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We report a temperature- and size-dependent compositionally tuned microstructural landscape for Ag-46 atom % Cu alloy nanoparticles. The microstructural and morphological changes were established through the technique of in situ transmission electron microscopy. The nanoparticles were synthesized by laser ablation of alloy target in an aqueous medium. The as-synthesized particles predominantly contain nanosized grains of a single-phase solid solution with grains having sizes ∼3 ± 0.5 nm. For particles with smaller sizes (∼20 nm), the solid solution decomposes and grains coarsen on heating to yield predominantly bicrystals containing two phases of Ag-rich and Cu-rich solid solution. The microstructure of the larger particles (≥40 nm) evolves through segregation of Ag and their preferential growth near the surface of the particles. This leads to a core–shell-like composition distribution at a certain range …