Superelasticity and Shape Memory in Micro‐ and Nanometer‐scale Pillars

Abstract

Micro-and nanoelectromechanical systems (MEMS and NEMS, respectively) are being developed intensively and constitute a new paradigm of technological development for the present century. With a growing world-wide market in excess of one hundred billion dollars, MEMS and NEMS pose a challenge for the science and technology of microfabrication [1] and have already found usage as sensors and actuators across numerous industrial sectors, [2] from automotive, aerospace, and telecommunications [3] to emerging biomedical technologies. [4] In parallel, the development of multifunctional and smart materials [5] is converging with miniaturization technologies, enabling a new generation of smart MEMS (SMEMS). Among the different smart materials targeted for use in SMEMS, shape memory alloys have attracted considerable interest [6,7] because they offer the highest output work density (about 10 7 J m -3 ), and exhibit specific desirable thermomechanical effects owing to the reversibility of their thermoelastic martensitic transformation. [8] In particular, integrating into MEMS components that exhibit superelasticity, or one-way or two-way shape memory, would enable a new generation of functional microdevices. In the last decade, great effort has been devoted to the production of shape memory thin films, which could be integrated into the planar technology of microsystems. Shape memory thin films have been mainly produced by sputtering, and exhibit both superelastic and shape memory properties. The effort has largely been focused on the Ti-Ni system (reviewed by Miyazaki and Ishida [9] ), and combinatorial methods are also being applied to develop new compositions not presently used in bulk shape memory applications.[10] However, although such sputtered films are often made with thicknesses between 0.5 and 15 lm, the desirable thermomechanical effects of superelasticity and shape memory have only been tested and exploited for applications using their largest dimensions above millimeter size. [6,7,9] Thus, there remain important unanswered questions about the nature of these thermomechanical effects at the scales relevant to MEMS and NEMS applications. In the present work the first objective is to design and produce simple shape memory alloy features of micro-and nanometer-scale dimensions, smaller in every dimension than a typical thin film thickness currently obtained by sputtering-around 5 lm. The second objective is to demonstrate on such features the thermomechanical properties of superelasticity and shape memory at the nanometer scale. We report that stress-induced or thermal martensitic transformation can take place at the nanometer scale in a reversible way; individual martensite variants below 25 nm thick appear in these experiments. The presented results demonstrate that in Cu-Al-Ni shape memory alloys, completely recoverable superelasticity, in a high number of cycles, is achievable at the very fine scales pertinent to MEMS devices. From a practical perspective, a first issue to consider is how we can demonstrate an...

Micro-Scale Cyclic Bending Response of NiTi Shape Memory Alloy

“…One should note that the curves have irregular shapes, however; the same shapes were evident in all companion experiments, eliminating the possibility of scatter in data. Indeed, these observations agree well with the previous findings of work on CuAlNi pillars 22,23) and wires, 24) and NiTi micro wires. 25) Specifically, consecutive compressive loading of the CuAlNi pillars and wires led to the narrowing of hysteresis.…”

Anomalous Behavior During Nano‐Compression Superelastic Tests on Cu‐Al‐Ni Shape Memory Alloy Micro Pillars

“…Therefore, prior to proceed with cycling tests, we carried out a series of six loading cycles at increasing load up to 1250 µN, which is the maximum applied load in the present cycling case. During these first few nano‐compression cycles, a local plastic deformation occurs beneath the sphero‐conical indenter, giving place to a settling point at the top of the pillar, and once the settling point is established the pillar experiences essentially uniform compressive load, as was described in previous works …”

“…These two properties, shape memory and superelasticity, were extensively studied at macroscopic scale and have found number of applications . The advent of the trend of miniaturization has led to focus research efforts on SMA at the micro and nano metric scale, seeking to obtain active components that can be used in the development of new and sophisticated sensors and actuators that can be framed in the concept of the smart micro electro‐mechanical systems (SMEMS), or its nanometric analogs, the SNEMS. In this sense, most efforts were made in the production and characterization of SMA thin films, from which many devices such as microwrappers, microgrippers, microcages, microvalves and micro switch, among others, were already developed for MEMS applications; see the reviews in this field .…”

“…In this scenario, our pioneering works showing the complete recovery of superelasticity and shape memory in Cu‐Al‐Ni SMA, as well as a size effect responsible for an ultrahigh damping at micro and nanoscale, opened a new way for the applications of SMA in SMEMS. To progress, a complete characterization of Cu‐Al‐Ni SMA at small scale was required, and then the selection rules of martensite by in‐situ transmission electron microscopy, and recently a size effect on the critical stress for superelasticity, were reported.…”

Shape memory and superelasticity in polycrystalline Cu–Al–Ni microwires

“…These transformation temperatures are close to those reported for Cu-Al-Ni single crystals of the same alloy composition. 18 The enthalpies for the martensitic and reverse transformations are ϳ330 J / mol ͑5.7 J/g͒ and ϳ360 J / mol ͑6.2 J/g͒, respectively, which are consistent with prior values reported for similar alloys, e.g., ϳ389 J / mol by Otsuka et al 5 and 267-362 J/mol by Gastien et al 8 We carry out isothermal uniaxial tensile tests of the SMA wires using a dynamic mechanical analyzer ͑TA Instruments Q800͒. Each end of the wire is mounted in an adhesive compound at the two gripping ends, which are clamped mechanically.…”