Size Independent Shape Memory Behavior of Nickel–Titanium

Abstract

Shape memory alloys represent a class of so-called ''smart'' materials that can be returned to their original shape after deformation, either spontaneously or through the application of heat. While several alloys are capable of shape memory behavior, nickel-titanium (NiTi) is the most extensively researched due primarily to its relatively large deformation recoverability, [1] as well as its high strength, [2] corrosion resistance, [3] biocompatibility, [4] and high intrinsic damping. [5] The shape memory effect for NiTi results from a reversible martensitic phase transformation, in which the crystal structure shifts from a B2 (austenite) to a B19 0 (martensite) phase in a shear-like manner. Depending upon composition and processing history, the stress-induced martensitic phase transformation is capable of two responses: pseudoelasticity and shape memory behavior. Pseudoelasticity occurs when the martensite is unstable at the testing temperature and spontaneously reverts back to austenite upon unloading, recovering the previously accumulated deformation. Shape memory behavior occurs when the martensite is stable at the testing temperature, requiring heat to revert to austenite and recover the strain associated with the phase transformation.Monotonic uniaxial stress-strain testing of shape memory NiTi is well-known to exhibit four stages of deformation, each dominated by a specific mechanism as a function of increasing strain: (I) elastic deformation of austenite, (II) austeniteto-martensite phase transformation, (III) elastic deformation of martensite, and (IV) martensite plasticity. [6] The martensite phase transformation is exemplified by a critical stress at which the phase transformation initiates, followed by a decrease in stress/strain slope signifying the propagation of the martensite throughout the sample. [1] Nominal values of the critical martensite initiation stress, transformation slope, and transformation strain are heavily influenced by processing history, microstructure, and crystallographic orientation. Because the phase transformation is temperature dependent, COMMUNICATION[*] Dr.While shape memory alloys such as NiTi have strong potential as active materials in many small-scale applications, much is still unknown about their shape memory and deformation behavior as size scale is reduced. This paper reports on two sets of experiments which shed light onto an inconsistent body of research regarding the behavior of NiTi at the nano-to microscale. In situ SEM pillar bending experiments directly show that the shape memory behavior of NiTi is still present for pillar diameters as small as 200 nm. Uniaxial pillar compression experiments demonstrate that plasticity of the phase transformation in NiTi is size independent and, in contrast to bulk single crystal observations, is not influenced by heat treatment (i.e., precipitate structure).808

Shape Memory Alloy (SMA)‐Based Microscale Actuators with 60% Deformation Rate and 1.6 kHz Actuation Speed

“…Among these, SMAs have strong advantage in micro/nanoscale actuation, as they can generate a large force within small volume . Furthermore, the unique shape memory behavior is even preserved in the submicrometer range . For these reasons, numerous studies have been performed to determine how to apply shape memory effects (SME) to microscale actuation.…”

Size effects in Al nanopillars: Single crystalline vs. bicrystalline

“…While so far most groups have focused on studying size effects in monolith systems such as single crystals, several researchers have looked at more complex microstructures, such as nano-crystalline [43], nano-twinned [44,45], nanolaminate [46,47], and alloyed materials [48,49], to name a few. For example, Jang et al found a "smaller is weaker" phenomenon in nanocrystalline Ni-W pillars with an average grain size of 60 nm [50], while Mara et al found that the presence of closely spaced Cu-Nb nano-laminates overrides the size effect, with the strength being a function of the laminate spacing rather than the sample dimensions [46].…”

Protocols for the Optimal Design of Multi‐Functional Cellular Structures: From Hypersonics to Micro‐Architected Materials

“…Over the past 5 years, a multitude of room‐temperature uniaxial compression and tension experiments have been performed on a wide range of single‐crystalline metallic nano‐pillars and nano‐dogbones, including fcc metals (Ni and Ni‐based superalloys, Au, Cu, and Al), bcc metals (W, Nb, Ta, and Mo), hcp metals (Mg and Ti), tetragonal low‐temperature metals (In), Gum metal, nanocrystalline metals (Ni), shape memory alloys (NiTi and Cu‐Al‐Ni), and a variety of metallic glasses. For samples with nonzero initial dislocation densities (i.e., excluding whiskers and nano‐fibers), a strong size effect on the flow strength was ubiquitously demonstrated as the sample size approached μm and sub‐μm dimensions .…”