In this perspective, we discuss the future of aluminum and steel with changes in electricity price and carbon intensity. Falling prices of renewable energy could mean that aluminum becomes the low-cost option for many applications, making aluminum decarbonization more cost-eDective and important than that of steel. With a production cost ratio of two to one, rather than four to one or so today, the substitution of aluminum for steel, already underway in certain industries, would only accelerate. While cars today contain, on average, 12% aluminum and 54% steel, cars in this electrified future might be made primarily with aluminum. Aluminum-dominant machinery would be next for substitution, and a transition of construction materials -particularly aluminum-reinforced building materials for steel rebar -could follow soon behind. A world where we can make green steel cost-competitively might, paradoxically, be a world where few want it. We caution that using aluminum in place of steel achieves carbon savings only with process changes in the aluminum industry, and could easily be counterproductive without these improvements. For the Bayer process, improvements include the use of heat pumps or waste heat for bauxite digestion, the addition of electric furnaces for calcination, and co-location with aluminum smelting for tighter heat integration. The Bayer process has a energy intensity at present of 14.7 GJ/t Al2O3 that far exceeds the thermodynamic minimum (1.3 GJ/t Al2O3). For the Hall-Héroult process, inert anodes would enable aluminum production without CO2 evolution, albeit at higher voltage, and wettable cathodes could allow alternative cell designs for greater eDiciency and reduced cost. Substitution of aluminum for steel is unlikely to be limited by the availability of aluminum-containing ores. Primary production of steel from iron ore is 1500 Mt/year, requiring 2500 Mt ore/year, and primary production of aluminum is 70 Mt/year, requiring 350 Mt bauxite/year. Bauxite reserves are estimated between 55,000 and 75,000 Mt, or around 200 years at current levels. This can accommodate a many-fold increase in aluminum demand, even without clays and other aluminum-containing minerals that are subeconomic today. Substitution of aluminum for steel could be constrained, however, by materials requirements for particular applications. Aluminum is less dense, has a lower yield strength, and has a higher thermal expansion coeDicient than steel. Within the construction sector (50% of steel, or 950 Mt/y), steel is used in sections (150 Mt/year), rebar (375 Mt/y), sheet (265 Mt/y), tubes (95 Mt/y), and railroad tracks (20 Mt/y). Challenges associated with using aluminum instead of steel as a construction material include flammability and heat resistance, hydrogen gas evolution, and a coeDicient of thermal expansion that is dissimilar from concrete, complicating aluminum-based rebar. Within the transport sector (16%, or 300 Mt/y), 7 main applications include cars (80 Mt/y), trucks, and ships. Use of more aluminum and plastic for passenger cars, and less steel, is already underway and seems poised only to accelerate, with fewer impediments than construction and indirect CO2 emissions savings during vehicle use from lightweighting. Substantial substitution in even a few of these areas -for instance, cars -would be enough to double or triple the production of aluminum (currently 70 Mt/y primary production), in line with the elasticity and electricity cost calculations presented in this analysis.