Fatigue resistant elastocaloric effect in TiNi via texture-precipitate synergy

Domain: neuroscience
Published: 2026-01-29
Editorial outcome: major_revision

Abstract
Shape memory alloys with both a large elastocaloric effect and exceptional fatigue resistance are key candidates for next-generation solid-state cooling technologies. Here, we report a textured TiNi alloy containing engineered Ti₄Ni₂O precipitates, fabricated via controlled directional solidification, that achieves a large adiabatic temperature change of −15.9 K after 10 million compressive cycles. Microstructural analyses reveal columnar B2 grains aligned with the solidification direction and a uniform dispersion of high-density Ti₄Ni₂O precipitates. A strong crystallographic texture is observed, enabling a large transformation strain of over 6% under compressive loading, as mapped by digital image correlation. In-situ loading X-ray diffraction confirms a continuous increase in intensity of B19′ martensite under stress, while in-situ cooling transmission electron microscopy captures numerous nucleation and progressive growth of the B19′ martensite from B2/Ti₄Ni₂O interfaces. High-resolution TEM further reveals localized lattice distortions at these interfaces, promoting directional and confined growth of B19′ martensite. This progressive transformation, which preserves functional reversibility, combined with the mutual texture–precipitate architecture that enhances mechanical stability, gives rise to ultra high fatigue life. These findings highlight the promise of textured TiNi with tailored precipitate architecture for long-term elastocaloric applications and provide a design strategy for developing fatigue-resistant SMAs through microstructural and textural engineering.

Introduction
Refrigeration is a cornerstone of modern infrastructure, enabling technologies from food preservation to electronics and medical systems1. However, conventional vapor compression systems are nearing their theoretical efficiency limits and rely on refrigerants with high global warming potential, raising serious environmental concerns2,3. These challenges have spurred the development of sustainable, high-efficiency solid-state cooling technologies. Among these, elastocaloric cooling, which leverages the reversible martensitic transformation in shape memory alloys (SMAs), has emerged as a particularly promising solution due to its high energy efficiency and environmentally benign nature4–8.

The elastocaloric effect is driven by the latent heat exchange associated with stress-induced martensitic transformation9, with performance typically quantified by the adiabatic temperature change ΔTad10,11. TiNi-based SMAs are among the most extensively studied due to their excellent mechanical and functional properties12,13. However, real-world elastocaloric applications require these materials to maintain functionality over millions of mechanical cycles14. TiNi alloys, in particular, are susceptible to both structural fatigue—leading to fracture—and functional fatigue, characterized by a decline in elastocaloric performance over time15,16. Thus, achieving both a large elastocaloric response and long-term cyclic durability remains a fundamental challenge.

A variety of strategies have been explored to address this issue, including compositional tuning, grain refinement, precipitation hardening, and composite architectures17–24. Among these, precipitation engineering has shown particular promise21,22,25. Strengthening phases such as TiCu17, Ti₂Ni26,27, Ti₃Ni₄ 22, and TiNi₃ 21 have been introduced to enhance fatigue life by promoting transformation reversibility at matrix–precipitate interfaces. However, excessive precipitation can hinder martensitic transformation, thereby diminishing the elastocaloric response21,22. Thus, optimizing the balance between mechanical strengthening and transformation compatibility is essential.

Crystallographic orientation also plays a critical role in dictating the functional response of TiNi alloys. Shape memory and superelastic behaviors are both strongly orientation-dependent28,29, and this extends to elastocaloric performance. Techniques such as cold rolling and directional solidification have been used to induce strong texture, with significant variation in elastocaloric response observed along different loading directions30–32. Recent reports of high elastocaloric performance in highly textured Heusler alloys and Ti-based SMAs—including NiMnTi and Ti-Nb-Zr-Ta—as well as enhanced stability in tweed-textured Fe-Pd single crystals, highlight the power of texture control33–40. Yet, limited attention has been paid to integrating crystallographic texture with microstructural engineering to improve fatigue resistance in elastocaloric TiNi alloys.

Notably, TiNi alloys exhibit strong mechanical asymmetry: while <001>B2-oriented grains show limited transformation under tension, they exhibit large recoverable strains under compression41,42. Moreover, introducing coherent precipitates with specific orientation relationships has been shown to promote reversible martensitic transformation and suppress fatigue damage by enabling martensite nucleation via localized lattice distortions17,21,22. These insights suggest that co-designing texture and precipitate architecture could be a powerful strategy to simultaneously achieve high elastocaloric performance and exceptional fatigue resistance.

Here, we report a textured Ti₄₉Ni₅₁ (at.%) alloy with high-density, epitaxially aligned Ti₄Ni₂O precipitates, synthesized via controlled directional solidification. The alloy exhibits a large ΔTad of −15.9 K, sustained over 10⁷ compressive loading-unloading cycles. This performance is proposed to derive from synergistic effects of strong crystallographic texture and a distributed martensitic transformation enabled by interfacial lattice distortions at B2/Ti₄Ni₂O boundaries. These findings underscore the promise of texture–precipitate co-design in TiNi-based SMAs and offer a viable pathway for developing durable, high-performance elastocaloric materials for solid-state cooling.

Results
Textured microstructure with aligned Ti₄Ni₂O precipitates enables large and stable transformation strain

The microstructure of the directionally solidified Ti₄₉Ni₅₁ alloy was first characterized using scanning electron microscopy (SEM). Backscattered electron (BSE) images captured along and perpendicular to the solidification direction (SD) reveal a highly textured structure comprising columnar grains oriented along the SD (Fig. 1a, b). The grain lengths exceed millimeter with widths ranging from tens to hundreds of micrometers. Strong contrast variation between grains in the cross-sectional view (perpendicular to SD) indicates substantial orientation differences, whereas the longitudinal section (parallel to SD) exhibits minimal contrast, suggesting a preferred grain orientation along the SD. Notably, fine precipitates are uniformly distributed throughout the columnar grains, aligned along the SD, and without significant segregation near grain boundaries.

X-ray diffraction (XRD) confirms the presence of the B2 austenite phase along with a secondary phase indexed as Ti₄Ni₂O (Fig. 1c). No martensitic phase is detected at room temperature. The Ti₄Ni₂O phase adopts a cubic structure (space group Fd3̅m), similar to the Ti₂Ni phase, with only slight differences in lattice parameters43,44. To further identify the chemical composition of the precipitates, scanning/transmission electron microscopy (S/TEM) coupled with energy-dispersive X-ray spectroscopy (EDS) was performed. A representative bright-field TEM image (Fig. 1d) shows a submicron precipitate embedded in the matrix. Elemental mapping and line profile analysis across the interface (Fig. 1e) reveal that the precipitate is enriched in Ti and O but depleted in Ni, with an approximate atomic ratio of Ti:Ni:O = 4:2:1. This confirms the formation of Ti₄Ni₂O precipitates coherently embedded within the B2 matrix.

Electron backscatter diffraction (EBSD) analysis performed on the transverse cross-section (perpendicular to SD) reveals a strong crystallographic texture in both the B2 matrix and the Ti₄Ni₂O precipitates. The inverse pole figure (IPF) map (Fig. 2a) indicates a predominant orientation close to <001>B2 aligned with the SD. Several discrete submicron regions are identified as Ti₄Ni₂O precipitates (Supplementary Fig. S1), which also share the <001> orientation. Pole figure (PF) analysis confirms an identical orientation relationship between the matrix and precipitates: {001}<110>B2 // {001}<110>Ti₄Ni₂O (Figure S2). This mutual texture supports the epitaxial growth of Ti₄Ni₂O precipitates within the preferentially oriented B2 grains during directional solidification.

Given the orientation-dependent transformation behavior in TiNi alloys, the local transformation strain was analyzed based on the measured orientation distribution and the known crystallography of the B2 to B19′ martensitic transformation12. A two-dimensional transformation strain map derived from the grain orientation data (Fig. 2b, Supplementary Fig. S3) shows an average theoretical transformation strain of ~6.1% under compression along the SD, with minor variations across different grains due to slight orientation deviations. Theoretical averaged transformation strains for arbitrary loading directions were computed from EBSD data and indicate a clear orientation preference (Figure S3).

To assess the mechanical response, superelastic cycling tests were conducted at room temperature in both parallel and perpendicular directions relative to the SD. Samples tested along the SD exhibit a large recoverable strain of up to 6.2%, maintaining stable superelasticity over repeated cycles (Fig. 2c). In contrast, samples tested perpendicular to the SD accumulate significant residual strain within 20 cycles (Fig. 2c), indicating poor functional stability. Comparing experimental results with these theoretical averages in Fig. S3, the measured compressive strain along the SD (6.2%) corresponds to ~102% of the theoretical average (6.1%), with the slight excess (~0.1%) attributable to elastic strain under high applied stress. In contrast, the strain perpendicular to the SD decreases from 4.5% initially to a stabilized value of 3.2% after 20 cycles, corresponding to ~80% and ~57% of the theoretical average (5.6%), respectively. The parallel direction also shows a nonlinear, narrow hysteresis superelastic response without a pronounced stress plateau, accompanied by a reduced elastic modulus (~38.3 GPa) compared to the perpendicular direction (~63.2 GPa). This reduced modulus and diminished hysteresis suggest a regulated and anisotropic martensitic transformation under the texture-precipitate synergy. These results highlight the superior cyclic stability and recoverability of the textured direction, which is critical to long-term elastocaloric performance.

Elastocaloric effect with high fatigue life

The elastocaloric performance and fatigue resistance of the textured Ti₄₉Ni₅₁ alloy were evaluated via cyclic compressive loading along the textured direction (parallel to the SD) at room temperature. As shown in Fig. 3a, b, the alloy exhibits exceptional durability, enduring up to 10⁷ compressive cycles without fracture. Despite minor attenuation of the superelastic response over cycling, the stress–strain curves (Fig. 3a) show a substantial recoverable strain of 5.1% after 10⁷ cycles, with total residual strain of ~0.75%. Both the hysteresis and elastic modulus decrease gradually during cycling, suggesting evolving but stable transformation behavior. A crossover emerges at ~4% strain in the stress–strain curves after ~10⁶ cycles, originating from the onset of a two-step transformation (Figure S11). The ΔTad, a direct measure of the elastocaloric effect, was also monitored throughout the fatigue life (Fig. 3b). Initially, the alloy delivers a large ΔTad of −21.8 K. After an initial drop of 5.7 K within the first 2 × 10⁶ cycles, ΔTad stabilizes, decreasing by only 0.2 K over the remaining 8 × 10⁶ cycles. This stabilization indicates that most performance degradation occurs early in cycling, after which the elastocaloric response remains largely constant, highlighting excellent long-term functional reliability.

A comparative analysis of bulk SMAs reported in literature19,21–23,45–48 (Fig. 3c) underscores the exceptional balance achieved in this textured Ti₄₉Ni₅₁ alloy. Unlike typical SMAs that display a trade-off between elastocaloric strength and fatigue life, the present material simultaneously exhibits an ultra high fatigue life (10⁷ cycles), a substantial elastocaloric effect (ΔTad = −15.9 K), and a large coefficient of material (COPmat = 23.7 after fatigue, Figure S5). Moreover, a relatively low compressive driving stress of ≈900 MPa is sufficient to induce complete transformation in our textured Ti₄₉Ni₅₁ alloy compared with several strengthened TiNi systems19,21,22, reducing mechanical energy input and enhancing potential for practical applications, although further optimization is required for commercialization. Importantly, the recoverable strain remains significant even after extended cycling (Fig. 3d), further supporting the material’s robust functional endurance. These combined attributes—large and stable ΔTad, long fatigue life, high COPmat—establish the textured Ti₄₉Ni₅₁ alloy as a highly promising candidate for durable elastocaloric cooling applications.

Uniform and progressive martensitic transformation upon loading

To elucidate the origin of the ultra high fatigue resistance, the phase transformation behavior during loading was characterized in detail. Digital image correlation (DIC) mapping of strain fields (Fig. 4a) reveals a transformation strain distribution of over 6% under compression. While local strain variations are present, the overall transformation field is more spatially uniform compared with the pronounced strain localization and Lüders-like banding during stress-induced martensitic transformation (SIMT) typically observed in conventional TiNi alloys49,50. This observation aligns with the previously reported nonlinear stress–strain response lacking a distinct plateau (Fig. 2), consistent with macroscopically uniform transformation phenomena observed in select TiNi alloys51,52.

To further probe the transformation pathway, in-situ XRD analysis was performed under incremental compressive stress up to 900 MPa (Fig. 4b). Initially, diffraction peaks correspond exclusively to the B2 parent phase and Ti₄Ni₂O precipitates. As stress increases, peaks associated with B19′ martensite gradually appear and intensify, while those of the B2 phase diminish. Upon unloading, the diffraction profile nearly returns to its original state, confirming a highly reversible martensitic transformation with minimal retained martensite. The evolution of the (100)B2 and (100)B19′ peak intensities is plotted in Fig. 4c. The relative intensity of (100)B19′ increases smoothly with stress beyond 500 MPa, reaching a maximum at peak load, and then gradually diminishes during unloading. This gradual and continuous evolution confirms the absence of abrupt martensite nucleation or burst-like propagation, distinguishing the transformation from classical SIMT and indicating a diffuse, progressive transformation mechanism. To avoid confusion with the strict thermodynamic concept of a second-order transition, we use ‘progressive’ or ‘distributed’ transformation to denote a macroscopically smooth, multi-site B2 → B19′ transformation that proceeds over a wide stress interval while retaining two-phase coexistence at microscopic length scales. Such a progressive phase transformation under wide stress range minimizes internal stress concentrations and reduces microstructural damage, thereby enhancing transformation reversibility and fatigue life. The progressive martensite formation and reversion also help suppress defect accumulation that typically degrades performance in conventional SMAs.

These insights underscore the pivotal role of microstructure-controlled transformation pathways in achieving superior fatigue resistance, warranting further investigation into the nanoscale characteristics governing this behavior.

Promoted nucleation and confined growth of martensite under lattice distortions near B2/Ti₄Ni₂O interface

To clarify the underlying microstructural mechanism responsible for the observed transformation behavior, in-situ TEM observations were performed during cooling from 315 K to 105 K. As shown in Fig. 5a, the microstructure at 315 K—well above the martensitic transformation temperature—consists of a B2 matrix with Ti₄Ni₂O precipitates. The selected-area electron diffraction (SAED) pattern (Fig. 5a′) from the red-circled region confirm the presence of a single-phase B2 matrix between precipitates at this temperature. Upon cooling to 215 K, just below the martensitic transformation start temperature, nucleation of B19′ martensite is observed in proximity to B2/Ti₄Ni₂O interfaces (Fig. 5b). The corresponding SAED pattern (Fig. 5b′) reveals the coexistence of both B2 and B19′ phases in this region, suggesting that the interface serves as a favorable nucleation site. At 105 K—below the martensitic finish temperature—the matrix between precipitates is nearly fully transformed into martensite (Fig. 5c). The SAED pattern (Fig. 5c′) confirms the presence of a single-variant B19′ martensitic structure, devoid of twinning. These observations suggest that the B2/Ti₄Ni₂O interfaces facilitate the directional nucleation and confined growth of martensite, enabling a smooth and spatially uniform transformation throughout the microstructure. The absence of twinned martensite further supports the hypothesis of interface-mediated variant selection, likely governed by strain fields at the phase boundaries.

To investigate the structural features governing this behavior, high-resolution transmission electron microscopy (HR-TEM) was employed to examine the B2/Ti₄Ni₂O interface (Fig. 6). The bright-field image (Fig. 6a) and corresponding SAED pattern (Fig. 6a′) confirm a well-defined orientation relationship of {100}<011>B2 // {400}<011>Ti₄Ni₂O between the matrix and precipitate. A closer examination of the B2/Ti₄Ni₂O interface region (red square in Fig. 6a) is shown in Fig. 6b, accompanied by its Fast Fourier Transform (FFT, Fig. 6c), where the presence of streaking indicates substantial lattice distortion near the interface. Further analysis via inverse FFT (IFFT), generated by selecting the overlapping (100)B2 and (400)Ti₄Ni₂O reflections (red circle in Fig. 6c) is shown in Fig. 6d, revealing a high density of dislocations localized at the interface. The lattice strain distribution near the B2/Ti₄Ni₂O boundary was quantitatively mapped using geometric phase analysis (GPA)53, shown in Fig. 6e–g. Strain mapping reveals localized lattice distortions exceeding ±10% adjacent to the interface, confirming the presence of intense strain fields. These distortions arise due to the lattice mismatch and dislocation accumulation at the interface, effectively lowering the energy barrier for martensite nucleation54. As such, these regions serve as ideal nucleation sites and provide a guiding framework for the formation and confined propagation of specific martensitic variants during cooling or mechanical loading. The combination of coherent crystallographic alignment and strain-concentrated interfacial zones enables a progressive, directional, and spatially uniform transformation process. This mechanism suppresses transformation-induced defects and enhances reversibility, ultimately contributing to the high fatigue resistance observed in the textured Ti₄₉Ni₅₁ alloy. The results highlight the critical role of microstructural design—particularly interface engineering—in achieving advanced functional properties in shape memory alloys.

Methods
Sample presentation

Ti₄₉Ni₅₁ (at.%) ingots were synthesized via vacuum arc melting of high-purity Ti and Ni, with trace oxygen intentionally introduced to promote the formation of Ti₄Ni₂O precipitates. The oxygen contents of the starting Ti and Ni materials were measured by inductively coupled plasma mass spectrometry (ICP-MS) to be 0.061 wt.% and 0.007 wt.%, respectively, resulting in a total oxygen concentration of 0.029 wt.% in the as-fabricated alloy. To ensure chemical homogeneity, each ingot was re-melted five times prior to casting into a water-cooled copper crucible. Directional solidification was achieved by installing a water-cooled system beneath the crucible base, establishing a vertical thermal gradient during solidification. For comparison, a commercial polycrystalline Ti₄₉Ni₅₁ (at.%) alloy was also prepared. Specimens for both microstructural characterization and property evaluation were sectioned from the ingots using spark cutting.

Mechanical and elastocaloric testing

Compression tests were performed on cylindrical specimens with dimensions of 4 mm in diameter and 8 mm in height using an Instron 5969 universal testing machine. Tests were conducted at room temperature under a quasi-static loading-unloading strain rate of 10⁻³ s⁻¹, with strain tracked via a video extensometer. Fatigue performance was evaluated on an Instron 8801 fatigue test system under sinusoidal loading (10 Hz) for up to 10⁷ cycles, with a maximum stress of 900 MPa, consistent with the superelastic measurements. For elastocaloric measurements, ΔTad during unloading was captured by a K-type thermocouple welded to the specimen surface. Samples were preloaded to 900 MPa at a strain rate of 10⁻³ s⁻¹, held briefly for thermal equilibrium, and then rapidly unloaded at a strain rate of 0.2 s⁻¹ to ensure near-adiabatic conditions.

Structural and compositional analysis

All the samples for structural and compositional analysis were mechanically polished. SEM samples were etched using a mixture of 10% HF, 45% HNO₃, and 45% H₂O (by volume). TEM foils were prepared via twin-jet electropolishing in a solution of 20% H₂SO₄ and 80% CH₃OH (by volume) at 253 K. Phase identification was conducted using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. Room-temperature microstructures were characterized using a FEI VERIOS 460 scanning electron microscope. Transmission electron microscopy investigations, including bright-field imaging, selected-area electron diffraction, energy-dispersive X-ray spectroscopy, and high-resolution TEM, were carried out on a JEOL JEM-F200 microscope. Crystallographic orientation and texture analyses were performed using a ZEISS Gemini 500 SEM equipped with an electron backscatter diffraction detector. EBSD data were processed using AztecCrystal software. Fast Fourier transformation, inverse fast Fourier transformation, and geometrical phase analysis were conducted using DigitalMicrograph software (version 3.43.3213.0).

In-situ transformation behavior analysis

Thermal transformation behavior was analyzed using a Netzsch 214 differential scanning calorimeter (DSC) across a temperature range of 153–263 K, at a constant heating and cooling rate of 10 K/min. To map strain evolution during compressive loading, digital image correlation was employed in conjunction with a high-speed camera. Tests were performed on rectangular specimens (3 × 3 × 8 mm³), loaded to 900 MPa at a strain rate of 10⁻³ s⁻¹ and unloaded to <10 MPa. In-situ loading XRD was conducted on a Rigaku MicroMax-007HF system equipped with a Mo rotating anode source (λ = 0.7093 Å). In-situ TEM cooling observations of martensitic transformation were carried out using a Thermo Fisher Talos-F20 microscope, equipped with a Gatan 636 liquid nitrogen cooling holder. The TEM foil was held for 10 mins at each observation temperature to maintain thermal equilibrium.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This outstanding manuscript represents a major advancement in the field of solid-state cooling materials. By skillfully combining controlled crystallographic texturing with epitaxial precipitate engineering, the authors achieve an exceptional balance between a large elastocaloric effect (∆Tₐd = –15.9 K) and unprecedented fatigue life (>10⁷ cycles) in a bulk TiNi₅₁ shape memory alloy. Through meticulous in-situ experimental investigations and comprehensive microstructural characterization, they reveal a novel continuous martensitic transformation mechanism, driven by interfacial strain fields at the B2/Ti₄Ni₂O interfaces.\nThis study not only addresses a critical and long-standing challenge in the elastocaloric community, namely, the intrinsic trade-off between transformation performance and fatigue durability, but also provides profound mechanistic insights that are likely to influence future SMA design strategies for functional and energy applications. The level of scientific quality, experimental rigor, and conceptual novelty demonstrated in this work fully justifies publication in Nature Communications without hesitation. I strongly recommend acceptance after minor revisions.\nTo further enhance the clarity and completeness of the manuscript, the following minor points should be considered:\n1. The authors mention a reduction in hysteresis with cycling, which plays a crucial role in maintaining elastocaloric stability. It would be informative to include a figure or numerical summary illustrating how hysteresis evolves over fatigue cycles, for example, a plot of hysteresis width versus cycle number to clearly visualize this beneficial trend.\n2. In some figures and text (e.g., Mechanical testing method and Fig. S4 caption), \"loading-unloading\" is sometimes written as \"loading–unloading\" (with dash vs. hyphen). Please standardize throughout.\n3. Similarly, crystallographic notation, particularly regarding directions and phases, occasionally varies, for instance, {001} <110>Ti₄Ni₂O (page 4 line 131) vs. <011>{400}Ti4Ni2O (page 6 line 242). A consistent subscripting and notation format across the manuscript would enhance the precision of presentation.\n4. Minor language polishing could further streamline the clarity of expression. For example, the phrase \"the functionality, including superelasticity and elastocaloric effect, is strongly dependent on crystal orientation\" (caption of Fig. 2) could be more succinctly rephrased as: \"The superelastic and elastocaloric responses are strongly dependent on the crystal orientation.\"\nOverall, this work constitutes a landmark contribution to the development of fatigue-resistant elastocaloric materials. The insights into texture–precipitate co-design, the clear demonstration of a continuous and highly reversible martensitic transformation mechanism, and the exceptional fatigue durability metrics all mark this study as highly impactful. The findings are poised to have significant influence not only within the elastocaloric and SMA communities but also across broader fields concerned with phase transformation materials and sustainable cooling technologies.\nI have no hesitation in recommending the acceptance of this manuscript after minor editorial polishing, if deemed necessary.", "reviewer_2": "The reviewed paper focus on the development and characterization of a TiNi-based shape-memory alloy that exhibits significant elastocaloric effects with quite good reproducibility during a very large number of cycles through its stress-induced martensitic transformation.\nTo design this material, the authors have synergically combined the two strategies recently proposed to improve elastocaloric properties of shape-memory materials. The former is based on the growth of precipitates with suitable morphology that permits smoothing out the elastic interaction between martensitic variants that gives rise to a significant decrease of the transition hysteresis. The second is based on tailoring an appropriate texture causing the transition to progress continuously as the applied stress increases. Overall, the paper is well written and well organized and the material developed has been very well characterized using numerous experimental techniques. These results provide a good understanding of the combined influence of both precipitates and texture on the transition properties.\nMy negative viewpoint about the paper refers to the fact that due to the designed microstructure, the transition takes place over a very wide range of stresses. The consequence of this fact is that a large stress must be applied to induce the entire transition and take advantage of the whole transformation latent heat, which is at the origin of the large elastocaloric response of the material. In the case of the present work a stress of 900 MPa is required. While this value is considered as a {\\it low driving stress} by the authors, a simple calculation indicates that for a sample of 12 mm^2 (as the one used in the experiments) a mechanical force larger than 10^4 N (more than one tonne!) is required to induce the whole transformation. In my opinion, this makes the material of little interest for practical applications of refrigeration. Therefore, although the material designed in this work is interesting and indeed represents a step forward in the development of new elastocaloric materials, it is still far from being suitable to be used efficiently in new refrigeration devices that can replace current ones based on vapor compression technology. The authors should clearly indicate that much more efficient materials will be needed before the elastocaloric effect can be used in commercial refrigeration applications.\nIn many places along the text the material is specified with the formula TiNi_{51}. This could be understood as a material with a composition such that for every 51 atoms of Ni there is 1 atom of Ti (which, as far as I know, is not a shape-memory material). In fact, I suppose that the composition should be 49 at. % Ti and 51 at % Ni. Therefore, I suggest that instead of TiNi_{51} the material be specified as Ti_{49}Ni_{51}.\nThe authors indicate that the measured temperature decrease induced by adiabatic release of stress, increases from -21.8 K to -15.1 K in the first 2 x10^6 cycles and only increases by 0.2 K in the next 8x10^6 cycles. While this is correct, a simple inspection of Figure 3b suggests a continuous evolution with cycling that is faster than the one described by a logarithmic law. Can this evolution be explained in terms of changes of the microstructure?\nIn conclusion, I think that after a revision that takes into account the points raised above, the paper could deserve publication in Nature Communications.", "reviewer_3": "Please see the attached pdf file \"comments to authors\".", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "We sincerely thank the reviewers for their thoughtful evaluation of our manuscript. We\ngreatly appreciate the recognition of the significance, novelty, and rigor of this work.\nThe constructive comments and suggestions have been extremely valuable in refining\nthe clarity, precision, and overall presentation of the manuscript. Below we provide\ndetailed responses to each point, together with the corresponding revisions\nimplemented in the manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "It has been a great pleasure to review the revised version of the manuscript (NCOMMS-25-29512A / Large Elastocaloric Effect with Ultrahigh Fatigue Resistance in TiNi via Texture-Precipitate Synergy]. I wish to commend the authors for their exceptional efforts in addressing the critiques raised during the initial review. The responses to my comments, as well as to those of the other reviewers, are nothing short of outstanding. Their responses are not merely defensive but are insightful, thoughtful, and demonstrate a profound understanding of the SMA field. The willingness to engage critically with the suggestions, and where necessary, to refine their interpretations, reflects a commendable scientific rigor and honesty. The manuscript has been elevated from an already strong submission to a truly excellent and robust piece of work. The revised version now presents a clear, convincing, and highly significant story that is well-supported by the data. I believe it meets the high standards of Nature Communications and will be of considerable interest to its readership. I have no further requests for revision and wholeheartedly recommend acceptance of this manuscript in its current form. Congratulations to the authors on a fine piece of research.", "reviewer_2": "The authors have responded satisfactorily to all the points listed in my previous report. Therefore, I recommend publication of the paper.", "reviewer_3": "The authors have corrected the terminology of continuous phase transformation. However, overall, the practical significance of this work is limited as the material requires quite high stress (900 MPa). The so-claimed new transformation behaviour and mechanism are in fact not distinct from traditional NiTi. The mechanistic interpretation on the cyclic stability remains rather superficial and is just based on speculative claims. I still do not believe this manuscript reach a novelty and academic depth for publication of nature communication. The key comments are as below:\n1. The authors still claim that the transformation mechanism in the textured Ti₄₉Ni₅₁ alloy differs in both macroscopic manifestation and microscopic origin from those reported in conventional TiNi alloys. This is not true at all. Macroscopically (in Fig. 2(c)), the textured Ti₄₉Ni₅₁ still exhibits a stress-strain curve typical of conventional NiTi under compression. The so-claimed progressive transformation that proceeds over a wide stress interval (600 – 900 MPa) is not true, since the transformation of conventional NiTi occurs in a similar stress interval under compression. The DIC images in Fig. 4 clearly show inhomogeneous strain fields, which means macroscopically inhomogeneous transformation. The hysteresis of the textured Ti₄₉Ni₅₁ is smaller, but this does not constitute a novelty. Microscopically, the heterogeneous martensite nucleation at precipitates is well established both in experiment and in theory. To me, the only novelty is that the authors used Ti₄Ni₂O precipitates that were rarely reported before. But this improvement is just incremental.\n2. The authors attributed the high cyclic stability to 4 points, which I believe is not convincing or new. I comment each point below:\n(i) Variant confinement reduces interface length and mobility per event.\nThe authors claimed that precipitate-regulated single-variant domains are shorter, more strongly pinned, and less mobile. Each interface contributes less to plastic slip, reducing the overall dislocation multiplication rate. In my opinion, however, although variant confinement reduces the length of each individual interface, it significant increases the total number of interfaces or the total volume fraction of the interface, which may increase the total amount of dislocation accumulated.\n(ii) Precipitates suppress dislocation motion near interfaces.\nThe precipitation strengthening is a well-documented mechanism.\n(iii) Distributed heterogeneous nucleation alleviates stress concentration.\nI highly doubt this interpretation. From microelasticitity theory of phase transformation (AG Khachaturyan, Theory of structural transformations in solids, 2013), the martensite should exhibit the highest degree of stress concentration at the nucleation stage (i.e. single variant stage). The stress concentration is minimized only when martensite develops a twinned structure at the later growth up stage. This is because at the early stage, the interface energy dominates so that the single-variant martensite adopts a sphere-like morphology to reduce the interface energy, at the cost of high elastic strain energy (thus high stress concentration). At the later stage, the interface energy becomes less important, and the martensite will be self-twinned to reduce the elastic strain energy (thus low stress concentration). The authors’ claim is contractionary to this classic theory and is speculative in nature (no experiment evidence supports this).\n(iv) Early-cycle defect accumulation saturates, then stabilizes transformation.\nMy comment: this defect saturation is a well-documented mechanism.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "We would like to express our deepest gratitude to the Reviewers for their careful evaluation of our revised manuscript entitled “Large Elastocaloric Effect with Ultrahigh Fatigue Resistance in TiNi via Texture–Precipitate Synergy”. We are sincerely thankful for the reviewers’ thoughtful comments and constructive suggestions, which have been invaluable in further improving the scientific depth, clarity, and rigor of our work.\nBelow, we provide detailed, point-by-point responses to each reviewer’s comment."}, {"round": 3, "reviewer_comments": {"reviewer_3": "I did not see any substantive revisions to the manuscript. The authors have not added sufficient experimental or computational evidence to clarify the roles of precipitates and texture in cyclic stability. In addition, the authors have no clear idea at all on what macro- and micro- scale deformations are. For example, the DIC strain map clearly shows macroscopic inhomogeneous phase transformation similar to conventional NiTi (see the scale bar in Fig. 4a is 2 mm), and thus the strain field cannot be described and claimed as “microscale”. In short, the key conclusions are not supported or adequately interpreted.\nI understand that elastocaloric cooling materials like shape memory alloys is a seasonal topic, but it is quite disappointing for readers to read a paper that lacks academic depth and rigor in such an esteemed journal.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "We sincerely thank the reviewer for the continued careful evaluation of our revised manuscript (Manuscript ID: NCOMMS-25-29512B). We regret that our previous revisions and responses did not fully address the concerns raised by Reviewer #3. We appreciate the opportunity to provide further clarification. Below, we provide a detailed clarification of the reviewer’s comments and summarize the corresponding revisions made to the manuscript to improve rigor, precision, and interpretative clarity."}]

Decision letter