As the relative costs of steel and aluminum change, one metal may substitute for the other (Figure ). The true substitutional elasticity of steel and aluminum is unknown and, to some degree, unknowable: it depends on the material and system demands of innumerable industries and applications. Substitution can be modeled, however, with values for self and cross-elasticities for the two metals (see Methods). The substitution ratio of aluminum for steel -that is, the mass of aluminum that could be used in place of a ton of steel, or vice versa -varies by application but can be bounded between gravimetric substitution (1 t aluminum / t steel) and volumetric substitution (0.35 t aluminum / 1 t steel). Comparisons of bending and tensile strength support estimates of 0.5-0.7 t aluminum / t steel, and we estimate a ratio of 0.59 t aluminum / t steel from the automobile industry (Fig. ). As the price of electricity decreases, the larger share of aluminum costs related to energy means that the cost of aluminum production will decrease more than steel. In the limit of nearly free renewable electricity, aluminum/steel production cost ratios could decrease from around 4:1 today to 2:1 in the future. For context, Figure shows historical steel and aluminum prices and price ratios from 1990 to present. In turn, our assumed substitutional rates and elasticities suggest that the demand for aluminum could increase dramatically at low electricity cost (Figure ), not only in automobiles but also in equipment, transportation, and construction applications where steel has historically been the metal of choice.