Humans have been making metal alloys for thousands of years, and most of us can conjure a rough mental image of the process – it involves red-hot molten metals being mixed, poured, and shaped in a sweltering workshop or factory. This approach still works perfectly well for the traditional metals we see every day, like steel. But advanced metals with special chemical and mechanical properties, ones that scientists are investigating to use in energy technologies like long-lasting batteries and the extreme-temperature engines for aerospace vehicles, need a more refined approach.
Researchers from the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered a new way to produce these materials, called high-entropy alloys (HEAs), at near-room temperatures. Their technique, described in a paper published today in Nature, also gives users much more control of the alloy’s crystal structure and overall morphology compared with existing methods, opening the door for a new paradigm of custom-made HEAs.
First discovered about 20 years ago, these materials have generated a lot of excitement thanks to their record-breaking strength and toughness, giving them many potential applications in mechanical engineering. HEAs could also serve as potent catalysts, boosting the efficiency and durability of batteries and fuel cells and reducing our reliance on imported rare minerals. HEAs’ unique properties stem from their balanced recipes of different elements. While typical metal alloys are composed of a high proportion of one element with lower amounts of additional elements mixed in (for example, steel is ~97–99% iron with tiny amounts of carbon and other elements), HEAs are made of equal or nearly equal ratios of elements, creating an internal crystal structure with more entropy – meaning it is more disorganized.
Alloys with Abundant Applications
HEAs are a diverse class of material with wide-ranging properties that depend on their composite elements. But overall, the alloys are known to have extreme durability against mechanical strain, corrosion, radiation, and both high and low temperatures – all factors that push the limits of traditional metals. Scientists at Berkeley Lab and UC Berkeley have been instrumental in testing the capabilities of these interesting materials, including discovering that one HEA is the toughest known material.
In addition to energy storage devices and spacecraft, our researchers and others worldwide are exploring HEAs for use in CO2 reduction to make fuels and other chemicals, fuel cells, sensors and other electronics, drills for digging geothermal energy wells, and even biomedical devices.
And although this disorganization is key to their function, engineers still need to be able to tune the materials to certain specifications and form them into different shapes. The existing methods to make HEAs involve heating the elements to high temperatures so that the atoms have a lot of kinetic energy, mixing the different elements into one lump, then rapidly dropping the temperature by various methods. The drastic temperature change is thought to be necessary to lock in a state of internal disorganization from the energetic atoms.
The Berkeley Lab team’s approach achieves the same high-entropy result at much lower, constant temperatures (a breezy 25–80 degrees Celsius, or 77–186 F) by mixing the elements that will make up the HEA into the metal gallium when it is in liquid form. The elements are introduced in a water-based solution in their chloride forms. When the acidic liquid meets the liquid gallium at a pleasantly warm-to-hot temperature, the elements very quickly shed their chlorine atoms and mix together, then solidify into an HEA alloy. According to team leader Haimei Zheng, a senior scientist in the Materials Science Division and adjunct professor at UC Berkeley, the incredible speed of the reaction and mixing at the liquid-liquid interface is what traps the much-desired entropy.
The new phenomenon was discovered by first author Qiubo Zhang, a postdoctoral researcher in Zheng’s group, when he was conducting experiments with the team’s liquid-cell transmission electron microscopy (TEM) platform, a technology they have advanced which allows scientists to study electronic and chemical reactions occurring in liquid environments in real time, at the atomic level. While using a liquid cell TEM to observe liquid gallium, he noticed Cu ions from the CuCl2 aqueous solution were getting sucked into the gallium and forming alloys.