Atomic-scale research on stress-induced ITB→9R→DT structural transformation at microcrack tips in FCC metal

Deformation twins (DTs), as a core carrier of plastic deformation in face-centered cubic (FCC) metals, play a significant role in coordinating dislocation movement and facilitating stress release. This study, through in-situ tensile experiments in a transmission electron microscope (TEM), reveals the atomic-scale dynamic evolution mechanism of DTs at the microcrack tip in pure aluminum. The results show that the incoherent twin boundary (ITB), as a key precursor structure, evolves into the long-period 9R structure (with a critical width of ≥9 ∑3{111} planes) by periodically emitting Shockley partial dislocations (1/6 〈112〉), and ultimately transforms into a stable twin. Under stress, the microcrack tip nucleates and grows twins through a stepwise nucleation pathway of “ITB → 9R → DT”, which has a significantly lower energy barrier than direct twin nucleation, enabling the system to achieve stepwise energy release. Further investigations reveal that DTs grow in length by emitting thin lamellar ITB distortion zones from their leading edge and in thickness by forming step structures along the twin side. Twin thickness is a reliable criterion for size increase, while length changes are unreliable due to the dynamic annihilation of the lamellar distortion layer. This mechanism elucidates, from an atomic-scale kinetic pathway, the widespread occurrence of deformation twinning in high stacking fault energy FCC metals and provides new insights for enhancing the material's resistance to crack propagation.