Two teams of researchers from North Carolina State University and Qatar University have developed a new ‘high-entropy’ metal alloy that has a higher strength-to-weight ratio than any other existing metallic material.
High-entropy alloys are materials that consist of five or more metals in approximately equal amounts. These alloys are currently the focus of significant attention in materials science and engineering, and the research teams have been successful in combining lithium, magnesium, titanium, aluminium and scandium to make a nanocrystalline high-entropy alloy that has low density, but very high strength.
“The density is comparable to aluminium, but it is stronger than titanium alloys,” stated NC State’s Professor Carl Koch, who is the senior author of a paper on the work titled ‘A Novel Low Density, High Hardness, High-Entropy Alloy with Close-packed Single-phase Nanocrystalline Structures’, published in the journal Materials Research Letters. “It has a combination of high strength and low density that is, as far as we can tell, unmatched by any other metallic material. The strength-to-weight ratio is comparable to some ceramics, but we think it’s tougher – less brittle – than ceramics.”
“We still have a lot of research to do to fully characterise this material and explore the best processing methods for it,” added Koch. At this point, the primary problem with the alloy is that it is made of 20% scandium, which is extremely expensive. Due to the high vapour pressures of several of the elements (Li, Mg), the researchers used mechanical alloying to prepare the alloy powders instead of melting and casting. The elemental metal powders of better than 99% purity were obtained from Alfa Aesar.
The mechanically alloyed powder, which did not have the high N, O impurity levels, was observed to have a single-phase fcc crystal structure in the as-milled condition. It had a nanocrystalline grain size estimated by the Scherrer formula to be about 12 nm.