Abstract: The superior dynamic mechanical properties of high-entropy alloys (HEAs) have attracted great interest, while there have been limited studies on the adiabatic temperature change as well as the deformation mechanism in the additively manufactured HEAs under impact loading. In this work, the deformation mechanism of laser powder bed fusion (LPBF)-fabricated CoCrFeMnNi HEAs under high-velocity impact loading was elucidated through multiple microstructural characterization in conjunction with molecular dynamics simulation. Different from CoCrFeMnNi alloys made by thermomechanical processing, both yield strength (YS) and strain hardening behavior of LPBF-fabricated HEA samples are more sensitive to the strain rate in the range of 0.001/s to 5000/s. The YS of the LPBF-fabricated HEAs shows an increasing trend from ∼452 MPa at 0.001/s to ∼685 MPa at 5000/s. The strain hardening capacity also increases with the increase of strain rate from 0.001/s to 3000/s. When the strain rates are over 3000/s, high strain hardening capacity is derived from strong dislocation multiplication induced by a high density of deformation twin boundaries. An intriguing mechanical response of the LPBF-fabricated HEAs emerges during loading: the strain hardening rate slightly decreases when the strain rate values increase from 3000/s to 5000/s, which is ascribed to the potential temperature rise-induced dislocation dynamic recovery. Further, in comparison to as-cast HEAs, LPBF-fabricated HEAs show significantly enhanced twinning behavior at high strain rates. The difference is related to unique as-printed microstructure of LPBF-fabricated HEAs: high-density dislocation increasing flow stress, cellular boundaries inducing dislocation dissociation and subsequent nano-twins at high strain rates. These substructures bring enhanced twinning behavior and strain rate-dependent high-velocity impact behavior in LPBF-fabricated HEAs. Our work provides a new insight into the dynamic impact behavior of additively manufactured HEA materials.

Key words: Laser powder bed fusion, High entropy alloy, Dynamic loading, Cellular structure, Twinning behavior