Simultaneous enhancement of strength and ductility in a NiCoCrFe high-entropy alloy upon dynamic tension: Micromechanism and constitutive modeling

The deformation responses of NiCoCrFe high-entropy alloy (HEA) under quasi-static (1 x 10(-4) - 1 x 10(-1) /s) and dynamic (1,000-6,000/s) tension were investigated. A good combination of high strength and ductility is obtained under dynamic tension. The yield strength and true ultimate tensile strength is increased from 217 to 830 MPa at 1 x 10(-4) /s to 440 MPa and more than 1,000 MPa at 6,000/s, respectively. In addition, the engineering fracture strains maintain 60%-85% over a wide range of strain rates. The enhancements of strength and ductility originate from (1) the significant strain-rate sensitivity (SRS) mainly due to the presence of short-range orders/clusters (SROs/SREs) as well as phonon drag effect of dislocations, and (2) the extraordinary work-hardening capacity due to dynamically formed nanoscale twins upon high strain-rate tension. The temperature and strain-rate dependence of the yield strength of the alloy are well modeled based on the thermally activated mechanism. Additionally, considering nanoscale twin boundaries as local sites for nucleating and accommodating dislocations, the dislocation density evolution model is modified and subsequently introduced into Taylor hardening model to accurately capture the hardening behavior of the current NiCoCrFe HEA. Hence, the distinguished work-hardening capacity under dynamic tension can be mainly ascribed to the low dislocation recovery rate and remarkable twin-induced dislocation generation.