Combinatorial search of thermoelastic shape-memory alloys with extremely small hysteresis width

Abstract

Reversibility of structural phase transformations has profound technological implications in a wide range of applications from fatigue life in shape-memory alloys (SMAs) to magnetism in multiferroic oxides. The geometric nonlinear theory of martensite universally applicable to all structural transitions has been developed. It predicts the reversibility of the transitions as manifested in the hysteresis behaviour based solely on crystal symmetry and geometric compatibilities between phases. In this article, we report on the verification of the theory using the high-throughput approach. The thin-film composition-spread technique was devised to rapidly map the lattice parameters and the thermal hysteresis of ternary alloy systems. A clear relationship between the hysteresis and the middle eigenvalue of the transformation stretch tensor as predicted by the theory was observed for the first time. We have also identified a new composition region of titanium-rich SMAs with potential for improved control of SMA properties.

Transmission electron microscopy study of low-hysteresis shape memory alloys

“…Hysteresis is the macroscopic manifestation of the energy dissipated during a phase transformation and which can result in the creation of defects that accumulate over cycling to eventually become the sites of crack initiation [1]. In this way, it is closely related to the fatigue life, an important parameter for applications, especially for systems subjected to cyclic loading.…”

“…This result is supported by the latest developments in the geometric nonlinear theory of martensite (GLNTM) [3,4] which explains the formation of martensite microstructures, the shape memory effect and the role of crystalline symmetry and phase compatibilities. This theory gives several conditions in order for SMAs to show extremely low hysteresis [1,3,5]. The first condition is no volume change during phase transformation which in mathematical terms translates to 1 2 3 = 1, where U is the transformation matrix that maps the martensite lattice to the austenite 1 2 3 are the ordered eigenvalues of U.…”

Growth, microstructure and thermal transformation behaviour of epitaxial Ni-Ti films

“…One open research question is if epitaxial NiTi films can be used to contribute to a more fundamental understanding of the basic behaviour of NiTi SMA, as sketched above for magnetic SMA. Moreover, epitaxial NiTi films might even improve the functionality compared to polycrystalline NiTi films, which have already been developed towards microactuators [12], stents for brain surgery [13], elastocaloric refrigeration [14], or used for the combinatorial search for low hysteresis materials [15,16].…”

A low volume‐fraction limit for martensic microstructures in shape‐memory alloys

“…We focus our analysis on an interface at which two different variants of martensite form microstructures with volume fraction 1−θ and θ. It has been conjectured that a low volume fraction in one of two martensic variants corresponds to a small width of the thermal hysteresis of the phase transition [4], [7]. We consider a scalar-valued, two dimensional, geometrically linearized simplification of the energy with a 'hard' austenite-martensite interface, namely…”