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Abstract Shape memory alloys (SMAs) absorb and release large amounts of latent heat during martensitic transformation, making them ideal candidates for applications involving thermal energy storage and management. In this study, Cu–Zn–Al SMAs were investigated as lower-cost alternatives to NiTi-based SMAs for solid–solid phase change materials. The alloys were fabricated using an unconventional method of melting and solidification of the constituent elements sealed in quartz tubes under a pressurized Ar atmosphere. The alloys synthesized were found to exhibit superior figure of merit values for thermal energy storage, as compared to conventional solid–liquid phase change materials and NiTi-based SMAs, with thermal conductivity between 59 and 75 W/mK and latent heat values ranging from 3 to 6.5 J/g. Transformation temperature ranges (Af–Mf) less than 20 °C were achieved within a wide operating temperature between − 145 °C and 100 °C. In addition, select CuZnAl compositions yielded excellent cyclic stability with only ± 2 °C shifts in transformation temperatures after 20 thermal cycles. The present study demonstrates the feasibility of CuZnAl SMAs for use in high heat flux thermal energy storage and management applications at a wider range of temperatures.
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Data-driven study of composition-dependent phase compatibility in NiTi shape memory alloys
The martensitic transformation in NiTi-based Shape Memory Alloys (SMAs) provides a basis for shape memory effect and superelasticity, thereby enabling applications requiring solid-state actuation and large recoverable shape changes upon mechanical load cycling. In order to tailor the transformation to a particular application, the compositional dependence of properties in NiTi-based SMAs, such as martensitic transformation temperatures and hysteresis, has been exploited. However, the compositional design space is large and complex, and experimental studies are expensive. In this work, we develop an interpretable piecewise linear regression model that predicts the parameter, a measure of compatibility between austenite and martensite phases, and an (indirect) factor that is well-correlated with martensitic transformation hysteresis, based on the chemical features derived from the alloy composition. The model is capable of predicting, for the first time, the type of martensitic transformation for a given alloy chemistry. The proposed model is validated by experimental data from the literature as well as in-house measurements. The results show that the model can effectively distinguish between B19 and regions for any given composition in NiTi-based SMAs and accurately estimate the lambda_2 parameter.
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- Award ID(s):
- 2119103
- PAR ID:
- 10542580
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Materials & Design
- Volume:
- 244
- Issue:
- C
- ISSN:
- 0264-1275
- Page Range / eLocation ID:
- 113096
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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NiTiHf is a class of promising high-temperature shape memory alloys (SMAs) that find many applications. However, their complex martensitic microstructure and attendant thermomechanical properties are not well understood. In this work, we used solution-treated (precipitate-free) and aged (precipitate-bearing) Ni50.3Ti29.7Hf20 (at.%) SMAs as a model system. We observed that the presence of precipitates refines the martensite plates, reduces the number of martensite variants, and changes the orientation relationship between the martensite plates compared with the solution-treated counterpart. Furthermore, the aged samples exhibited higher transformation temperatures, narrower phase transformation temperature windows, improved thermal stability, and retained or even improved actuation strain. The improved thermomechanical properties observed in the aged samples are attributed in part to the reduction of the number of martensite variants and the change in martensite and twin interface characteristics, both of which are induced by the presence of precipitates. The findings of this study offer new information on the processing-property-microstructure relationship in NiTiHf-based SMAs. These insights can guide future materials design efforts, facilitating the development of advanced SMAs tailored for specific high-temperature applications.more » « less
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A phase-field model for thermomechanically-induced fracture in NiTi at the single crystal level, i.e., fracture under loading paths that may take advantage of either of the functional properties of NiTi–superelasticity or shape memory effect–, is presented, formulated within the kinematically linear regime. The model accounts for reversible phase transformation from austenite to martensite habit plane variants and plastic deformation in the austenite phase. Transformation-induced plastic deformation is viewed as a mechanism for accommodation of the local deformation incompatibility at the austenite–martensite interfaces and is accounted for by introducing an interaction term in the free energy derived based on the Mori–Tanaka and Kröner micromechanical assumptions and the hypothesis of martensite instantaneous growth within austenite. Based on experimental observations suggesting that NiTi fractures in a stress-controlled manner, damage is assumed to be driven by the elastic energy, i.e., phase transformation and plastic deformation are assumed to contribute in crack formation and growth indirectly through stress redistribution. The model is restricted to quasistatic mechanical loading (no latent heat effects), thermal loading sufficiently slow with respect to the time rate of heat transfer by conduction (no thermal gradients), and a temperature range below 𝑀𝑑, which is the temperature above which the austenite phase is stable, i.e., stress-induced martensitic transformation is suppressed. The numerical implementation of the model is based on an efficient scheme of viscous regularization in both phase transformation and plastic deformation, an explicit numerical integration via a tangent modulus method, and a staggered scheme for the coupling of the unknown fields. The model is shown able to capture transformation-induced toughening, i.e., stable crack advance attributed to the shielding effect of inelastic deformation left in the wake of the growing crack under nominal isothermal loading, actuation-induced fracture under a constant bias load, and crystallographic dependence on crack pattern.more » « less
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Abstract BackgroundSuperelastic shape memory alloys (SMAs) such as nickel-titanium, also known as Nitinol, recover large deformations via a reversible, stress-induced martensitic transformation. ObjectivePartitioning the deformation into the contributions from superelasticity and plasticity and quantifying the interaction between these mechanisms is key to modeling their fatigue behavior. MethodsWe capture these microscopic interactions across many grains using a combination of scanning electron microscopy digital image correlation (SEM-DIC) and electron backscatter diffraction (EBSD). Modeling our data as a statistical distribution, we employ a Gaussian Mixture Model (GMM) soft clustering framework to understand how these mechanisms interact and evolve as a function of global strain. ResultsOur findings show that, under globally-applied uniaxial tensile loading, plasticity bridges deformation in regions where competing positive and negative martensitic shear bands intersect. Early stage transformation-induced plasticity is concentrated at these intersections and forms concurrently with the Lüders-like martensitic transformation front, often appearing with a zig-zag pattern that is linked to compound twinning at the martensite-martensite interface. At higher strains, austenite slip is activated as a second mechanism of plastic deformation. ConclusionsWe propose that this plastic bridging mechanism underpins the prestrain effects previously reported in the literature, where higher prestrains can enhance the fatigue strength of superelastic materials within a given loading mode.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.matdes.2024.113096
