Dynamic mechanical properties and deformation mechanism of (NiCoV)95W5 medium entropy alloy

Medium-entropy alloys (MEAs), renowned for their outstanding strength and ductility, possess great potential for high strain-rate applications. This study focuses on a NiCoV-based MEA system, and proposes a novel alloy design strategy to fabricate the (NiCoV)(95)W-5 alloy by introducing 5% (atomic percent) high-melting-point tungsten through vacuum arc melting coupled with thermomechanical processing. Split Hopkinson pressure bar (SHPB) experiments are conducted to elucidate the dynamic response mechanism and deformation behavior under high strain rates (2000-6000 s(-1)). The results show that due to severe lattice distortion, the enhanced phonon drag effect at elevated strain rates results in a substantial increase in yield strength from 720 MPa (10(-3) s(-1)) to 1887 MPa (6000 s(-1)), an increase of 162%, accompanied by a relatively high strain-rate sensitivity (m = 0.42). Microscopic analysis reveals the multi-scale cooperative deformation mechanism of the alloy system under high strain rate. When the strain rate is 2000 s(-1), the alloy exhibits a low dislocation density dominated by dislocation planar slip. As the strain rate increases to 4000 s(-1), the increased flow stress and deformation promote the proliferation and entanglement of a large number of dislocations into high-density dislocation cells. The accumulation of dislocation stress leads to the coordinated deformation of precipitates and releases stress concentration at the phase interface. When the strain rate further increases to 6000(-1), severe plastic deformation will lead to the formation of nanotwins within the matrix, which is the main strain hardening. This study elucidates the dynamic response mechanism of NiCoV MEA mediated by tungsten doping, providing a guidance for designing novel structural materials with excellent dynamic mechanical responses.