Researchers from the U.S. Department of Energy's Oak Ridge and Lawrence Berkeley National Laboratories have created a new metal alloy made up of five elements that gets tougher, stronger and more ductile as temperatures drop.
Called "high-entropy alloy," it contains chromium, manganese, iron, cobalt and nickel but doesn't behave like ordinary alloys in that not one element is dominant while the others offer complementing characteristics. When the researchers tested the alloy, it crystallized into a single-phase, face-centered cubic material with exceptional tolerance to damage, over one gigapascal of tensile strength and off-the-charts values in fracture toughness that no other metallic alloys can compare to.
According to Robert Ritchie, a materials scientist from the Lawrence Berkeley National Laboratory and lead author for the study, the idea behind not one element dominating the material, like what high-entropy alloys are showing, is that the level of configurational entropy rises alongside the number of alloy elements. This counteracts compound formation, stabilizing the alloy so it ends up in a single phase similar to pure metal.
High-entropy alloys are not exactly new but this is the first time that the alloy was created with enough quality to allow for testing. To achieve this, researchers at the Oak Ridge National Laboratory combined highly pure elemental starting materials using a drop-casting and arc-melting process, producing the alloy in sheets about 10 mm thick. Once the samples have been tested for microstructures and tensile properties, they were sent to researchers at the Berkeley Lab for toughness and fracture testing.
What's truly remarkable about high-entropy alloys though is that unlike most metallic alloys, they did not become more brittle and lose ductility when temperatures were low. Researchers attribute the alloy's superb toughness, ductility and cryogenic strength to nano-twinning, a phenomenon in which the arrangements of atoms in neighboring crystal structures mirror each other during deformation.
"These nano-twins are created when the material undergoes plastic deformation at cryogenic temperatures. The result of nano-twinning deformation is a continuous strain hardening, which acts to suppress the localized deformation that causes premature failure," explained Ritchie.
Called "A fracture-resistant high-entropy alloy for cryogenic applications," the study was published in the journal Science. Other authors include: Edwin Chang, Dhiraj Catoor, Anton Hohenwarter, Bernd Gludovatz and Easo George.
Other examples of alloys are bronze (made from combining tin and copper), steel (produced from mixing carbon and iron) and non-corrosive steel (created by adding nickel and chromium to steel's components).