Materials Science and Engineering A 378 (2004) 175–179 Local influence of material processing on phase transitions in NiTi shape memory alloys investigated by IR thermography J. Gibkes a,∗ , W. Siegert b , D. Dietzel a , I. Delgadillo-Holtfort a , B.K. Bein a , J. Pelzl a a Solid State Spectroscopy, Institute of Experimental Physics III, Ruhr-University, Bochum D-44780, Germany b Faculty of Mechanical Engineering, Institute of Materials, Ruhr-University, Bochum D-44780, Germany Received 2 June 2003; received in revised form 22 October 2003 Abstract Local variations of the phase transition temperatures are important for the functionality of actuators based on NiTi shape memory alloys. Mechanical distortions induced by process treatment can suppress the phase transition from the austenitic to the martensitic structure. In this work, local changes of the phase transition have been investigated by time-dependent IR thermography. For the quantitative interpretation of the thermographical measurements, information about the IR emissivity is important. In this work a new thermal wave method based on two detection techniques is presented, which can be used to measure both the IR emissivity and the thermal transport properties. © 2004 Elsevier B.V. All rights reserved. Keywords: IR thermography; Phase transition; Emissivity 1. Introduction In many applications, shape memory alloys (SMA) are used for mechanical switch mechanisms. An example of a decoupling mechanism, which is analysed in this work, is that of an actuator to be used to hold the solar panels of a space satellite during transport into space. After transport, the actuator is used only once to expand the solar panels (one way effect). Problems may arise due to mechanical distortions induced by process treatment during fabrication, which can suppress the phase transition from the martensitic to the austenitic structure in parts of the actuator. To study the effects of mechanical distortions, the local changes of the phase transition have been investigated by time-dependent infrared thermography. For the quantitave interpretation of the measured temperature profiles, the emissivity ε of the sample surface has to be known. In the following, a new tech- nique is presented to measure the emissivity by comparing the results of different photothermal methods. Additionally, a brief description of the interpretation of the thermographic measurements is presented, developed for the quantitative analysis of the phase transitions. ∗ Corresponding author. Tel.: +49-234-32-28618; fax: +49-234-32-14336. E-mail address: gibkes@ep3.ruhr-uni-bochum.de (J. Gibkes). 2. Investigation of the temperature evolution at positions with and without phase transition The local variations of the phase transition temperature are important for the functionality of actuators based on NiTi shape memory alloys. Mechanical distortions induced by process treatment can contribute to suppress the phase transition from the martensitic to the austenitic structure. We have measured the effect of phase transitions on the temperature evolution of an electrically heated actuator (Fig. 1c) by time-dependent infrared thermography. For the temperature measurement, an IR camera from JENOPTIC (Varioscan 3011) with a temperature range from -40 to 1200 ◦ C, a temperature resolution of 30 mK, a geometri- cal resolution of 1.5 mrad and a time resolution of 0.8 s has been used. The camera is a single detector camera with a nitrogen-cooled MCT (HgCdTe) detector which is working in the wavelength spectrum between 8 and 12 m. From the time-dependent IR images registered by the camera we can extract the temperature evolution at differ- ent positions of the sample surface (Fig. 1a and b). At the positions P1 and P2 the actuator material was affected by mechanical distortions related to the mechanical treatment process, whereas the positions P3 and P4 are not affected by mechanical treatment. 0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.343