Shape Memory and Superelastic Ceramics at Small Scales

Abstract

Shape memory materials are a class of smart materials able to convert heat into mechanical strain (or strain into heat), by virtue of a martensitic phase transformation. Some brittle materials such as intermetallics and ceramics exhibit a martensitic transformation, but fail by cracking at low strains and after only several applied strain cycles. Here we show that such failure can be suppressed in normally brittle martensitic ceramics by providing a fine-scale structure with few crystal grains. Such oligocrystalline structures reduce internal mismatch stresses during the martensitic transformation, and lead to robust shape memory ceramics capable of many superelastic cycles to large strains; here we describe samples cycled up to 50 times, and samples which can show strains over 7%. Shape memory ceramics with these properties represent a new class of actuators or smart materials with a unique set of properties that include high energy output, high energy damping, and high temperature usage.One Sentence Summary: Fine-scale shape memory ceramics capable of many actuation cycles to strains up to 7%.Main Text: Shape memory materials are solid-state transducers, able to convert heat to strain and vice versa. They exhibit two unique properties: 1) the shape memory effect, which is the ability to transform to a "remembered" pre-defined shape upon the application of heat and 2) superelasticity, which is the ability to deform to large strains recoverably, while dissipating energy as heat. The underlying mechanism in crystalline shape memory materials is a thermoelastic martensitic transformation between two crystallographic phases that can be induced thermally (shape memory effect) or with the application of stress (superelasticity) (1, 2).The ability to transduce heat and strain renders shape memory materials useful in a wide variety of actuation, energy damping, and energy harvesting applications (3-7). To be of practical use, the material must be able to accommodate the extreme deviatoric strains associated with the

Nanoarchitectured materials composed of fullerene-like spheroids and disordered graphene layers with tunable mechanical properties

“…3b). The large linear elastic strain rivals the maximum strain (8%) of shape-memory materials such as NiTi alloys and zirconia ceramics 26,27 . Furthermore, although the material is elastically 'soft' (highly compressible), it has an ultrahigh strength over 1 GPa because of the remarkably wide elastic regime.…”

“…Under triaxial deformation, type-II GC exhibits both ultrahigh compressive strength (above 1 GPa) and superelasticity (recoverable linear strain up to 6%, comparable to, or even beyond, common shape-memory alloys and ceramics 26,27 ). To understand the remarkably high compressibility and superelastic behaviour of type-II GC, we need to examine the interaction of the structural components in GC.…”

Microstructure, crystallization and shape memory behavior of titania and yttria co-doped zirconia

“…13 Since small grains tend to F o r P e e r R e v i e w 3 suppress the martensitic transformation, when such polycrystals are subjected to applied stress, cracks are generally initiated before achieving observable shape deformation via the transformation. On the other hand, in our recent work 14 , we showed that for small-volume samples with a length scale reduced to a few micrometers, zirconia ceramics will have an oligocrystalline structure and can exhibit significantly enhanced shape memory properties without cracking. In that work, we used Ce-containing zirconia with a grain size of about 2 µm.…”

In-situ studies on martensitic transformation and high-temperature shape memory in small volume zirconia

“…The ability to experience significant shape deformation and recovery has made shape memory materials highly attractive for military, medical, aerospace and robotics applications [6e8]. Recently we reported a new class of shape memory ceramics: micron-sized single-crystal zirconia [9]. These ceramics exhibit strains comparable to metals but have higher strength and potentially higher operating temperatures than metals [10,11] for applications like solid state actuators [12].…”