Annealing twin boundary widening drives 9R phase transformation in a gradient VCoNi medium-entropy alloy

VCoNi medium-entropy alloy (MEA), with medium stacking fault energy (SFE), typically develops a high density of annealing twins. Yet, its room-temperature plasticity is widely viewed as being governed by full dislocation slip, with annealing twin boundaries (ATBs) regarded as Hall-Petch strengtheners akin to high-angle grain boundaries. Here, we show that, at room temperature, an unexpected 9 R phase can nucleate directly within ATBs. This response is enabled by a deliberately engineered gradient dislocation structure with depth-dependent slip-band spacing, which generates an exceptional structural gradient (∼10.4 GPa/mm) together with high residual compressive stress (∼928 MPa). These coupled factors intensify stacking-fault activity and promote dislocation–twin interactions that progressively widen ATBs. Once the boundary width reaches a critical value of ∼4 nm is reached, 9 R nuclei emerge within the widened region. The synergistic interplay of planar disordered faulting, 9 R transformation, and dislocation multiplication sustains stable work hardening, delivering an ultrahigh yield strength above 1.2 GPa while retaining >20% uniform elongation. Our findings identify annealing twins as active strain-hardening elements in MEAs and reveal 9 R-transformation engineering as a promising strengthening paradigm for medium-SFE alloys.