Rapid amorphization of CrMnFeCoNi high-entropy alloy under ultrasonic vibrations

Due to the distinct design concept, high-entropy alloys (HEAs) exhibit unusual properties and lead an emerging new field. In this work, we show that a typical face-centered cubic crystalline phase CoCrFeNiMn HEA can be readily transformed into the amorphous phase under the ultrasonic vibration treatment (UVT) at a frequency of 20000 Hz. The nanoscale hierarchical features include twins, stacking faults bands, hexagonal-close packed phase bands and even amorphous bands can be obviously identified in samples treated by different UVT energies. The dominant mechanism of ultrasonic vibration-induced amorphization is that the grain refinement promotes the formation of amorphous phases when the defect density at the grain boundaries reaches a critical level. In addition, the mechanical instability is easily induced by ultrasonic vibration at high strain rate to generate amorphous phase inside the grains. As a consequence of UVT, the HEA samples revealed significant mechanical performance improvement owing to the microstructure evolution especially the generation of amorphous phase, such as yielding strength and hardness. This rapid amorphization process provides not only a candidate strengthening mechanism for HEA, but also a novel approach to unveil the pending crystal-amorphous transition problem.