Mechanical Behaviors and Deformation Constitutive Equations of CrFeNi Medium-Entropy Alloys Under Tensile Conditions from 77 K to 1073 K

Concentrated multicomponent alloys (CMCAs) or high/medium-entropy alloys (HEAs/ MEAs) possess outstanding comprehensive properties, causing them to have the potential to be the next generation of structural materials. Phenomena occurring under dynamic tensile loading or high/low temperature of such alloys have been hardly investigated. However, its understanding is essentially needed in their application in automotive, aerospace, and military industries. Meanwhile, the suitable constitutive equations of CMCAs under such cases have been rarely investigated. In this work, the thermodynamic behavior of equiatomic CrFeNi MEA with single-phase fcc structure has been systematically investigated at strain rates from 10(-3) s(-1) to 1800 s(-) 1 and temperatures from 77 K to 1073 K. The results showed that as the deformation temperature decreased from 1073 K to 77 K, the yield stress was improved significantly from 125 MPa to 415 MPa. Meanwhile, the uniform elongation increased from 2% to 82%. The abnormal uniform elongation appearing at 673 K was closely related to dynamic strain aging. As the strain rate increased from 10(-3) s(-1) to 1800 s(-1) at a constant temperature of 77 K, the strength increased significantly (e. g., the yield stress increased from 415 MPa to 595 MPa), and the uniform elongation remained unchanged, still maintaining 68% at 1800 s(-1). After deformation, there were no second phases attributed to a large Ni amount in the alloys. Some deformation twins appeared at 77 K. Based on the experimental results, the relationship between yield stress and temperatures/strain rates could be successfully revealed using the ZA model. Moreover, regression analysis and constraint optimization established two phenomenological constitutive models (JC and KHL models) and three physically-based constitutive models (PB model, ZA model, and NNL model). JC and PB models had the highest and lowest description accuracy, respectively. Besides, the JC model was hard to describe the case that the work hardening decreased due to the change of temperature or strain rates, and the PB model was unsuitable in characterizing the complex work hardening behaviors.