The availability of new analytic technologies has set metallurgists to work reevaluating the potential for alloys long thought to be too difficult to produce or manage. The availability of new forming possibilities (particularly additive technologies) has given product designer the same inspiration.
Nickel-titanium alloys, frequently called Nitinol, were discovered almost 70 years ago by U.S. Navy researchers, but they are intriguing now because of their shape-memory and superelasticity characteristics. Product designers are selecting NiTi for surgical implants, anti-scalding fixtures, and motorized actuators, for example. They are seen as potentially effective materials for solid-state refrigeration units, like heat pumps that transfer heat from one location to another.
There is a challenge in evaluating these materials because of their unusual thermal properties. Researchers at Hong Kong Polytechnic University sought to track thermal behavior of NiTi materials under mechanical stress.
“It is well-established that the temperature of some special shape-memory alloys varies under applied stress,” explained Dr. Ruien Hu, scientific officer at PolyU. “We previously used thermocouples to measure temperature changes, but their single-point measurement capability was insufficient to capture the non-uniform temperature distribution across specimens particularly given the heterogeneous nature of NiTi alloys.”
Infrared cameras seemed to offer a better approach, and so Hu and his colleagues selected the FLIR X8583 camera unit, outfitted with advanced lenses, filters, and software. The camera offered the high-speed, 1280 × 1024 thermal imaging necessary to visualize inhomogeneous surface-temperature distributions during mechanical loading cycles.
“Our experimental set-up involves using a tensile testing machine to conduct tensile and compression tests on shape memory alloys,” Dr. Hu detailed. “Simultaneously, the FLIR camera records and analyzes the temperature variations under stress. This capability is essential for understanding the elastocaloric response.”
The researchers selected an alloy with a nearly equiatomic composition of nickel and titanium, and an additive manufacturing method (laser powder bed fusion, or LPBF) to produce the test parts. They discovered significant inhomogeneity in the elastocaloric effect (i.e., the reversible heating and cooling that takes place under stress), with up to 4.2° K (-268.95 C° / -452.11 F°) variance across a single sample.
Using the FLIR camera set-up, including a midwave infrared lens and custom software, the researchers identified the role of the microstructural inhomogeneity in temperature distribution; captured full-field thermal responses during cyclic mechanical testing; and recorded new learnings about the alloy’s superelasticity and fatigue. Their findings are contributing to broader research into “advanced solid-state cooling technologies and adaptive material design.”
“Infrared imaging revealed that the surface temperature distribution was inhomogeneous during compression-induced elastocaloric effects,” Dr. Hu noted. “This was mainly due to the non-uniform microstructure of the NiTi alloys fabricated by LPBF.”
The FLIR thermal imaging captured striped high- and low-temperature regions, helping the team link grain size, dislocation density, and precipitate distribution to the functional performance of the material.
“The high resolution of the FLIR infrared camera is particularly advantageous for our research, as our specimens are relatively small. Additionally, the camera's exceptional frame rate allows us to capture rapid temperature changes with precision critical for our analysis,” Dr. Hu said.