Hardening-softening of Al0.3CoCrFeNi high-entropy alloy under nanoindentation

The competition and balance mechanism between work hardening resulting from the surge in dislocations and material softening caused by plastic deformation during contact loading in Al0.3CoCrFeNi high-entropy alloy is unclear. The structural transformation and strain localization of Al0.3CoCrFeNi high-entropy alloy during nanoindentation are investigated using molecular dynamics, and the hardening-softening mechanism is discussed. The simulations demonstrate that the prismatic dislocation loop with independent nucleation is discovered for the first time in the 111 orientation. Dislocation multiplication and cross-slip lead to an increase in indentation resistance, resulting in work hardening. Free dislocation slip and dislocation annihilation accommodate plastic strain will reduce indentation resistance, resulting in plastic softening. Twin boundaries can effectively block dislocation propagation, which contributes to hardening. However, twin boundaries cause a softening effect in the later stage of plastic deformation owing to two reasons: (1) the formation of steps and the partial slip where the slip plane and Burgers vector are parallel to the twin boundary, (2) steps and local damage zones in twin boundaries become new nucleation sites for dislocations. The phase-field-crystal method confirms that Al0.3CoCrFeNi high-entropy alloy has the highest hardness when the twin spacing is 2.467 nm.