An official website of the United States government
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.
more »
« less
This content will become publicly available on August 5, 2026
CuZnAl Shape Memory Alloys for High Heat Flux Thermal Energy Storage
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.
more »
« less
- Award ID(s):
- 2119103
- PAR ID:
- 10630037
- Publisher / Repository:
- Springer
- Date Published:
- Journal Name:
- Shape Memory and Superelasticity
- ISSN:
- 2199-384X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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
-
This study explores the latent thermal energy storage potential of an organic phase change material with porous copper foam and its applicability in electronic cooling under varying heat load conditions. The organic phase change material, n-eicosane, is known for its inherently low thermal conductivity of 0.15 W/mK, rendering it vulnerable during power spikes despite its abundant latent heat energy for phase transition from solid to liquid. Porous copper foams are often integrated into n-eicosane to enhance the composite’s thermal conductivity. However, the volume fraction of the phase change material in the porous foam that optimally improves the thermal performance can be dependent on the boundary condition, the cut-off temperature, and the thickness. A finite difference numerical model was developed and utilized to ascertain the energy consumption for the composite of n-eicosane with two kinds of porous copper foam with varying porosity under different heat rates, cut-off temperatures, and thickness. In addition, the results are compared with a metallic phase change material (gallium), a material chosen with a similar melting point but significantly high thermal conductivity and volumetric latent heat. For validation of the numerical model and to experimentally verify the effect of boundary condition (heat rate), experimental investigation was performed for n-eicosane and high porosity copper foam composite at varying heat rates to observe its melting and solidification behaviors during continuous operation until a cut-off temperature of 70 ◦C is reached. Experiments reveal that heat rate influences the amount of latent energy storage capability until a cutoff temperature is reached. For broad comparison, the numerical model was used to obtain the accessed energy and power density and generate thermal Ragone plots to compare and characterize pure gallium and n-eicosane - porous foam composite with varying volume fractions, cutoff temperature, and thickness under volumetric and gravimetric constraints. Overall, the proposed framework in the form of thermal Ragone plots effectively delineates the optimal points for various combinations of heat rate, cutoff point, and aspect ratio, affirming its utility for comprehensive design guidelines for PCM-based composites for electronic cooling applicationsmore » « less
-
In the world of soft-robotic medical devices, there is a growing need for low profile, non-rigid, and lower power actuators for soft exoskeletons and dynamic compression garments. Advanced compression garments with integrated shape memory materials have been developed recently to alleviate the functional and usability limitations associated with traditional compression garments. These advanced garments use contractile shape memory alloy (SMA) coil actuators to produce dynamic compression on the body through selective heating of the SMA material. While these garments can create spatially- and temporally-controllable compression, typical SMA materials (e.g., 70°C Flexinol) consume considerable power and require considerable thermal insulation to protect the wearer during the heating phase of the SMA actuation. Alternative SMA materials (e.g., NiTi #8 by Fort Wayne Metals, Inc.) transform below room temperature and do so using no applied electrical power and generate no waste heat. However, these materials are challenging to dynamically control and require active refrigeration to reset to material. In theory, low-temperature SMA actuators made from materials like NiTi #8 may maintain additional dynamic actuation capacity once equilibrated to room temperature (i.e., the material may not fully transform), as the SMA phase transformation temperature window expands when the material experiences applied stress. This paper investigates this possibility: we manufactured and tested low-temperature NiTi coil actuators to determine the magnitude of the additional force that can be generated via Joule heating once the material has equilibrated to room temperature. SMA spring actuators made from NiTi #8 consumed 84% less power and stabilized at significantly lower temperatures (26.0°C vs. 41.2°C) than SMA springs made from 70°C Flexinol, when actuated at identically fixed displacements (100% nominal strain) and when driven to produce equal forces (∼3.35N). This demonstration of low-power, minimal-heat exposure SMA actuation holds promise for many future wearable actuation applications, including dynamic compression garments.more » « less
-
Structurally stabilized composites are promising for using phase change materials in high‐temperature thermal energy storage (TES). However, conventional skeleton materials, which typically comprise 30–50 wt% of the composite, mainly provide sensible heat storage and contribute minimally to overall energy density. This study introduces a new class of redox‐active oxide‐molten salt (ROMS) composites that overcome this limitation by combining sensible, latent, and thermochemical heat storage in a single particle. Specifically, porous, redox‐active Ca2AlMnO5+δ(CAM) complex oxide particles were demonstrated as a suitable support matrix, with the pores filled by eutectic NaCl/CaCl2salt. X‐ray diffraction confirms excellent phase compatibility between CAM and the salt. Scanning electron microscopy/energy dispersive X‐ray spectroscopy and nano X‐ray tomography show good salt infiltration and wettability within the CAM pores. Thermogravimetric analysis reveals that a 60 wt% CAM/40 wt% salt composite achieves an energy density of 267 kJ kg−1over a narrow 150 °C window, with ≈50 kJ kg−1from thermochemical storage. Additionally, the composite shows higher thermal conductivity than salt alone, enabling faster energy storage and release. ROMS composites thus represent a novel and efficient solution for high‐performance TES.more » « less
