City University of Hong Kong (CityU) researchers report that they have developed a novel strategy to design new high-strength alloys that are also ductile and flexible. The strategy overcomes the critical issues of the strength-ductility trade-off dilemma, paving the way for developing innovative structural materials in future.
Multiple-principal element alloys, generally referred as high-entropy alloys (HEAs), are designed with equal or nearly equal quantities of five or more metals. They are currently the focus of attention in materials science and engineering due to their potential for structural applications. Yet most of the alloys share the same key detrimental feature: the higher the strength, the less the ductility and toughness.
Recently, a research team led by Professor Liu Chain Tsuan, University Distinguished Professor of the Department of Materials Science and Engineering at CityU, has found a breakthrough solution to this daunting decades-long dilemma — by making high-entropy alloys both strong and yet also very ductile through massive precipitation of nanoscale particles. The research has just been published in the latest issue of the journal Science, titled “Multicomponent intermetallic nanoparticles and superb mechanical behaviours of complex alloys.”
“We are able to make a new high-entropy alloy called Al7Ti7 ((FeCoNi)86-Al7Ti7) with a superior strength of 1.5 GPa and ductility as high as 50% in tension at ambient temperature. Strengthened by nanoparticles, this new alloy is five times stronger than that of the iron-cobalt-nickel (FeCoNi)-based alloy,” says Professor Liu.
“Most conventional alloys contain one or two major elements, such as nickel and iron to manufacture,” he explains. “However, by adding additional elements of aluminum and titanium to form massive precipitation in the FeCoNi-based alloy, both the strength and the ductility have significantly increased, solving the critical issue of the trade-off dilemma for structural materials.”
Moreover, high-strength alloys usually face plastic deformation instability, known as the necking problem, meaning that when the alloy is under high pressure, its deformation would become unstable and very easily lead to necking fracture (localized deformation) with very limited uniform elongation. But the team has further found that by adding “multicomponent intermetallic nanoparticles,” meaning complex nanoparticles made of atoms of different elements, it can greatly strengthen the alloy uniformly by reducing the deformation instability.
And they have found the ideal formula for these complex nanoparticles, which consist of nickel, cobalt, iron, titanium, and aluminum atoms. Professor Liu explains that each nanoparticle measures 30 to 50 nm only. The iron and cobalt atoms, which replace some of the nickel atoms, help to reduce the valence electron density and improve the new alloy’s ductility. On the other hand, replacing some of the aluminum with titanium largely reduces the impact of moisture in air, to avoid induced embrittlement.
https://www.sciencedaily.com/releases/2018/11/181127092520.htm
