Black body cavity apparatus for measuring the emissivity of Nickle-Titanium based shape memory allows

Nickel-Titanium based shape memory alloys (SMAs) have various applications in biomedical, actuators, aerospace technologies, and elastocaloric devices, because of their shape memory and super elastic effects.1 These effects are induced in SMAs by a re- versible phase transformation from martensite to austenite. This phase transformation can be achieved by applying stress or heating and cooling. It is extremely difficult to measure the temperature of the NiTi elements with contact thermometers without dis- turbing the phase transformation parameters. To enable the determination of tempera- ture distribution in NiTi components with infrared thermography, a newly constructed apparatus is presented to measure the emissivity of NiTi SMAs. For this purpose, a state-of-the-art infrared camera (type: ImageIR 8380, InfraTec GmbH, Germany) was employed to analyze the emissivity behavior during the phase transformation. The emissivity of the SMA was systematically studied by comparing it with a reference- quality black body cavity (BBC) having emissivity better than 0.9995. The calibration measurements reveal that the maximum deviation of the BBC temperature measured with the infrared camera was only 0.15% from the temperature measured with the con- tact thermometers. The apparatus was also validated by measuring the emissivity of a polished aluminum sample and the obtained results were in a good agreement with the literature data at high temperatures. Finally, the emissivity of two rough and one polished NiTi samples was measured covering the temperature range from 313 K to 422 K with a experimental uncertainty of about 0.01. The emissivity of NiTi varies between 0.1 and 0.28. In the martensite phase, it increases rapidly with the increase in temperature, while decreases slowly in the austenite phase. Moreover, it is investigated that the emissivity of NiTi depends on the microstructure, and surface roughness of the samples.