Selectively realizing metastable phases is challenging, despite their vast number, due to the interplay of kinetics and thermodynamics, which has to be realized through processing conditions. Here, we demonstrate selective and controlled realization of metastable phases through size confinement effects. This is realized during diffusion-controlled filling of nanocavities. By nanomolding AuSi alloy and performing crystal structure and elemental analysis of molded alloy nanowires, we observe three different phase states, i.e., hybrid nanophase, bamboo-shaped, and uniform alloy phases, corresponding to different nanowire sizes. The surprising uniform alloy phase for nanowires with diameter below $\sim 20\mathrm{nm}$ exhibits perfect single crystalline structure with uniform distribution of Au and Si compositions. A phase state-energy map is constructed to rationalize the findings by considering the competition between mixing enthalpy, configuration entropy, strain energy and interfacial energy. Our Letter demonstrates the universal applicability of nanomolding in fabricating metal-metal and metal-nonmetal metastable phases. In contrast to state-of-the-art rapid cooling techniques, this strategy is simple and practical, thereby enabling precise selection of novel materials with tailored functionalities.

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• Creep
• Crystal phenomena
• Phase separation
• Alloys