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Title of Journal: Shap Mem Superelasticity

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Abbravation: Shape Memory and Superelasticity

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Springer International Publishing

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DOI

10.1002/cbic.201600312

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ISSN

2199-3858

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Thermal Stabilization of NiTiCuV Shape Memory Allo

Authors: Marvin Schmidt Johannes Ullrich André Wieczorek Jan Frenzel Andreas Schütze Gunther Eggeler Stefan Seelecke
Publish Date: 2015/06/20
Volume: 1, Issue: 2, Pages: 132-141
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Abstract

The paper presents novel findings observed during the training process of superelastic elastocalorically optimized Ni–Tibased shape memory alloys SMA NiTiCuV alloys exhibit large latent heats and a small mechanical hysteresis which may potentially lead to the development of efficient solidstatebased cooling processes The paper starts with a brief introduction to the underlying principles of elastocaloric cooling illustrating the effect by means of a typical thermodynamic cycle It proceeds with the description of a custombuilt testing platform that allows observation of temperature profiles and heat transfer between SMA and heat source/sink during highloadingrate tensile tests Similar to other SMA applications a training process is necessary in order to guarantee stable performance This wellknown mechanical stabilization affects the stress–strain hysteresis and the cycledependent evolution of differential scanning calorimetry results In addition it can be shown here that the training is also accompanied by a cycledependent evolution of temperature profiles on the surface of an SMA ribbon The applied training leads to local temperature peaks with intensity number and distribution of the temperature fronts showing a cycle dependency The homogeneity of the elastocaloric effect has a significant influence on the efficiency of elastocaloric cooling process and is a key aspect of the specific material characterizationSuperelastic shape memory alloys have been known for biocompatibility 1 2 3 and good damping properties 4 5 Ni–Tibased stents have been established in the market for years 6 and the damping properties have been studied in a large number of scientific works 7 8 In addition to these established fields elastocaloric cooling has recently started to attract interest as a further possible application for superelasticity and is currently being investigated as part of the German Science Foundation DFG Priority program SPP 1599 Ferroic Cooling 9 The interest is based on the large latent heats of the stressinduced phase transformation between austenite and martensite which can potentially be used for the development of novel cooling processes The elastocaloric effect in SMAs was already described by Refs 10 11 12 and in addition to the magnetocaloric electrocaloric and barocaloric effect it represents a further possible physical mechanism for cooling processes based on ferroic materials 13 The high potential 14 of these solidstate cooling processes and the potential to establish environmentally friendly alternatives to conventional vaporcompressionbased processes increase the interest in these technologiesMagnetocaloric cooling is the most established ferroic cooling technology 15 16 17 However an increased interest in the elastocaloric effect is noticeable as well 18 19 20 21 22 The comparison of different materials shows that Ni–Tibased shape memory alloys provide the most promising elastocaloric properties The latent heats of binary Ni–Ti alloys 22 J/g are significantly higher than the latent heats of other SMAs eg CuZnAl 62 J/g and CuAlNi 68 J/g 14 These heats are in direct correlation with the absorbable heat during an elastocaloric cooling process 21 23 The efficiency of an elastocaloric cooling process depends on the latent heat the mechanical work and the homogeneity of the elastocaloric effect Even though commercially available superelastic Ni–Ti alloys suitable for elastocaloric cooling already exhibit large latent heats of 15 J/g 24 the mechanical hysteresis is comparatively high and can be reduced significantly by means of optimized alloy compositions In order to improve elastocaloric material properties new Ni–Tibased alloys have been developed 25 featuring small mechanical hysteresis and high cyclic stability Furthermore the elastocaloric cooling cycle requires heat transfer at high and low temperature level To this end superelastic material behavior at low temperatures is required which can be achieved with low martensite to austenite transformation temperatures A strong correlation between the transformation temperature and the latent heats can be observed in many Ni–Tibased alloy systems leading to small latent heats for low transformation temperatures 26Based on an extensive study of more than 70 alloy compositions 26 a quaternary NiTiCuV alloy was developed with optimized elastocaloric properties and will be investigated within this work The investigation of the elastocaloric cooling process and the elastocaloric material properties requires a scientific test setup capable of performing cooling cycles and simultaneous measurement of mechanical parameters strain and strain rate as well as thermal parameters temperature change and homogeneity of elastocaloric effects Such a test platform was developed 27 and has been used to investigate cooling properties of NiTiCuVIn this paper we put a special emphasis on the wellknown mechanical stabilization of the material during training and to the best knowledge of the authors for the first time also document thermal stabilization behavior by visualizing the cycledependent evolution of temperature profiles In addition to these investigations we round off the picture with data for the stabilization of the caloric properties of NiTiCuV investigated by means of differential scanning calorimetry DSCElastocaloric cooling processes have specific material requirements and new test methods are required in order to investigate properties of materials that could be used for elastocaloric cooling The following chapter gives a brief introduction to the underlying principles for elastocaloric cooling Furthermore a custombuilt testing platform is presented enabling the investigation of elastocaloric materials and cooling processesAdiabatic elastocaloric cooling process following a Brayton cycle The four process phases are shown by means of IR images The images show the temperature change of the heat source/sink after 40 cooling cycles of an experiment while studying a NiTiCuV ribbon sample


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