Enhancing elastocaloric regenerative performance using functionally graded structures: a model-based efficiency evaluation

Abstract Elastocaloric (eC) cooling technology, leveraging the latent heat associated with phase transformations in shape memory alloys, presents a promising alternative to traditional refrigeration systems. In this study, a three-dimensional phenomenological constitutive model based on Gibbs free energy is employed and coupled with an energy balance equation in cylindrical coordinates as a necessary step toward system-level simulation of eC tubes. After calibration against experimental data from the literature, the model is used to investigate an innovative functionally graded (FG) superelastic tube in which the transformation temperature (TT) is intentionally varied along the length. The primary objective is to determine whether a spatially tailored transformation-temperature profile can counteract the strong temperature gradients established during regenerative cycling. The simulations demonstrate that such grading promotes a more uniform phase transformation under constant loading and directly improves the material-level coefficient of performance. Notably, the graded profile reduces hysteresis losses by 7.85% and 10.88% at 650 MPa and 750 MPa, respectively through a gradual, distributed transformation that lowers the local critical stresses for forward and reverse transformation. The improvement in hysteresis in the FG design arises from the spatial variation in TTs across the tube, which leads to a gradual and distributed phase transformation rather than a sharp transition. In the model, this grading reduces the critical stresses required for both forward and reverse transformations in different tube segments. While the model does not directly alter the hysteresis loop size, the lowered transformation stresses effectively narrow the hysteresis loop by reducing the stress gap between the forward and reverse paths. This mechanism is embedded in the model influencing martensitic volume fraction, and subsequently through the stress–strain response. The FG design, therefore, enables a smoother transformation response with reduced internal energy dissipation, which manifests as improved (reduced) hysteresis. This work demonstrates the transformative potential of FG structures in creating practical eC systems.