The alloys can still form intermetallics, the metals don’t have to be in precisely equal proportions to get interesting properties, and entropy doesn’t seem to be the most important factor, Ritchie says. Researchers are now beginning to question the entropy explanation. By increasing the relative proportions of the elements and introducing more internal entropy, the formation of such compounds becomes energetically much less favourable, giving the resultant alloys improved physical properties without the drawbacks of conventional mixtures. Typically, when several metals are combined, enthalpy drives them towards forming intermetallic compounds that are hard and brittle. The term high-entropy alloys arises from the high level of internal disorder that occurs when the principal elements are mixed. High-entropy alloys, however, show promise in providing both high strength and high ductility. “So if you put them in an engine, the first pigeon you ingested would shatter the engine,” Ritchie says. The trouble is, as materials get stronger that way, they become less ductile, making them brittle. It is possible to make metals that can withstand higher temperatures by using alloys with very strong atomic bonds in the crystal. “We’d kill just to get a five-degree increase in temperature in the engine.”

Engines could run several hundred degrees hotter than existing metals allow, Ritchie says, if only the materials were available.

But to keep the engine parts from being damaged by high temperatures, the engines also use some of the fuel, which could otherwise be devoted to flying the plane, to compress air so that it can be used to cool the parts, thus decreasing efficiency. Jet engines convert heat energy from burning fuel into thrust, and the greater the temperature difference between the heat source and the cool air outside the engine, the more thrust the engines can get from a given amount of fuel. “The whole development of the gas-turbine engine has been basically driven by the development of higher and higher temperature materials,” he says. One of the biggest challenges in structural materials is creating substances that can maintain their integrity at high temperatures, such as those found in aerospace applications, says Robert Ritchie, a materials scientist at the University of California, Berkeley, who studies the mechanical behaviour of such alloys at the US Department of Energy’s Lawrence Berkeley National Laboratory. The resulting crystal structures can endow the alloy with a combination of useful properties, such as strength, toughness and resistance to corrosion. Unlike conventional alloys, which usually feature one main metal mixed with small quantities of others, high-entropy alloys typically use five metals mixed in roughly equal proportions. A solution to this problem might come from a burgeoning field of metallurgy: high-entropy alloys. But they can’t get too hot - above 1,150 ☌, the nickel superalloy in their turbines starts to soften and bend, which could quickly lead to engine failure. As they get hotter, they burn fuel more efficiently, which means they can fly farther on the same volume of propellant. Sydney/SPLĪircraft engines work better at higher temperatures. Nanocrystalline microstructures, similar to those shown here in an aluminium alloy, can impart desired properties such as enhanced strength and hardness in high-entropy alloys.

High entropy