Alloy Predict material properties of stainless steel

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Predicting the material properties of metal alloys has so far been difficult. An international research team, including the Munich University of Applied Sciences (Germany), has now succeeded in calculating the complex physical properties of alloys and predicting their changes during laser light processing.

A life without alloys is hard to imagine: spoons, aluminum foil, knives, car parts, mobile phone and computer circuit boards, stents, and construction steel are made from chemical compounds that contain at least one metal. "Although alloys are used everywhere, there are still significant gaps in knowledge about their physical properties," says Prof. Dr. Heinz P. Huber, head of the Laser Center at the University of Applied Sciences in Munich (HM) and professor at the Faculty of Applied Sciences and Mechatronics. The material properties of "high-entropy alloys" are particularly difficult to predict: these materials contain various metals in equal proportions whose positions in the crystal lattice can vary arbitrarily. The maximum disorder—entropy—results in the fact that the electrical and magnetic properties of these materials have so far been hardly predictable.

Companies that, for example, perforate alloys for implants or phone circuit boards with ultra-short pulse lasers have so far relied on trial and error in controlling their machines.

Prof. Dr. Heinz P. Huber

Processing alloys: material properties not predictable

"This lack of knowledge has practical implications," emphasizes Huber. "Companies that, for example, perforate alloys for implants or mobile phone circuit boards with ultrashort pulse lasers have so far relied on trial and error to control their machines." To control manufacturing in a needs-based and precise manner, it would be necessary to know how, when, and to what depth the laser's energy is absorbed. The prerequisite for this would be a process that determines material properties such as temperature and density in real-time at the exact point the laser targets. However, the fundamental physical understanding required for such monitoring has been lacking until now.

Gallery

Quantum mechanics makes disorder calculable

An international research team has now succeeded in closing this knowledge gap. The work, whose first author is Dr. David Redka from the University of Applied Sciences in Munich (Germany), was recently published in Nature Communications. The predictions of the material behavior of high-entropy alloys are based on complex quantum mechanical calculations. With their help, Redka was able to explain the influence of maximum disorder and strong electronic interactions on their electrical and optical properties.

Thanks to the experiments and theoretical calculations, we were able to separate the influences of chemical entropy and strong electronic interactions on the material properties.

Dr. David Redka

The theoretical predictions were supported by experiments at the Swiss EPFL and the Paul Scherrer Institute (PSI), where scientists used high-energy X-rays to examine an alloy of chromium, manganese, iron, cobalt, and nickel. This metal mixture is used in research as a prototype for high-entropy alloys. "Thanks to the experiments and theoretical calculations, we were able to separate the influences of chemical entropy and strong electronic interactions on the material properties," explains Redka.

Entropy of stainless steel identifiable in the light spectrum

Strong electronic interactions affect the absorption of visible light when the surface is illuminated. Entropy, in turn, influences electrical resistance and the absorption of infrared light. Light reflected from the surface also reveals the state of the alloy: the higher the temperature and the denser the molten material, the more the refractive index changes, and thus the color of the reflected light beam also changes.

The researchers at the Laser Center now aim to use the results of the work to develop new methods for process control in material processing with ultrashort pulse lasers: "Our long-term goal is to determine the temperature and density in real-time using a light beam directed onto the surface of an alloy and reflected there," summarizes Huber. "This would make it possible, for the first time, to develop sensors that allow monitoring during processing with high-energy laser radiation."

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