Composition-driven fibrous-to-lamellar transition of discontinuous precipitates in Cu-Ni-Si alloys

• Higher solute content significantly accelerates discontinuous precipitation kinetics. • The DP orientation relationship is identified as [110]Cu∥[010]δ and (111)Cu∥(301)δ. • 3DIL model explains the transition via competition between driving force and strain energy. • Higher driving force activates a secondary growth direction, causing the 1D-to-2D transition. Discontinuous precipitation (DP), a common transformation in the later stages of aging, was traditionally considered detrimental to mechanical properties. Recently, a new paradigm has emerged, viewing DP products as in-situ composite reinforcements to achieve high-performance copper alloys. However, the rational utilization of this approach depends on a deeper understanding and control of the DP product morphology (e.g., fibrous or lamellar), the selection mechanism for which remains elusive. This study systematically investigates the effect of solute concentration (Ni content from 3.0 to 7.0 wt%) on DP behavior in Cu-Ni-Si alloys using multi-scale characterization and the three-dimensional invariant line (3DIL) model. We observe a definitive transition from a one-dimensional (1D) fibrous to a two-dimensional (2D) lamellar morphology of δ-Ni 2 Si precipitates with increasing solute concentration. This transition is quantitatively explained by the interplay between the chemical driving force and the anisotropic elastic strain energy. The 3DIL model reveals that a greater chemical driving force enables the activation of a secondary, higher-strain-energy growth direction, leading to the 1D-to-2D morphological transition. These findings establish a “compositional lever” for controlling DP morphology, providing a new theoretical basis for the targeted engineering and rational utilization of DP microstructures in advanced alloys.