Thermo-Mechanical behavior at Nano-Scale and Size Effects in Shape Memory Alloys
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Thermo-Mechanical behavior at Nano-Scale and Size Effects in Shape Memory Alloys Jose San Juan1,2, Maria L. Nó3 and Christopher A. Schuh2 1
Dept. Física Materia Condensada, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apdo. 644, 48080 Bilbao, Spain. 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA. 3 Dept. Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apdo. 644, 48080 Bilbao, Spain. ABSTRACT Shape Memory Alloys (SMA) undergo reversible martensitic transformation in response to changes in temperature or applied stress, exhibiting specific properties of superelasticity and shape memory. At present there is a high scientific and technological interest to develop these properties at small scale, to apply SMA as sensors and actuators in MEMS technologies. In order to study the thermo-mechanical properties of SMA at micro and nano scale, instrumented nano indentation is being widely used for nano compression tests. By using this technique, superelasticity and shape memory at the nano-scale has been demonstrated in micro and nano pillars of Cu-Al-Ni SMA. However the martensitic transformation seems to exhibit a different behavior at small scale than in bulk materials and a size effect on superelasticity has been recently reported. In the present work we will overview the thermo-mechanical properties of CuAl-Ni SMA at the nano-scale, with special emphasis on size effects. Finally, the above commented size effects will be discussed on the light of the microscopic mechanisms controlling the martensitic transformation at nano scale. INTRODUCTION Recently, there has been growing interest in the possible use of shape memory alloys (SMA) in micro and nano-scale structures and devices, for example as sensors or actuators in micro electro-mechanical systems (MEMS). With a growing world-wide market in excess of one hundred billion dollars, MEMS and NEMS constitute a new paradigm of technological development for the present century and have already found usage as sensors and actuators across numerous industrial sectors [1]. The development of multifunctional and smart materials [2] is converging with miniaturization technologies, enabling a new generation of smart MEMS or SMEMS. Among the different smart materials targeted for use in SMEMS, shape memory alloys have attracted considerable interest [3,4] because they offer the highest output work density, about 107 J/m3 [5], and exhibit specific desirable thermo-mechanical effects such as superelasticity and shape memory, due to the reversibility of their thermoelastic martensitic transformation [6]. So it is expected that integrating into MEMS components that exhibit superelasticity, one-way or two-way shape memory, would enable a new generation of SMEMS. In addition, the development of more precise and reliable MEMS and NEMS requires improve their endurance against hazardous environmental vibrations [7-9]. This could be achieved by incorpora
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