Spall response of medium-entropy alloy CrCoNi under plate impact

Dynamic mechanical properties of high-entropy alloys (HEAs) or medium-entropy alloys (MEAs) under high strain rate loading are of increasing interest for their promising applications in dynamic extreme environments. Here, spall damage of a coarse-grained CrCoNi MEA with a face-centered-cubic structure is firstly investigated by combining plate impact experiments, and molecular dynamics (MD) simulations. Yield strength, spall strength and spall-induced re-acceleration at different impact velocities are derived from free-surface velocity history measurements. In addition, soft-recovered samples after spallation are characterized with scanning electron microscopy and electron backscatter diffraction. The CrCoNi MEA exhibits the highest spall strength (around 4 GPa) among the MEAs/HEAs ever reported, with similar or better impact ductility compared to the reported MEAs/HEAs. With increasing impact velocity, the dominant damage mode of the CrCoNi MEA undergoes a gradual transition from intergranular damage, to mixed intergranular and intragranular damage, and then to intragranular damage. MD simulations reveal multiple deformation mechanisms (dislocation slip, stacking faults and twinning) for the CrCoNi MEA under impact loading, and show consistent spall damage mode transition with the experimental observations. The excellent combination of spall strength and ductility for the CrCoNi MEA is mainly attributed to the pronounced intragranular void nucleation followed by ductile void growth and coalescence, ensuring broad application prospects of the CrCoNi MEA in impact scenarios.