Abnormal acoustoplasticity originating from ultrasound-controlled evolution of multiscale ordering in complex concentrated alloys

• Ultrasound-assisted heat treatment (UATT) markedly improves the mechanical properties of CrCoNi alloy, achieving an unprecedent acoustic hardening ratio of up to 52%. • By tuning the ultrasound amplitude, UATT can promote the dislocation multiplication and drive the evolution of multiscale ordered microstructures across multiple scales—from atomic-scale short-range order to long-range ordered phases (L1 2 ) and ultimately to micron-scale Cr 2 O 3 precipitates. • Quantitative analysis reveals that the hardening contribution from ultrasound-induced ordered structures can, under certain conditions, significantly surpass that from dislocation proliferation. This finding challenges the conventional dislocation-dominated view of acoustic hardening. • Integrated experiments and Monte Carlo/molecular dynamics (MC/MD) simulations elucidate that the ultrasound-induced ordering arises from the enhanced atomic diffusion (facilitated by a high vacancy concentration) and inhomogeneous atomic environments caused by dislocation multiplication. Contrary to the traditional Blaha effect in ordinary metals and alloys, where ultrasonic excitations induce softening, this study unveils an abnormal acoustoplasticity effect in the CrCoNi complex concentrated alloy (CCA), where ultrasound induces hardening up to 52% by increasing the ultrasonic amplitude. Two effects contribute to this unusual phenomenon in CrCoNi. First, as ultrasound amplitude increases, dislocations multiply, leading to work hardening, as opposed to annihilation leading to softening as in ordinary metals and alloys, due to the very low stacking fault energy in CrCoNi, which makes dipole annihilation on stress reversal difficult. Secondly, ultrasound vibrations trigger novel disordering-ordering transitions in CrCoNi, progressing from short- to long-range ordering, resulting in phases including L1 2 and Cr 2 O 3 nano-precipitates as well as large (∼2.37 μm) Cr 2 O 3 precipitates. Combined experiments and Monte Carlo/molecular dynamics (MC/MD) simulations suggest that the enhanced ordering stems from two mechanisms: (i) the multiplied dislocations attract atomic segregation at their cores, and (ii) the incomplete dipole annihilation driven by stress reversals elevates the concentration of vacancies, which enhances atomic diffusion for ordering. These ordered phases may facilitate higher dislocation nucleation rates and hinder dislocation mobility, further promoting dislocation multiplication in the CrCoNi alloy, which has low cross-slip potential. Quantitative analysis reveals that the contribution of ordering to hardening even surpasses Taylor hardening at an ultrasound amplitude of 11 μm. This work shows an unexpected acoustoplastic response in CCAs arising from the coupling of dislocation-based mechanisms and ultrasound-induced ordering, offering a novel pathway for customizing the mechanical properties in these alloys.