Growth-state-dependent deformation stability of discontinuous precipitation in Cu-Ni-Si alloys

초록

Discontinuous precipitation (DP) in Cu-Ni-Si alloys can either promote strengthening or diminish it through fragmentation, yet the factor governing these contrasting effects remains unclear. Here, the growth of individual DP colonies in a Cu-4Ni-1Si alloy was tracked using in situ EBSD and quantitatively correlated with their deformation stability during cold drawing. Contrary to the classical view of DP as a strain-free transformation product, the growing colonies progressively accumulated lattice rotation, geometrically necessary dislocations (GNDs), and low-angle grain boundaries (LAGBs). The colony interiors reached a median GND density of 2.2 & times; 1014 m(-2), more than one order of magnitude higher than that of the matrix. HRTEM combined with geometric phase analysis further revealed that, whereas the early-stage S-Ni2Si/Cu interface remained close to an elastically accommodated semi-coherent state, prolonged aging increased the interfacial mismatch beyond the geometrically predicted value. The additional lattice distortion was stored within the S-Ni2Si fibers, while the ductile Cu matrix plastically accommodated the resulting incompatibility. This growth-inherited structural state governed the subsequent deformation response: later-stage fibers accommodated only approximately one-third of the plastic strain carried by early-stage fibers and progressively fragmented during drawing. Consequently, the strengthening rate was highest at a low DP fraction of approximately 10% and lowest at the largest DP fraction of 45%. The deformation stability of DP is therefore determined not by its fraction alone, but by the structural state established during colony growth, providing a practical criterion for the thermomechanical design of highstrength, high-conductivity Cu-Ni-Si alloys.

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