Objective 1: Processing and fabrication of elastocaloric materials based on Cu–Al–Mn

Within Objective 1, carried out in Work Package 1 (WP1), we investigated the fabrication and elastocaloric performance of Cu–Al–Mn alloys, which are among the most promising elastocaloric materials (eCM). The work focused on targeted processing, fabrication, and evaluation of alloys to achieve desired properties, namely an austenite finish temperature in the range −10 °C ≤ Af ≤ 0 °C, sufficient material stability, and a pronounced elastocaloric effect.

A series of Cu–Al–Mn alloys with varying Al and Mn ratios were prepared by arc melting and casting into a copper mould. The abnormal grain growth (AGG) technique was applied to approach a single-crystal-like microstructure. The results showed that alloys without additional elements do not allow precise tuning of Af, as it lies either above or below the target range. Although AGG enabled the development of coarse-grained or quasi-single-crystalline microstructures, a true single crystal could not be achieved.

To improve control over transformation temperatures, Sn was introduced as an alloying element. It proved to be a key parameter, enabling systematic lowering of As and Af, with Af shifting into the sub-zero range already at low Sn contents. Sn also refines the grain structure, providing a favorable starting point for further processing. During cyclic heat treatment, AGG was effectively triggered in early cycles and then saturated; the microstructure became heterogeneous with very large grains (up to mm scale), indicating a quasi-single-crystalline state, although full single crystallinity was not achieved.

The functional performance was evaluated using DSC, electrical resistance measurements, and mechanical testing. An adiabatic temperature change above 7 K was achieved (e.g., CuAl8.5Mn9.5Sn1), with good agreement between DSC and R(T) and stable transformation behavior within a limited temperature range. However, key limitations were identified due to the polycrystalline microstructure, including high transformation stresses, retained martensite, and functional fatigue. A scientific paper on Cu–Al–Mn–Sn alloys is currently in preparation.

Overall, Objective 1 has been largely achieved, as we developed Cu–Al–Mn(–Sn) alloys with tunable transformation temperatures and measurable elastocaloric response, and established key processing routes. Due to limitations of AGG in achieving single crystals, future work will focus on the Bridgman method.

Objective 2: Evaluation and identification of the most promising elastocaloric materials

Within Objective 2, we obtained several shape memory materials from international partners: polycrystalline Ni–Ti–Cu–V (Ingpuls), single-crystal Cu–Al–Mn (Furukawa), and Cu–Al–Ni and Cu–Al–Be (Nimesis). These materials exhibit good superelasticity in the literature but had not been systematically evaluated for elastocaloric performance and functional fatigue.

The tested Ni–Ti–Cu–V alloy did not prove suitable for elastocalorics. Although DSC showed appropriate transformation temperatures (Af ≈ −7 °C) and latent heat (~15 J/g), the transformation was weak and rapidly degraded. The maximum elastocaloric effect was ~10 K (COP ≈ 7), but at very high stresses (~1000 MPa). This is likely due to non-optimized microstructure, high defect density, and compressive loading conditions.

Similarly, the tested single-crystal Cu–Al–Mn showed rapid functional degradation. Despite initial superelasticity at low stress (~100 MPa) and low hysteresis (~20 MPa), the elastocaloric effect dropped to ~3 K after ~10 cycles. Microstructural analysis revealed sub-grain boundaries where retained martensite formed.

In contrast, single-crystal Cu–Al–Ni and Cu–Al–Be showed excellent performance and were shown to be one of the most efficient elastocaloric materials. Both exhibited stable superelasticity at 100–200 MPa, low hysteresis (≤5 MPa for Cu–Al–Ni, ~20 MPa for Cu–Al–Be), and no degradation. Cu–Al–Be reached ΔT ≈ 10 K (COP ≈ 11), while Cu–Al–Ni achieved up to 16 K (COP ≈ 18), among the best reported results. True single crystallinity was confirmed. A scientific paper on Cu–Al–Ni is in preparation, while a phenomenological modeling and fatigue testing of these alloys are ongoing.

Additionally, a comprehensive review of elastocaloric materials has been conducted, and a review paper is in preparation. The results clearly show that single-crystal microstructure is key for achieving high and stable performance of Cu-bases alloys.

Objective 3: Development of advanced elastocaloric geometries

Objective 3 is addressed in Work Package 3 (WP3), focusing on the development of advanced elastocaloric geometries enabling compression without buckling, high specific surface area (>10 cm²/g), and small hydraulic diameter (<0.3 mm).

Currently, numerical modeling and parametric optimization of spiral regenerator geometries are being performed using shell-based finite elements with nonlinear (geometric and material) analysis capabilities [1]. The model is implemented in AceFEM (Wolfram Mathematica).

Parametric studies and experimental validation of spiral geometries reinforced with corrugated structures are ongoing. This approach increases geometrical stiffness and enables stable compressive loading while maintaining high surface area. Validation is currently performed on Ni–Ti, with future extension to Cu-based alloys from WP2. Results confirm the suitability of the approach and the potential of such geometries.

[1] PORENTA, Luka, PLANTARIČ, Adam, TUŠEK, Jaka, BROJAN, Miha. Shell-based finite element modelling for predicting buckling stability of superelastic SMA tubes. V: International Conference on Martensitic Transformations : ICOMAT 2025 : Prague, 7-12 September 2025 : program and abstracts. Prague: FZU, 2025. Str. 79. https://www.icomat2025.org/book-of-abstracts/. [COBISS.SI-ID 263971331]

Objective 4: Evaluation of cooling and heating performance of elastocaloric regenerators

Objective 4 starts in M18 (April 1, 2026) and focuses on the evaluation of the most promising elastocaloric regenerators developed in previous work packages. These combine optimized materials (WP1–WP2) and advanced geometries (WP3).

In the first phase, their performance will be analyzed numerically to simulate operation and optimize parameters. In the second phase, experimental validation will be performed using an existing test system, evaluating temperature span, cooling power, and efficiency. These results will provide a direct link between simulations and real performance and form the basis for further development and scaling of elastocaloric cooling devices.