José Manuel Torralba, IMDEA MATERIALS

Elinvar is a unicorn, a new material that once seemed impossible, created by humans using principles that border on magic: the principles that make high-entropy alloys possible.

Elinvar has an exceptional property that overturns every textbook rule: it is a metal that becomes stiffer when heated. Where will we use it? It’s not yet possible to produce it on a large scale. It’s just one example of a new era being forged in cutting-edge laboratories and research centres around the world, the era of high-entropy alloys.

The Defense Advanced Research Projects Agency (DARPA) and the U.S. Air Force Research Laboratory (AFRL) have invested significant resources in studying them. Their report, Defining Pathways for Realizing the Revolutionary Potential of High Entropy Alloy, involved the best laboratories, universities, and specialised researchers in the United States. It’s quite possible that whoever controls the materials of the future will dominate the world, as has always been the case throughout human history.

The political power of copper

Around 8,000 BCE, a curious human picked up a piece of native copper while walking through the countryside. When he struck it, he discovered that it could deform, harden, and take on different shapes. Humanity was in the Neolithic era, and many stone and wooden tools were replaced with copper ones. Not only tools, efficient copper weapons were also developed. The peoples who discovered this secret dominated the world. The Copper Age, the Chalcolithic period, had begun.

A few thousand years later, around 5,000 BCE, somewhere near present-day Turkey, perhaps inspired by the mesmerising glow of a campfire at dusk, other humans discovered that liquid copper could be extracted from certain types of sand (malachite and azurite), and that this copper could be moulded into complex shapes.

The Earth is a storehouse of metals

Humanity had discovered that the Earth is a vast pantry of metals, and that, with the right knowledge, materials with magical properties could be extracted.

After another 2,000 years, around 3,000 BCE, not far from that same region, copper workers discovered that by adding a little tin, the resulting material became harder and more resistant. Thus was born a revolutionary formula — the alloy — in which the whole is far greater than the sum of its parts. And with it came the Bronze Age.

Throughout history, those who understood the secrets of materials have dominated the world. That’s why we name historical periods after the material that enabled the creation of tools and weapons more powerful than those of their enemies: stone, copper, bronze, iron… silicon, carbon?

Technological limits halted the miracle of alloys

The potential for creating alloys seemed infinite, but there is a limit to the properties that can be achieved by adding elements to a metal. Beyond a certain number of components, instead of improving, the resulting alloy develops negative properties, for example, brittleness.

Since ancient times, metallurgists have known there is a limit to how many alloying elements, and in what proportions, can be used.

At the end of the 19th century, the English geologist and microscopist, Sir Henry Clifton Sorby, developed a microscope that could reveal the microstructure of metals, uncovering the structure of steel. He discovered that the loss of desirable properties was linked to the appearance of complex phases (intermetallic compounds), usually brittle, which appeared when certain proportions of alloying elements were reached.

It was understood that the optimal properties of an alloy depend on maintaining one or two simple phases (usually cubic, where atoms are arranged in cubes).

The four rules that defined the limits

In 1938, the chemist and metallurgist, William Hume-Rothery, formulated four rules, the Hume-Rothery rules, explaining why this happens. If two alloying elements don’t satisfy these rules, they won’t form a single phase in which their atoms occupy the same cubic lattice. Instead, more complex phases arise, degrading the material’s properties.

These four rules relate to fundamental atomic properties: size, electronegativity, valence, and the tendency to organise into cubes of the same size and configuration, and they governed alloy design for more than 5,000 years.

High entropy

But in 2004, everything changed. Two research groups, led by Brian Cantor and Jien-Wei Yeh (working independently and simultaneously, as often happens with major scientific breakthroughs), discovered and demonstrated that if the entropy of a mixture is high enough, beyond a certain threshold, a single-phase alloy can form in which atoms occupy the positions of the cube indiscriminately.

Entropy measures the degree of disorder among a material’s molecules. Considering this disorder, Cantor and Yeh broke the limits that had held for millennia — showing that it’s possible to form multi-component single-phase alloys without following the Hume-Rothery rules, vastly expanding the number of viable alloy combinations. Thus was born the concept and origin of high-entropy alloys (HEAs).

Initially, only mixtures in which all components were in equal proportions were considered, but the concept quickly expanded to include multi-metal mixtures that weren’t necessarily equiatomic (with equal atomic numbers).

This revolution in metallurgy accelerated as scientists began evaluating the properties of HEAs. The resulting materials can rival or surpass the best-known alloys in areas such as high-temperature resistance, magnetic properties, and hydrogen storage, etc.

In barely 18 years, the field has grown from Cantor’s and Yeh’s first two papers to over 5,000 related publications in the past year alone.

We are witnessing a discovery on the scale of bronze or steel. The materials we have today are already at their limits in most technological applications — and even small improvements could enable giant leaps. High-entropy alloys promise exactly those improvements.

The properties that could change the world

The combinations of elements for creating HEAs are almost limitless. However, despite advances in simulation techniques, the gap between discovery and practical application can span many years, especially in fields like biomedicine and aerospace.

Many possible element combinations include critical metals: scarce, expensive, and potentially harmful if mishandled. Finding alternatives is another major challenge in materials science.

Multicomponent alloys could also offer solutions for recycling metals that are difficult to separate in electronic waste. Research, including work from the IMDEA Materials Institute, has already shown that HEAs can be developed using commodity alloys originally made for other purposes.

Are we witnessing the dawn of a new human era, the Age of High-Entropy Alloys? It will take centuries to know for sure, but it certainly looks promising.

José Manuel Torralba, Full Professor at the Carlos III University of Madrid, IMDEA MATERIALS

The article was originally published in The Conversation. Read the original (content in Spanish).