CoCrFeNiAlx系高熵合金的动态力学性能和本构关系

高熵合金，以其独特的合金设计和优异的综合性能，成为当下材料研究的热点。本文利用高真空电弧熔炼法成功制备出了CoCrFeNiAl_x（x=0, 0.6, 1）系高熵合金，并通过分离式霍普金森压杆对其进行一系列不同应变速率下的动态压缩试验。通过X射线、扫描电镜和透射电镜分析，深入探索了该合金系的晶体结构、微观组织和变形特征。最后，利用修正后的Johnson-Cook (J-C)本构模型，获得了该体系高熵合金的动态本构关系。

Dynamic mechanical properties and constitutive relations of CoCrFeNiAlx high entropy alloys

High-entropy alloys (HEAs), due to their unique alloy-design concepts and excellent comprehensive properties, are becoming a research hotspot nowadays. However, previous reports were scarcely focused on the dynamic mechanical loading of the HEAs, that is the applied strain rates often were limited in the quasi-static regime. In this research, CoCrFeNiAl_x HEAs were successfully prepared by vacuum arc melting pure elements in a high-purity argon atmosphere on a water-cooled Cu hearth. Each ingot was remelted at least five times to ensure its chemical homogeneity. Cylindrical samples with a diameter of three millimeters were then synthesized by copper-mould suction casting. Quasi-static compressive tests at room temperature were conducted by using an Instron 5969 testing machine, wherein the test specimens with an aspect ratio of 1∶1 were cut from the cylindrical samples along the longitudinal direction by electrical discharge machining. As a comparison, dynamic compression experiments with various strain rates were carried out at room temperature by the split Hopkinson pressure bar (SHPB). For characterization, crystal structure, microstructure and deformation characteristics were investigated in detail by a combination of X-ray diffraction (XRD), scanning-electron microscopy (SEM), and transmission-electron microscopy (TEM) analyses. The XRD results reveal that simple solid solution structures, in forms of face-centered cube (FCC) and/or body-centered cube (BCC), were obtained in the current alloys. All the alloys exhibit positive strain-rate sensitivity and excellent work-hardening ability. Interestingly, three isolated deformation mechanisms were detected by TEM analysis, that is combined dislocation slip plus deformation twinning dominats the plastic deformation in the CoCrFeNi alloy (with FCC structure) for both quasi-static and dynamic loading conditions, however, such phenomenon was only observed in the CoCrFeNiAl_0.6 alloy (with FCC plus BCC structure) under dynamic loading. As for the CoCrFeNiAl alloy (with BCC structure), single dislocation slip accounts for the plastic deformation in both quasi-static and dynamic loading conditions. Moreover, the dynamic constitutive relations of CoCrFeNiAl_x HEAs were obtained by the modified John-Cook (J-C) constitutive model.