Researchers have fabricated a high-performance elastocaloric nickel-titanium (NiTi) alloy using additive manufacturing. The alloy retains its shape after one million compressive cycles, and it could potentially be used in green cooling devices. They report their work in a recent issue of Science.

NiTi, an alloy that is equal parts nickel and titanium metal, exists in two phases with distinct crystal lattice structures: a high-temperature austenite phase and a low-temperature martensite phase. As a shape memory alloy (SMA), NiTi can reversibly transition between the two phases, although the transition to the martensite phase occurs at a higher temperature than the austenite phase transition. An elastocaloric compound in the austenitic phase is exposed to a heat load. Stress is applied, which causes the elastocaloric to transition to the martensite phase. The material is held in that phase until it cools. The strain is then removed, the material returns to the austenite phase, and the cycle can be repeated.

“One issue with conventionally processed NiTi alloys is the degradation of the strain recovery; essentially, NiTi alloys, after many cycles, lose their ability to remember their shape,” says Beth Last of Pennsylvania State University, who was not connected with the study. 

For this work, a team of scientists from the University of Maryland collaborated with scientists at Ames Laboratory to fabricate the NiTi alloy, using a form of additive manufacturing called laser-directed-energy deposition (L-DED). Using L-DED, powders of nickel and titanium were mixed in the presence of a laser, which created a molten alloy that was printed into various shapes, such as tubes and honeycombs. The L-DED process gave control over the ratio of nickel and titanium, which allowed the researchers to create a composite structure on the nanoscale. The resulting material was a matrix of the binary alloy (Ni_51.5Ti_48.5) that contained microdomains of nickel-rich Ni₃Ti.

The stress-strain hysteresis curve of the NiTi nanocomposite alloy was almost linear and much narrower than that seen in other alloys, including NiTi alloys created by melt-casting. To understand the material’s response under strain, a research group from the Colorado School of Mines performed in situ synchrotron x-ray diffraction on the NiTi nanocomposite alloy during one compressive cycle. When combined with simulations, the researchers could determine the optimal ratio of the binary and nickel-rich alloys and understand “the stress transfer mechanism that occurs within the nanocomposite,” says Ichiro Takeuchi of the University of Maryland and the corresponding author of the work published in Science.

The NiTi nanocomposite alloy fatigue study was performed at the University of Maryland, led by Huilong Hou. During the study, the nanocomposite was subjected to one million compression cycles. Fatigue-studies on materials are typically conducted for one million cycles; however, this had not been done for three-dimensional (3D)-printed NiTi alloys with elastocaloric performance. “One million cycles corresponds to a 10 year life of commercial cooling equipment,” Takeuchi says. Remarkably, the response of the material on the first and one-millionth cycle were very similar. 

Takeuchi and his colleagues attributed the durability of their NiTi alloy to the fact that energy lost due to hysteresis (DE) was very small compared to the total input energy (E). The ratio between the two, called the dissipated energy fraction (DE/E), was an order of magnitude smaller for the NiTi alloy in this work compared to many other elastocaloric materials.

Because the researchers have demonstrated a range of shapes using the L-DED process, one of the next steps is to build a cooling device using the NiTi nanocomposite. “These materials could double as metallic refrigerants and heat exchanges,” says Takeuchi, “We can now design components to use in actual cooling devices.”

The NiTi nanocomposite could be used in applications other than cooling. According to Last, “The exceptional cyclic stability observed in these L-DED NiTi alloys really is remarkable and could be utilized for high cycle applications which have been previously excluded due to the non-stable response of conventionally processed NiTi alloys.”

Read the abstract in Science.