Authors: John T. Sypek
Source: OpenCommons - UConn (University of Connecticut) (article, 2019-01-01)
Field: Materials Science — Electronic, Optical and Magnetic Materials

Mechanical Properties of the Intermetallic Compound CaFe2As2 at Small Length Scales Nanotechnology has paved the way for the research and development of new classes of materials and how we use them. With the desire to decrease the size of devices all while having advanced properties, research in this field and the development of small-scale experimental techniques has become significantly important. Materials can deform in elastically or plastically when an external load is applied. If elastically deforming a material, the material can then fully recover when unloading and there is no permanent damage to the shape or the structure of the material. When plastically deforming, this means that the materials shape cannot be recovered and that there is permanent damage done to the material. However, in some unique cases, even after a large amount of plastic deformation, the shape of a material can be recovered through a reversible phase transformation. Due to this reversible phase transformation, it is possible to have a high recoverable strain. This phenomenon is called superelasticity (or pseudo-elasticity).\nShape memory effect is closely related to superelasticity because shape memory materials have the capability to recover their original shape after a significant amount of deformation when they are subjected to certain stimuli, for instance, heat or magnetic fields, which induces the reversible phase transformation. Superelastic (shape memory) performance is usually measured by the maximum elastic strain or the maximum absorption of deformation energy prior to yielding (the modulus of resilience). When these values are larger, superelastic materials can exhibit a better actuation power, i.e., return the higher work to an external environment, which is usable to generate the mechanical motion for device switching, precise robotic motion, telescope lens control, etc.\nHowever, the performance of conventional shape memory materials, such as Ni-Ti alloys, is often limited by the energetics and geometry of the martensitic-austenitic phase transformation. In order to achieve the better superelastic performance, it is necessary to identify a class of materials with a different structure and different superelasticity mechanism. Here, we report the discovery of a unique shape memory behavior in CaFe2As2, which exhibits unprecedented superelasticity with over 13% recoverable strain, over 3 GPa yield strength, repeatable stress-strain response even at the micrometer scale, and cryogenic linear shape memory effects near 50 K. These properties are achieved through a reversible uni-axial phase transformation mechanism, the tetragonal/orthorhombic-to-collapsed tetragonal phase transformation by making and breaking As- As bonds. This uniaxial process is entirely distinct from the conventional shear-based superelastic mechanism, martensite-austenite phase transformation of conventional shape memory alloys and ceramics. Notably, this large elastic strain could make strain-engineering possible and would lead to the development of mechanically switchable functional materials for cryogenic shape memory devices and cryogenic actuators operating under uniaxial mechanical loading, which is a favorable switching loading mode in engineering devices. Note that the ThCr2Si2-structure, and its hybrid structures, are considered to be one of the most populous of all crystal structure types. Thus, our observation can be extended to search for a large group of superelastic and strain-engineer-able functional materials, and, more broadly, will lead to various research opportunities in materials science, solid-state physics, device engineering, and machine-learning-based materials research. Intermetallic ; Materials science ; Length scale ; Metallurgy ; Physics ; Mechanics ; Alloy Iron-based superconductors research ; Rare-earth and actinide compounds ; Inorganic Chemistry and Materials