The evolution of the internal microstructure of a nickel-titanium sample is measured while it is mechanically tested. The colors correspond to crystallographic orientation, and the orientations are clustered into grains, or crystals. Source: Colorado School of Mines.

Visualizing shape memory alloys in real-time

Using the most powerful 3D microscopes available today, researchers associated with the Colorado School of Mines successfully imaged the interior microstructure of shape memory alloys in three major experiments, shedding new light on this underutilized material.
 
^ The evolution of the internal microstructure of a nickel-titanium sample is measured while it is mechanically tested. The colors correspond to crystallographic orientation, and the orientations are clustered into grains, or crystals. Source: Colorado School of Mines.

Article by Daniel Sweet
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Shape memory alloys are well known for their ability to be crumpled up and then spring back to a “remembered” original shape. But the advanced material remains drastically underutilized in commercial applications, uses that could include morphing the shape of airplane structures to make flight more efficient or deploying communication dishes and solar arrays in space.

“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,” explained Dr. Ashley Bucsek, the primary scientist behind the latest research into SMAs. “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 bridge that gap, Bucsek and her fellow researchers put nickel-titanium — the most widely used and available SMA — under powerful 3D microscopes, allowing them to visualize the interior microstructure of SMAs in three dimensions and in real-time. Then they conducted three experiments into various topics in SMA micromechanics.

Three experiments

In SMAs, it is often the high-symmetry phase called “austenite” that is stable at a higher temperature, but if enough stress is applied or the temperature is decreased, it will phase transform to a low-symmetry phase called “martensite.” The first experiment conducted by Dr. Bucsek’s team looked to predict the specific variety of martensite that would form.

“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,” Dr. Bucsek said.

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

“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,” Dr. Bucsek said “These findings will guide future researchers in employing twin rearrangement in novel multiferroic technologies.”

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

The target of the final experiment was a phenomenon in which special high-angle grain boundaries emerge inside austenite grains when SMAs are actuated. During actuation, phase transformation from austenite to martensite then back to austenite is induced by heating, cooling and then reheating the SMA while under a constant load.

“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,” Dr. Bucsek said. “Our results showed that these grain rotations can occur after just one cycle in moderate condition. But because of the low volume and heterogenous dispersion of the rotations, a bulk volume is required to observe them.”
 

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