Scripta Materialia 162 (2019) 68–71 Contents lists available at ScienceDirect Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat Degradation of the recoverable strain during stress controlled full transformation cycling in NiTi shape memory alloys Yahui Zhang a, c , * , Weichen Li b, c , Ziad Moumni c , Jihong Zhu a , Weihong Zhang a, ** , Sheng-Yi Zhong b a State IJR Center of Aerospace Design and Additive Manufacturing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China b School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China c IMSIA, UMR 8193 CNRS-EDF-CEA-ENSTA, Université Paris Saclay, 828 Boulevard des Maréchaux, Palaiseau Cedex 91762, France ARTICLE INFO Article history: Received 6 September 2018 Received in revised form 21 October 2018 Accepted 22 October 2018 Available online xxxx Keywords: Shape memory alloys Residual martensite Cyclic loading Fatigue ABSTRACT This paper investigates the degradation of the recoverable strain during full transformation cycling in shape memory alloys. Results show that such degradation occurs due to: (i) untransformed austenite during full transformation, (ii) residual martensite after complete unloading, and (iii) plastic strain produced in the lat- est reverse transformation. Particularly, the plastic strain tends to zero when the stabilized state is reached. Furthermore, the degradation is frequency-dependent in the sense that a higher loading frequency results in a larger degradation of the recoverable strain. Finally, it is shown that fatigue lifetime decreases when transformation degradation increases. © 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Pseudoelastic shape memory alloys (SMAs) have the ability to accommodate large recoverable inelastic strain as a result of stress-induced phase transformation between austenite and ori- ented martensite. Thanks to this special property, SMAs have been widely used in a variety of applications ranging from civil engineer- ing to space industries, where SMA structures are always submitted to cyclic loading. The cyclic behavior of pseudoelastic SMAs is always affected by plastic deformation: when phase transformation takes place, a high-level local stress field is created as a result of the unmatched deformation in austenite-martensite interfaces, and this local stress fosters dislocation slips [1-6] (such dislocation slips can be triggered separately by forward and reverse martensitic trans- formations [7]), leading to macroscopic plastic deformations and a residual internal stress field inside the material. The internal stress field then assists the nucleation of martensite variants, thus gen- erating some residual martensites [8-14] while reducing the yield stresses required to trigger phase transformation during subsequent cycles [15-19]. As a consequence, the residual strain accumulates upon cycling, and the critical stresses for transformation and the hysteresis loop area decrease. For the strain controlled loading, * Correspondence to: Y. Zhang, State IJR Center of Aerospace Design and Additive Manufacturing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China. ** Corresponding author. E-mail addresses: zhang.yahui@outlook.com (Y. Zhang), zhangwh@nwpu.edu.cn (W. Zhang). the degradation mechanism is straightforward: since the maximum deformation is fixed, an accumulation of the residual strain results in a reduction of the recoverable strain [1,20]. However, the situation for stress control is more complicated: for partial transformation, transformation ratchetting is observed and explained by the accu- mulation of residual martensite [1]; as the critical stress for forward transformation decreases, more transformation is triggered leading to an increase in the recoverable strain [21]. The experimental results in [22] show that an increase in temperature makes phase trans- formation more difficult and thus leads to remarkable reduction of the recoverable transformation strain. In the case of stress controlled full transformation cycling, as will be shown in this work, the degra- dation of the recoverable strain can still appear although the trans- formation is completely finished. More effort should be dedicated to unveil the effects of plastic deformation (generated separately by forward and reverse transformations), residual martensite, untrans- formed austenite [8] and temperature on such degradation as well as on fatigue failure. This paper aims at elucidating the degradation mechanism via presenting a theoretical and experimental study on the recov- erable strain degradation during stress induced full transforma- tion cycling. To this end, cyclic tensile tests on pseudoelas- tic NiTi SMA wires with a diameter of 1 mm (50.8 at. % Ni, obtained from Xi’an Saite Metal Materials Development Co. Ltd., China) were conducted. Phase transformation temperatures of the material are measured using a differential scanning calorimeter https://doi.org/10.1016/j.scriptamat.2018.10.031 1359-6462/ © 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.