Dislocation-ledge coupling governs semicoherent precipitate growth

Semicoherent precipitates govern strength, stability and transformation pathways in structural alloys, yet the kinetic defect process underlying their three-dimensional growth has remained unresolved. Here we show that lath growth is driven by diffusion-enabled, non-conservative reorganization of closed interfacial dislocation networks coupled to nanoscale growth ledges. Phase-field-crystal simulations of a model face-centred cubic/body-centred cubic transformation reveal strongly anisotropic kinetics: end faces advance continuously along the long axis, whereas broad facets thicken by discrete ledge sweeps accompanied by mixed glide-climb reactions. O-lattice analysis predicts the defect network, explains the anisotropy through misfit-localization geometry, and shows how the same dislocation motion accommodates transformation strain. In situ transmission electron microscopy of austenite precipitates in duplex stainless steel captures rapid ledge propagation on habit planes, consistent with the predicted mechanism. These results identify the missing kinetic unit of semicoherent precipitate growth and establish a transferable defect-kinetics framework for morphology selection.