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Researchers Conduct 3D Experiments to Better Understand Shape Memory Alloy

Shape memory alloys are famous for their extraordinary properties—shape memory, superelasticity, and actuation—that allow them to be crumpled up and then return back to a "remembered" original shape.

Ashley Bucsek PhD '18 was the lead author on the three papers. (Image credit: Colorado School of Mines)

Nevertheless, the advanced material remains radically underutilized in commercial applications, uses that could include deploying solar arrays and communication dishes in space or morphing the shape of airplane structures to increase flight efficiency.

Scientists from Colorado School of Mines are involved in understanding how their multifaceted internal microstructures alter during shape memory behaviors and the findings of their first-of-their-kind experiments were recently reported by three major materials science and mechanics journals, Acta Crystallographica, Journal of the Mechanics and Physics of Solids and Scripta Materialia.

"Discovered over 70 years ago, the promise of shape memory alloys (SMAs) has led to over 10,000 patents in the U.S. and 20,000 worldwide. However, that promise has not been matched by its technological impact—only a limited number of these 20,000 SMA patents have been realized as commercially viable products," said Ashley Bucsek PhD '18, lead author of the three papers and now a President's Postdoctoral Fellow at the University of Minnesota. "The story is similar for many other advanced materials, taking decades to move from development to implementation. One reason for this gap between development and implementation is that researchers are literally just scratching the surface with conventional microscopy techniques, when most of the micromechanisms in SMAs are 3D, out-of-plane and sensitive to internal constraints."

To close that gap, Bucsek and her fellow scientists placed nickel-titanium—the most extensively used and available SMA—under some of the most powerful 3D microscopes presently available at the Cornell High Energy Synchrotron Source (CHESS) at Cornell University in upstate New York.

Specifically, she used far-field and near-field high-energy diffraction microscopy (HEDM), which come under the bandwidth of 3D X-Ray Diffraction methods, enabling her to picture the material's interior microstructure in three dimensions while it is reacting in real time.

Even though HEDM has been developed at CHESS and other synchrotrons around the world for over a decade now, the procedures for applying HEDM to studying advanced materials with features like low-symmetry phase mixtures and large crystal size disparities were essentially nonexistent. As a result, each of these three experiments required the development of novel experimental, data analysis and data visualization techniques to extract the desired information. Many of the results were surprising, shedding light on decades-old areas of contention in SMA micromechanics.

Ashley Bucsek, PhD, Study Lead Author and President's Postdoctoral Fellow, University of Minnesota

In SMAs, it is frequently the high-symmetry phase known as "austenite" that is constant at a higher temperature, but if sufficient stress is applied or the temperature is reduced, it will phase transform to a low-symmetry phase termed as "martensite."

The first paper, "Measuring stress-induced martensite microstructures using far-field high-energy diffraction microscopy," published in September in Acta Crystallographica Section A: Foundations and Advances, aimed to predict the specific type of martensite that would develop.

"Using this approach, we found that martensite microstructures within SMAs strongly violated the predictions of the maximum transformation work criterion, showing that the application of the widely accepted maximum transformation work criterion needs to be modified for cases where SMAs may have engineering-grade microstructure features and defects," Bucsek said.

The second experiment undertook load-induced twin rearrangement, or martensite reorientation, a reversible deformation mechanism by which materials can handle large loads and deformations without loss through rearrangements of crystallographic twins.

The paper, "Ferroelastic twin reorientation mechanisms in shape memory alloys elucidated with 3D X-ray microscopy," is scheduled for publication in March in the Journal of the Mechanics and Physics of Solids.

A specific sequence of twin rearrangement micromechanisms occurs inside macroscopic deformation bands as they propagate through the microstructure, and we showed that the strain localization inside these bands causes the lattice to curve up to 15 degrees, which has important implications on elastic strain, resolved shear stress, and maximizing the twin rearrangement. These findings will guide future researchers in employing twin rearrangement in novel multiferroic technologies.

Ashley Bucsek, PhD, Study Lead Author and President's Postdoctoral Fellow, University of Minnesota

Solid-state actuation is one of the most crucial applications of SMAs, used in several biomedical, microelectromechanical and nanoelectromechanical systems, active damping and aerospace actuation systems.

The aim of the final experiment was an occurrence wherein special high-angle grain boundaries combine inside austenite grains when SMAs are actuated. When actuation takes place, phase change from austenite to martensite then back to austenite is triggered by heating, cooling, and then reheating the SMA while under a constant load.

The paper, "3D in situ characterization of phase transformation induced austenite grain refinement in nickel-titanium," will be published in the March issue of Scripta Materialia.

Using electron microscopy, it has been observed that the austenite can exhibit large rotations when the sample is reheated, which is detrimental to both work output and fatigue. However, because of the small sample sizes required for electron microscopy, these rotations were observed very inconsistently, appearing but then not appearing under the same loading conditions, or appearing after a few cycles but then not appearing after a few thousand cycles. Our results showed that these grain rotations can occur after just one cycle in moderate condition. But because of the low volume and heterogeneous dispersion of the rotations, a bulk volume is required to observe them.

Ashley Bucsek, PhD, Study Lead Author and President's Postdoctoral Fellow, University of Minnesota

Bucsek's study was funded by the National Science Foundation (NSF) Graduate Research Fellowship, as well as the 2015 NSF CAREER Award of her PhD advisor and co-author, Aaron Stebner, Rowlinson Associate Professor of Mechanical Engineering at Mines. Extra funding to use the high-performance computers necessary to study the data came from the NSF XSEDE program.

"Dr. Bucsek's thesis work documented in these articles shows the importance of using 3D techniques to study the 3D structure of materials. She was able to observe and understand mechanisms that have been postulated and debated for over 50 years for the first time," Stebner said. "The biggest hindrance to adopting new materials, like most technologies, is fear of the unknown. Such understanding will undoubtedly lead to wider acceptance and application of these miraculous materials, as it improves our confidence in developing means to certify and qualify them."

The working of the Cornell High Energy Synchrotron Source, which was used to carry out the X-ray microscopy measurements, was also given by NSF.

Throughout her thesis work, Dr. Bucsek developed new, creative ways to apply HEDM methods to the study of shape memory alloy systems. Her ability to overcome challenges associated with data processing and interpretation enabled new insights to be gained into the micromechanics of shape memory alloy deformation.

Darren Pagan, Staff Scientist, CHESS

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