| High-entropy alloys(HEAs)exhibited outstanding properties such as high strength and hardness,temper softening resistance,wear resistance and corrosion resistance,which are widely used in aerospace,automotive equipment,rail transit and electronic products.However,it is hard to achieve strength-ductility trade-off in practical applications.Therefore,specific strategies need to be taken to regulate and optimize the microstructure and mechanical properties of HEAs to achieve the tradeoff between strength and ductility.Post-treatment such as rolling and annealing or the introduction of nano-reinforced materials into the matrix are two effective methods to improve the properties of the alloy.However,the existing research on the control of the properties of the HEAs by these two methods exhibit several problems,such as difficulty in the balance of strength and ductility,unclear rules of microstructure evolution,unclear mechanisms of structure-behavior-properties correlation,etc.Therefore,it is necessary to further reveal the structure-behavior-properties correlation of HEAs and realize effective strengthening construction.The main contents of this paper are as follows:(1)Aiming at the problem of strength-ductility trade-off of HEAs,cold rolling and annealing treatment were carried out for HEAs with different components.The influence of cold rolling and annealing process on the microstructure and mechanical properties of HEAs was systematically analyzed,the strength-ductility mechanisms of HEAs with different components were revealed,and the strengthening mechanisms of strength and ductility of HEAs were clarified.The enhancement of strength after cold rolling and annealing is mainly due to grain refinement strengthening,second phase strengthening and twin strengthening.The increase of ductility is caused by the joint action of dislocation wall and coherent phase boundary.The strength and ductility of HEAs can be improved simultaneously by reasonably controlling the parameters of cold rolling and annealing.(2)Through the research in the previous part,it is found that the second phase strengthening effect will promote strength-ductility trade-off of Co Cr Fe Ni Al system HEAs.Based on this,the structure-behavior-properties correlation of dual-phase Co Cr Fe Ni Al system HEA was studied by micropillars compression test.The mechanical behavior of compressive micropillar in FCC,BCC phases and FCC/BCC phase boundary of HEAs was studied.The size effect,stress fluctuation and strain hardening behavior of micropillar with different phases were analyzed.The effect of phase boundaries on the micromechanical properties of HEAs was clarified,and the strengthening mechanism of phase boundaries in two-phase HEAs was revealed.It was found that the flow stress in FCC,BCC and FCC/BCC increased with the decrease of the diameter of the micropillar.According to the study of stress fluctuation,it is found that the stress fluctuation of phase boundary micropillar is the largest,followed by FCC and BCC phase micropillars.According to the study of strain hardening,it is found that the strain hardening rate of BCC phase is the largest,followed by phase boundary micropillar and FCC phase micropillar.The modulation strengthening in BCC phase contributed to the increase of strain hardening rate.For micropillars with the same diameter,the strength relationship is: FCC/BCC>BCC>FCC.The incoherent phase boundary between FCC and BCC and the coherent phase boundary between BCC1 and BCC2 together strengthen the FCC/BCC micropillar to show the highest strength,and the higher Perls stress of BCC phase is the reason why its strength is higher than that of FCC phase.(3)In view of the two main errors that may occur during the micropillar compression test,that is,the taper of the micropillar and the eccentric load,proposed a calibration method for the above two errors,analyzed the difference between the stressstrain curves before and after calibration,and verified the accuracy and feasibility of the calibration method.The average of the top diameter and the bottom diameter of the micropillar after compression was used to calculate the true stress.First calibrated the inclination taper of the micropillar,and then calibrated the eccentric load.The calibrated stress-strain curves of micropillars with different taper or eccentric loads approximately coincide,indicating the feasibility of the calibration method.For micropillars with different tapers,the yield strength before calibration is much higher than that after calibration.The difference of flow stress before and after calibration increased with increasing the taper of micropillar,indicating that the error increased with taper.For the compression testing of micropillar under different eccentricity,the flow stress after calibration was significantly higher than that before calibration,resulting from the eccentric load brings additional bending strength.Because eccentric loading is a combination of compression and bending,the morphology of the micropillar after compression appeared bending,and the bending degree increased with eccentricity.The slip t-test of the stress-strain curves before and after the calibration of taper and eccentricity shows that all P values are less than 0.0001,indicating that there is a statistical difference between the calibrated curves and the measured curves.(4)In view of the further strengthening construction of the above dual-phase HEA,HEAs-based composites with different contents of graphene oxide were prepared.The lattice structure,grain size,crystal orientation and micro morphology of the composites were analyzed.The influence of graphene oxide on the surface mechanical properties of the HEAs including hardness and wear resistance were studied.The strengthening mechanism of graphene oxide on the HEAs was revealed.The mechanical behavior of the composite in micropillar compression testing was deeply explored.The strengthductility mechanisms of the composite were clarified.The grain size of the composite was lower than that of the HEA,and the doping of graphene oxide plays an important role in grain refinement.The hardness and wear resistance of the composites were enhanced first and then weakened with the increase of graphene oxide content.When the content of graphene oxide increased to 0.6 wt%,the hardness reached the maximum.When the content of graphene oxide is too high,it will agglomerate and show black holes in the electron microscope morphology,which reduces the mechanical properties of HEA/1.0 wt% GO.The yield strength and flow stress of the composite at micro scale were higher than those of the HEA matrix.The yield strength of the composites showed a trend of increasing first and then decreasing with the increase of graphene oxide.Compared with HEA/0.6 wt% GO,the yield strength of HEA/1.0 wt% GO decreased slightly.It is found that the interfacial micropillar with chromium carbide shows lower flow stress and strain hardening rate than that without interface,which ultimately reduces the strength of HEA/1.0 wt% GO. |
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