Substitutability: When No Alternative Exists

Substitutability is a critical dimension in mineral criticality assessments because it determines whether an economy can adapt if supply of a particular mineral is disrupted. If a viable substitute material exists, companies can switch to the alternative, reducing the impact of a supply shortage. If no substitute is available, or if substitution involves severe performance penalties, cost increases, or lengthy redesign cycles, the mineral's criticality is amplified significantly. The more irreplaceable a mineral is in its key applications, the greater the economic and strategic consequences of losing access to it.

How Substitutability Is Measured

Criticality frameworks evaluate substitutability through several lenses:

• Technical feasibility: Can another material perform the same physical or chemical function? For example, can a different metal provide the same magnetic performance, electrical conductivity, catalytic activity, or thermal resistance as the mineral in question?
• Performance penalty: Even when a substitute is technically possible, it may deliver inferior performance. Ferrite magnets can replace rare earth permanent magnets in some motor applications, but they produce significantly weaker magnetic fields, requiring larger, heavier motors with reduced efficiency.
• Cost competitiveness: A substitute that costs five times more than the original material is not a practical alternative for high-volume applications, even if it is technically feasible.
• Time to implement: Switching to a substitute material often requires redesigning products, requalifying manufacturing processes, and obtaining regulatory approvals. In aerospace and defense, material qualification can take 5-10 years.
• Supply chain maturity: A potential substitute is only useful if it can be produced at scale. If the substitute material itself has a constrained supply chain, switching to it simply transfers the vulnerability rather than eliminating it.

The European Commission's framework quantifies substitutability using a Substitution Index (SI) that ranges from 0 (fully substitutable) to 1 (no substitution possible). Each mineral is scored across its main applications, with the overall SI reflecting a weighted average based on consumption shares.

Examples of Low Substitutability

Several critical minerals have exceptionally low substitutability in their primary applications:

• Neodymium and dysprosium in permanent magnets: NdFeB (neodymium-iron-boron) permanent magnets are the strongest commercially available magnets and are essential for compact, efficient electric motors and generators. No other magnetic material comes close to their energy density. While motor designs that use electromagnets or switched reluctance technology can avoid permanent magnets entirely, they sacrifice power density, efficiency, and compactness, making them unsuitable for many EV and wind turbine applications.
• Cobalt in superalloys: Cobalt-based superalloys withstand extreme temperatures and stress in jet engine turbine blades and gas turbines. While nickel-based superalloys can partially substitute, they do not match cobalt alloys' performance at the highest operating temperatures. In battery applications, cobalt can be reduced or eliminated through chemistry changes (LFP, high-manganese cathodes), but this often involves trade-offs in energy density.
• Platinum group metals in catalysis: Platinum, palladium, and rhodium are used in automotive catalytic converters because of their unique ability to catalyze the conversion of harmful exhaust gases at operating temperatures. No commercially viable non-PGM catalysts have been developed for this application. In hydrogen fuel cells, platinum remains essential as a catalyst, though research is reducing the loading required per unit.
• Gallium in compound semiconductors: Gallium arsenide and gallium nitride have electronic properties, including high electron mobility, wide bandgap, and thermal stability, that silicon cannot match for applications in high-frequency communications, power electronics, and LED lighting. While silicon is dominant in general computing, it cannot substitute for gallium in these specialized applications.
• Helium in cryogenics: Helium is the only element that remains liquid at temperatures close to absolute zero, making it irreplaceable as a coolant for MRI machines, particle accelerators, and quantum computing systems. No chemical substitute exists for this physical property.

Partial Substitutability and Trade-offs

Many minerals fall into a middle ground where substitution is technically possible but involves significant trade-offs. Lithium iron phosphate (LFP) batteries substitute iron and phosphate for the cobalt and nickel used in NMC (nickel-manganese-cobalt) batteries, but with lower energy density. Sodium-ion batteries can potentially substitute for lithium-ion in stationary storage, but with lower energy density and an immature supply chain. Thallium-based high-temperature superconductors can theoretically substitute for rare-earth-based superconductors, but face their own supply and toxicity challenges.

Understanding these trade-offs is essential for realistic assessment of substitutability. A mineral is not truly substitutable unless the alternative can be deployed at scale, at competitive cost, and with acceptable performance, within a timeframe that matters for supply security.

The Role of Research and Innovation

Governments invest heavily in materials research aimed at developing substitutes for critical minerals. The U.S. Department of Energy's Critical Materials Institute, the EU's EIT RawMaterials, and Japan's NIMS (National Institute for Materials Science) all conduct research into alternative materials. Success stories include the development of reduced-rare-earth magnets, cobalt-free battery cathodes, and thin-film solar cells that minimize indium and tellurium use.

However, materials innovation is inherently slow. Moving a substitute material from laboratory demonstration to commercial-scale production typically requires 10-20 years of development, testing, qualification, and manufacturing scale-up. This timeline means that substitution is more of a long-term risk mitigation strategy than a rapid response to supply disruptions. In the near term, minerals with low substitutability remain among the most critical and warrant the most aggressive supply diversification and stockpiling measures.

Implications for Criticality Scoring

Minerals with low substitutability scores consistently rank among the most critical in every national assessment. Rare earths, PGMs, gallium, and cobalt (in its superalloy applications) are perennial high-scorers. As new technologies emerge and applications evolve, substitutability assessments must be regularly updated to reflect both new alternative materials and new uses that may lack substitutes. The dynamic interplay between demand growth and substitutability is particularly important: a mineral can become more critical even as its per-unit substitutability improves, if demand growth outpaces the deployment of substitutes.