Unveiling the atomic-scale origins of high damage tolerance of single-crystal high entropy alloys

High entropy alloys (HEAs) exhibit an unusual combination of high fracture strength and ductility. However, atomic mechanisms responsible for crack propagation in HEAs are still not clear, which limits further improving the damage tolerance. Here we investigate effect of crystal orientation on the crack-tip behaviors in single-crystal HEA CrMnFeCoNi using atomic simulations to explore fracture micromechanism. The formation of deformation twinning and activation of multislip systems are observed during the propagation crack with the (001)(110) orientation, consistent with the previous experiments. Under the (1¯10)(110) orientation, the amorphous region takes place throughout the crack growth, and is difficult to occur in traditional metal materials. Dissimilarly, for the (11¯1¯)(110) orientation, the blunting and slip bands occur at the front of the crack tip by switching the slip mode from the planar to wavy slip, observed in recent transmission electron microscopy experiments. The chemical disorder leads to the obvious fluctuation of flow stress, but hardly affects the deformation mechanism at the crack tip. Compared to traditional metals and alloys, the high local stress concentration induced by coupling effect of severe lattice distortion and tension strain leads to the structure transformation from crystallization to amorphization at the crack tip in HEA. While the presented atomic simulations and the associated conclusions are based on CrMnFeCoNi HEA, it is believed that the current deformation mechanism at crack tip could also be applied to other face-centered-cubic HEA.