Osaka Metropolitan University achieves fabrication and manipulation of high-entropy alloys via laser

A group at Osaka Metropolitan University revealed that they made fine particles of multi-element "high-entropy alloys" via laser irradiation underwater, identified the cause of specific elements being unevenly distributed, and confirmed that element ratios can also be manipulated. High-entropy alloys are expected to find broad applications as next-generation materials. It is said that this may prove useful for research and development into their fabrication methods.

Multi-element alloys such as brass (copper and zinc), stainless steel (iron and chromium), and duralumin (aluminum with copper and magnesium) possess characteristics that cannot be achieved with a single metal. Many of these traditional alloys add another metal to improve the characteristics of the main metal.

In 2004, the concept of "high-entropy alloys" mixing five or more elements in almost equal proportions was proposed (those with three or four elements are medium-entropy alloys). Such alloys are coming to be known for excellence in strength, heat resistance, corrosion resistance, and other properties. In recent years, research has been advancing to apply them as catalysts, electrode materials, biomaterials, and thermoelectric materials.

Professor Tomoyuki Yatsuhashi (Photochemistry) of Osaka Metropolitan University decided to investigate the effects of ultraviolet lasers, which are not researched much overseas, and alloy fabrication in water. He prepared a ternary alloy (FeCoNi) consisting of iron, cobalt, and nickel; a quaternary alloy (CrFeCoNi) adding chromium to this; and a quinary alloy (CrMnFeCoNi) further adding manganese. With each immersed in water, irradiating any of a near-infrared laser, a visible laser that appears green, or an ultraviolet laser 100 times per second produced "submicron particles" of alloys smaller than 1 micrometer and generally larger than 0.1 micrometers.

When observing these submicron particles with a transmission electron microscope, each element was distributed almost uniformly in ones with small diameters. However, starting around where the diameter exceeded 0.2 micrometers, iron (Fe), chromium (Cr), and manganese (Mn) were distributed on the outside, and cobalt (Co) and nickel (Ni) were distributed on the inside. Regarding element distribution, there were four types: "core-shell," "crescent-shaped segregation," "uniform distribution," and "phase separation." According to Yatsuhashi, results were similar with any laser. It is considered that factors such as how easily each element is oxidized are involved. Element distribution inside CrFeCoNi particles was investigated with a transmission electron microscope.

In addition, investigating the elemental composition of submicron particles via energy-dispersive X-ray spectroscopy, for those made with the ultraviolet laser, element proportions changed significantly as particle diameters became smaller. In FeCoNi particles, the proportion of Fe; in CrFeCoNi particles, the proportions of Cr and Fe; and in CrMnFeCoNi, the proportions of Cr, Mn, and Fe increased markedly compared to the original raw materials, respectively, while Co and Ni decreased. The same trend was observed when using the visible laser, but not to the extent of the ultraviolet laser. With the near-infrared laser, almost no composition change occurred.

Much of the research on high-entropy alloys has focused on creating alloys with a uniform elemental distribution. However, non-uniform distributions are also necessary for certain functions to emerge. In the present study, ultraviolet laser irradiation in water was found to alter the composition of the resulting alloy particles. "Our findings demonstrate a possible route for selectively concentrating specific elements at particular locations and in desired proportions by tuning the laser wavelength," Yatsuhashi said.

The research was published on June 1 in the electronic edition of Journal of Alloys and Compounds, an international academic journal.