Dynamic deformation behavior of single phase VNbTa medium-entropy alloys

Abstract

In this study, the deformation behavior of body-centered-cubic (BCC) VNbTa medium-entropy alloy (MEA) was studied under different strain rates ranging from 10−3 s−1 to 5000 s−1 using screw-driven testing machine and split Hopkinson pressure bar (SHPB), respectively. The alloy presents an outstanding synergy of strength and plasticity without any adiabatic shear bands even at a strain rate of 5000 s−1. Moreover, the yield strength of the alloy displays an obvious strain rate dependence, increasing by ∼94.4% from 842 MPa at 10−3 s−1 to 1637 MPa at 5000 s−1. The strain rate sensitivity (SRS) factor m and highest temperature rise under dynamic loading is calculated to be 0.2991 and 182 K, respectively, which means a strong strain rate hardening and thermal softening resistance. Microstructure evolution analyses demonstrate a transition of deformation mechanism from single dislocation slip controlling in low strain rate to a synergy of mechanical twinning and dislocation slip in high strain rate. Therefore, the excellent dynamic mechanical properties could be attributed to the synergistic effects of the strong strain rate hardening and thermal softening resistance, the mechanical twins, and the cell structures and microbands derived from dislocation slip controlling. In addition, the dynamic deformation behavior of VNbTa could also be described by a classical Johnson-Cook model. Combining the excellent cryogenic temperature mechanical properties (tensile strength of ∼1930 MPa and fracture elongation ∼27%), the present work further indicates that the VNbTa MEA has great potential for engineering applications under some extreme circumstances.