轧态CrCoNi中熵合金动态拉伸本构模型的建立和变形机理研究

Study on Dynamic Tensile Mechanical Behavior and Deformation Mechanisms of CrCoNi Medium Entropy Alloy at Room and Cryogenic Temperature

CrCoNi中熵合金在准静态拉伸下具有良好的强度和塑性,而其动态拉伸力学行为还有待研究。利用霍普金森拉杆分别对CrCoNi中熵合金试样进行了室温(298K)和低温(77K)下不同应变率的动态拉伸力学行为研究,建立了修正的J-C(Johson-Cook)本构模型对其塑性流动行为进行了较好的描述,通过断后样品的微观组织表征揭示了其变形机理。结果表明:室温下CrCoNi中熵合金的强度和塑性随着应变率增大逐渐提高。与准静态拉伸相比,动态拉伸应变率为1200~5000 s^(-1)时,试样的屈服强度增大至560 MPa到1150 MPa,伸长率增长至60%到90%;低温下强度表现出相似的应变率效应且强度较室温下更高,但韧性有所降低。变形机理结果表明:相比于室温准静态,室温动态拉伸下试样内部孪晶密度更大且交叉孪晶出现、FCC→HCP相变发生、纳米晶形成,三者共同作用促使CrCoNi中熵合金加工硬化提高;相比于室温动态拉伸,低温动态拉伸下试样孪晶密度过大导致孪晶增厚,且纳米晶形成,促使试样加工硬化进一步提高,而孪晶增厚加强了对位错的阻碍致使韧性降低。

The CrCoNi medium entropy alloy(MEA)has both good strength and ductility under quasi-static tension,but its dynamic tensile mechanical behavior needs further study.The split Hopkinson tensile bar is used to perform dynamic tensile tests on the CrCoNi MEA with different strain rates at room temperature(298 K)and cryogenic temperature(77 K),and a modified JC(Johson-Cook)constitutive model is established to describe the plastic flow behavior well.The deformation mechanisms of CrCoNi MEA are revealed by microstructure characterization.The tests results show that:the strength and ductility of CrCoNi MEA at room temperature gradually increase with the increase of strain rate.Compared with quasi-static tensile,when the dynamic tensile strain rate is 1200-5000 s^(-1),the yield strength increases to 560 MPa and to 1150 MPa,and the elongation increases to 60% and to 90%;At cryogenic temperature,the strength exhibits a similar strain rate effect and the strength is higher,but the elongation is reduced.The deformation mechanism results are:compared with quasi-static tensile at room temperature,the twins density of the samples under dynamic tension at room temperature increases and multiple twinning occurs,the FCC→HCP phase transition occurs,and the nanograins form,together promoting the improvement of work hardening of CrCoNi MEA;compared with dynamic tensile at room temperature,the excessive twin density of the samples under dynamic tensile at cryogenic temperature causes increase in twin thickness,and the nanograins form,which promote further increase in work hardening,but the increase of twin thickness enhances the hindrance to dislocations thus leading to a decrease in ductility.