Imbalances between metal supply and demand, real or anticipated, have inspired the concept of metal criticality. We here characterize the criticality of 62 metals and metalloids in a 3D "criticality space" consisting of supply risk, environmental implications, and vulnerability to supply restriction. Contributing factors that lead to extreme values include high geopolitical concentration of primary production, lack of available suitable substitutes, and political instability. The results show that the limitations for many metals important in emerging electronics (e.g., gallium and selenium) are largely those related to supply risk; those of platinum group metals, gold, and mercury, to environmental implications; and steel alloying elements (e.g., chromium and niobium) as well as elements used in high-temperature alloys (e.g., tungsten and molybdenum), to vulnerability to supply restriction. The metals of most concern tend to be those available largely or entirely as byproducts, used in small quantities for highly specialized applications, and possessing no effective substitutes.economic geology | materials science | substitution | supply risk | sustainability

It is indisputable that modern life is enabled by the use of materials in its technologies. Those technologies do many things very well, largely because each material is used for purposes to which it is exquisitely fitted. The result over time has been a steady increase in product performance. We show that this materials complexity has markedly increased in the past half-century and that elemental life cycle analyses characterize rates of recycling and loss. A further concern is that of possible scarcity of some of the elements as their use increases. Should materials availability constraints occur, the use of substitute materials comes to mind. We studied substitution potential by generating a comprehensive summary of potential substitutes for 62 different metals in all their major uses and of the performance of the substitutes in those applications. As we show herein, for a dozen different metals, the potential substitutes for their major uses are either inadequate or appear not to exist at all. Further, for not 1 of the 62 metals are exemplary substitutes available for all major uses. This situation largely decouples materials substitution from price, thereby forcing material design changes to be primarily transformative rather than incremental. As wealth and population increase worldwide in the next few decades, scientists will be increasingly challenged to maintain and improve product utility by designing new and better materials, but doing so under potential constraints in resource availability.criticality | material substitution | product complexity | metal life cycle | sustainability T he degree to which the materials of modern technology enable and improve our state of life is not adequately appreciated. A century ago, or even half a century ago, less than 12 materials were in wide use: wood, brick, iron, copper, gold, silver, and a few plastics. Today, however, substantial materials diversity in products of every kind is the rule rather than the exception. [A modern computer chip, for example, employs more than 60 different elements (1).] This use of materials is not a whim of the designer, but a carefully calculated effort to achieve increasingly high performance in products simple to complex. Rich Materials Palette of Modern ProductsThere are many examples of this evolution in material complexity (2), but a particularly vivid one is the increase in diversity of the "superalloy" metals used in aircraft turbine blades, as shown in Fig. 1. These nickel-rich alloys are corrosion resistant and stable at higher temperatures than most metals and alloys. As the figure shows, additional alloying elements have been added one by one over the years, and the alloys have assumed more complex structural forms as well. The result has been a relatively steady increase in the engine operating temperature over time, carrying with it increased engine efficiency and reduced greenhouse gas emissions. A similar story could be told for most of today's more advanced technologies and products.This utilization of materials pro...

Metals that are obtained largely or entirely as by-products are essential to modern technology, but their supplies may be constrained due to a variety of factors.