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

• Published in: International journal of plasticity 2020-01, Vol.124, p.226-246
• Main Authors: Zhang, T.W., Ma, S.G., Zhao, D., Wu, Y.C., Zhang, Y., Wang, Z.H., Qiao, J.W.
• Format: Article
• Language: English
• Subjects: Constitutive modeling; Dislocation density; Ductility; Dynamic loading; High entropy alloys; High-entropy alloys (HEAs); Mechanical properties; Nanotwins; Phonon drag; Strain rate sensitivity; Strain-rate sensitivity (SRS); Temperature dependence; Twin boundaries; Ultimate tensile strength; Yield strength; Yield stress
• ISSN: 0749-6419; 1879-2154
• DOI: 10.1016/j.ijplas.2019.08.013

The deformation responses of NiCoCrFe high-entropy alloy (HEA) under quasi-static (1 × 10−4-1 × 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 × 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/SRCs) 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. [Display omitted] •NiCoCrFe HEA possesses simultaneous enhancement of strength and ductility at high strain rates (greater than 1,000/s).•Strong strain rate sensitivity originates from short-range orders/clusters and phonon drag effect of dislocation.•Extraordinary strain hardening mainly stems from the dynamically formed nanoscale twins.•A strain rate-dependent model was established to bridge the gap between microstructure and mechanical properties.•Design idea of new alloys serving under cryogenic temperatures and dynamic impacts was given.