Metallurgical production traditionally involves two steps: extracting metals from ores and mixing them into alloys via liquid processing. This sequential approach has been successful since the Bronze Age, but today it reaches a limit because of the urgent demand for a sustainable economy: almost 10 % of all greenhouse gas emissions are due to the use of fossil reductants and high-temperature processing in the metallurgical sector. Here we present a H2-based redox synthesis strategy that revolutionizes traditional alloy making by merging metal extraction and alloying into one single solid-state process step. We apply this novel concept to the fabrication of Fe-Ni invar alloys, one of the most appealing ferrous materials but the dirtiest to produce: invar has uniquely low thermal expansion, enabling key applications spanning from precision instruments to cryogenic transport components. Yet, it is notoriously eco-unfriendly, with Ni causing 10 times higher CO2 emission than Fe per kilogram production, qualifying this alloy class as a perfect demonstrator case. Our sustainable method turns oxides directly into green invar alloys, far below the melting temperature, at zero CO2 footprint. Based on the governing thermodynamic-kinetic mechanisms, we propose a general design treasure map that unlocks tremendous sustainable alloy design opportunities. Our approach has the potential to reform metallurgy, by merging the traditional two-step alloy fabrication of metal extraction and liquid alloying into a sustainable one-step solid-state operation.