact test,the microstructure evolution mechanism of CoCrNiSi_0.3C_0.048 MEA at a higher strain rate was studied.The main research contents of this paper are introduced as follows:（1）Eight kinds of CoCrNiSi_0.3C_0.048MEAs with different microstructure and mechanical properties were prepared by cold rolling and short annealing.Among them,the three-level hierarchical precipitate microstructure(the primary Cr₂₃C₆ carbide,with the size of 2-10μm;the secondary Type-ⅠSi C precipitates,with the size of 200-500 nm;the tertiary Type-ⅡSi C precipitates,with the size of～50 nm)was introduced into CoCrNiSi_0.3C_0.048 MEA to achieve excellent strength and ductility coordination.Its yield strength and ultimate tensile strength can reach 1.34 GPa,1.50 GPa,and the total elongation is still 9.12%,which is apparently superior to most those reported in carbon-or silicon-doped MEA/HEAs.The results show that the crack branching and blunting in Cr₂₃C₆ carbides,dislocation-bypass mechanism around the non-shearable Si C precipitates,hetero-deformation-induced strengthening caused by the interface between matrix/three-level hierarchical precipitates,as well as stacking-fault networks and dense nanotwins activated by precipitates in the matrix contribute to strengthening and toughening behavior in the current MEAs.（2）Compressive mechanical behavior,microstructure evolution,and deformation mechanism of the homogenized CoCrNiSi_0.3C_0.048 MEA（C48-H）composed of FCC matrix and Cr₂₃C₆ carbides and the CoCrNiSi_0.3C_0.048 composed of FCC matrix and three-level hierarchical precipitate microstructure have been systematically studied MEA（C48-800-1h）at wide strain rate range.The results show that the strengthening effects of strain rate on yield strength and flow stress of the two alloys under dynamic conditions are more significant than ones under quasi-static conditions,exhibiting a stronger strain rate sensitivity.Moreover,within the current strain rate range,the strain rate sensitivity values of C48-H and C48-800-1h alloys containing large amounts of precipitates are much smaller than the ones of single-phase FCC-structured Co Cr Ni and CoCrNiSi_0.3 MEAs.（3）The compression mechanical properties of C48-H and C48-800-1h MEAs at a wide range of temperature and strain rates were tested,and the microstructure evolution and deformation mechanism of the two alloys were analyzed in the coupled environment of strain rate and temperature.It can be found that the serration behavior（Portevin-Le Chatelier effect）is observed on the true stress-strain curve of C48-H alloy at 400-600℃under the quasi-static deformation.With the increase in temperature,the Type-A serrations at 400℃gradually transform into the Type-C serrations at 600℃.However,the serration behavior was observed on the true stress-strain curve of C48-800-1h alloy only at 400℃.And as the strain increases,the amplitude of the serrations decreases until it disappears.For C48-800-1h alloy,abnormal stress peak（3rd strain aging effect）appears on the curve of flow stress variation with temperature under quasi-static deformation.Nevertheless,the abnormal stress peak caused by the 3rd strain aging effect is not obvious or even disappears at a high strain rate.For C48-H alloy,with the increase of strain rate,the temperature range of abnormal stress peak will move to a higher temperature region.（4）The microscopic mechanism of the dynamic strain aging phenomenon on the curve of flow stress variation with temperature under quasi-static and dynamic conditions was studied by analyzing the microstructure of the samples of C48-H and C48-800-1h MEAs deformed at different strain rates and temperatures.For C48-H alloy,the apparition of abnormal stress peak（3rd strain aging effect）in a specific temperature range under quasi-static and dynamic loading may be attributed to the fact that the deformation mode is mainly dislocation slip.The presence of interstitial C element and a large amount of Cr₂₃C₆ carbides intensifies the interaction with the dislocation,and then continuously pins the dislocation,leading to the emergence of dynamic strain aging.For C48-800-1h alloy,the main reason for the 3rd strain aging phenomenon under quasi-static conditions may be the existence of interstitial C atoms.During the continuous development of plastic deformation,a series of heterogeneous structures consisting of dense dislocation cells（DCs）,micro-bands（MBs）,stack faults（SFs）,dislocation clusters,and deformation twins（DTs）occur.These mixed structures intensify the interaction between the interstitial atoms and the moving dislocation,and then pin the dislocation,resulting in the dynamic strain aging phenomenon.The reason why the 3rd strain aging phenomenon does not occur under dynamic conditions may be that the motion of interstitial solute atoms is slower than that of dislocation,thus,the dislocation cannot be pinned in time.In addition,the precipitation of a large number of nano-sized Si C precipitates weakens the"pinning"effect of interstitial atoms under dynamic loading.（5）Taylor impact test was carried out for C48-800-1h MEA,and a wide range of strain and strain rate with gradient distribution of the Taylor sample was formed.The strain and strain rate-dependent microstructure evolution of C48-800-1h alloy was studied by analyzing the microstructure of the Taylor sample in different regions from the impact end to the free end.With the increase of strain and strain rate,the evolution process of the primary Cr₂₃C₆carbides is mainly the increase of cracks,crack branching and crack width.For the secondary Type-ⅠSi C precipitates,the evolution process is mainly the formation and propagation of cracks.However,the evolution process of the tertiary Type-ⅡSi C precipitation is mainly gradual intensification of dislocation-bypass mechanism.In the FCC matrix,the interaction among dislocation,twins,and three-level hierarchical precipitates is intensified with the increase of strain and strain rate.