Enhanced Martensitic Transformation Enthalpy and Modified Magnetic Properties in a NiTi/NiFeGa Bilayer Composite Processed by High-Speed High-Pressure Torsion

Abstract

Hybrid shape-memory materials combining structural and magnetic functionalities offer new opportunities for tailoring functional responses through mechanical coupling between dissimilar phases. In this work, a NiTi/NiFeGa bilayer composite was fabricated by high-speed high-pressure torsion (HSHPT), a severe plastic deformation technique enabling the consolidation of dissimilar materials. Although the constituent alloys exhibit martensitic transformations separated by nearly 135 K in their initial states, the HSHPT-processed hybrid displays a single dominant calorimetric transformation despite the absence of measurable chemical interdiffusion, consistent with a cooperative transformation response promoted by strong interfacial mechanical coupling. The microstructure was investigated by scanning electron microscopy, while the martensitic transformation and magnetic properties were characterized by differential scanning calorimetry and magnetometry. SEM observations revealed a well-defined bilayer architecture with a sharp chemically distinct interface. Despite the absence of measurable interdiffusion, the hybrid exhibited a transformation enthalpy approximately twice that of the NiTi constituent (≈20 J g⁻¹ vs. ≈10.35 J g⁻¹), accompanied by a broader thermal hysteresis. The magnetic response was also modified, exhibiting reduced saturation magnetization and increased coercivity. These findings demonstrate that HSHPT provides an effective route for designing hybrid functional materials with tunable thermomechanical and magnetic responses through controlled interfacial interactions between chemically distinct phases.

1. Introduction

Designing hybrid materials with enhanced transformation enthalpy and tunable magnetic properties represents a key objective in the development of advanced multifunctional systems for thermomechanical applications. Severe plastic deformation (SPD) techniques provide an effective route for tailoring the microstructure of metallic materials by producing ultrafine-grained or even nanocrystalline structures through intense plastic deformation under high hydrostatic pressure [1,2]. The resulting high density of lattice defects—including dislocations, vacancies, stacking faults, twins, and grain boundaries—plays a central role in controlling microstructure–property relationships. SPD has become a powerful strategy for developing advanced functional materials with tailored mechanical and physical properties [3,4,5]. Beyond grain refinement and the associated strength enhancement through the Hall–Petch mechanism [6], SPD can significantly influence phase transformations and modify a wide range of functional properties, including electrical [7], thermoelectric [8], magnetic [9], and corrosion behavior [10].

Among SPD techniques, high-speed high-pressure torsion (HSHPT) is particularly attractive because it enables both the consolidation of dissimilar materials and the refinement of coarse-grained microstructures into ultrafine-grained or nanocrystalline states [11,12,13]. The process combines high pressure with intense plastic deformation at high strain rates, leading to extensive grain refinement and the accumulation of crystalline defects that strongly influence microstructure–property relationships. Besides improving mechanical properties, HSHPT can also modify functional behavior; for example, changes in magnetic response, including shifts in Curie temperature and reduced magnetization in the austenitic state, have been reported [13]. These effects highlight the strong coupling between deformation-induced microstructural evolution and the physical properties of functional alloys.

Shape memory alloys (SMAs) can recover a predefined shape through a reversible martensitic transformation (MT) between austenite and martensite [14]. This diffusionless transformation underlies the shape memory effect and enables applications in actuators, sensors, and damping systems [15,16]. Among SMAs, NiTi remains the most widely used system due to its excellent functional stability and mechanical performance, which are preserved across different length scales and structural configurations [17,18,19,20,21,22]. Deformation-induced microstructural changes have been shown to influence both transformation thermodynamics and functional responses, including magnetic behavior where applicable [23,24,25]. Heusler-type magnetic shape memory alloys, which are usually described by the general formula X2YZ [26], may in some cases exhibit a martensitic transformation coupled with magnetic order. The prototypical system Ni₂MnGa displays magnetic-field-induced strains of up to 10%, as reported by Kari Ullakko et al. [27]. However, their intrinsic brittleness remains a major limitation for practical applications [28]. For this reason, the search for new Heusler alloys exhibiting martensitic transformation has intensified in recent years, encompassing systems such as Co–Ni–Ga [29,30], Ni–Mn–(In, Sn, Sb) [31,32,33,34,35,36], Ni–Fe–Ga-based [37,38,39] and all-d-metal alloys [40,41,42,43]. In particular these alloys have attracted increasing attention due to their favorable combination of structural and magnetic properties, making them promising candidates for multifunctional applications. It is generally accepted that martensitic transformation temperatures in these systems are highly sensitive to a wide range of parameters, including processing route, chemical composition and atomic ordering, internal stresses, and thermal treatments. As a result, various synthesis and processing strategies have been explored to optimize both the magnetic and mechanical performance of Heusler-type alloys [44,45].

To improve the performance of these materials’ specific properties, increasing attention has been directed toward SMA-based composites that combine complementary functionalities, leading to improved mechanical strength, fracture toughness, damping capacity, and superelastic performance [46,47,48]. In particular, composites integrating conventional SMAs with Heusler-type alloys offer promising routes for coupling thermoelastic and magnetic responses within a single material system. Nevertheless, the effect of severe plastic deformation on the interplay between martensitic transformation, magnetic behavior, and interfacial structure in such hybrid systems remains insufficiently understood, especially regarding its impact on transformation enthalpy.

NiTi/NiFeGa bilayer composites provide a suitable platform to address these aspects. By combining the shape memory behavior of NiTi with the magneto-structural characteristics of NiFeGa, these systems enable multifunctional responses relevant for applications in sensors, actuators, and MEMS devices [48]. Processing by HSHPT is expected to induce ultrafine-grained structures and strong interfacial coupling, which may significantly influence both transformation energetics and magnetic properties. In this context, the present study investigates a NiTi/NiFeGa bilayer hybrid composite processed by HSHPT, with particular emphasis on the origin of the enhanced transformation enthalpy and the modification of magnetic properties. The observed functional response is attributed to the microstructural characteristics of the composite, particularly the refined structure and interface characteristics.

2. Materials and Methods

The ingot with nominal composition Ni₅₇Fe₁₈Ga₂₅ (at.%.) shape memory alloy was prepared by arc melting under an argon protective atmosphere using high-purity elements (99.99%, purchased from Alfa Aesar, Karlsruhe, Germany). The melting procedure was repeated five times to ensure chemical homogeneity. Subsequently, the ingot was subjected to a thermal treatment in high vacuum at 1223 K for 25 h, followed by quenching in iced water. The alloy was cut into specimens of appropriate dimensions and subjected to severe plastic deformation (SPD) using the high-speed high-pressure torsion (HSHPT) technique. A detailed study of the alloy’s behavior under SPD conditions was previously reported in [49]. The HSHPT method represents a variation in the classical high-pressure torsion (HPT) technique, in which the material is simultaneously subjected to compressive and torsional strains. The HSHPT process and equipment have been described in detail in earlier works [11,12], and it is recognized as an advanced SPD technique derived from conventional HPT [3].

Thin Ni₅₇Fe₁₈Ga₂₅ (at.%) and Ni_50.3Ti_49.7 (at.%) discs, with dimensions of ϕ13.05 × 1.37 mm and ϕ22.30 × 1.28 mm, respectively, were first obtained by HSHPT from the initial bulk samples. The two discs were subsequently stacked in a sandwich configuration and processed by HSHPT to produce the hybrid composite. To achieve simultaneous bonding and structural refinement, the SPD parameters were carefully selected. The rotational speed of the upper punch was set to approximately 900 rpm, while an initial pressure of 20 bar was applied by the lower punch. The compressive force was monitored using the Hottinger Spider 8 equipment(Hottinger Baldwin Messtechnik, Darmstadt, Germany), with the hydrostatic pressure reaching values of ~1 GPa. The deformation parameters were controlled via a PLC XC 200 EATON system (Syracuse, NY 13208, USA). In this process, friction between the anvils and the sample is generated by the high-speed rotation of the upper anvil, contributing to the imposed shear strain. To obtain ultrafine-grained structures, three main deformation parameters were controlled: (i) the rotational speed of the upper punch (~900 rpm), (ii) the hydrostatic pressure (~1 GPa), and (iii) the friction conditions arising from the high pressure and relative displacement between the sample and anvils. The specimens were initially pressed at 5 MPa at room temperature (RT), followed by the application of a pressure of ~1 GPa, resulting in the formation of the hybrid material. The entire HSHPT process is very short, with a duration ranging from about 7 s for the first cycle to approximately 2 s for the third cycle. The resulting hybrid materials have the form of truncated conical specimens with a diameter of approximately 30 mm and a thickness of about 0.26 mm.

The effective strain of the hybrid composite was calculated using the following relation:

$${\epsilon }_{NiTi/NiFeGa}=ln\left({\displaystyle \frac{h_i}{h_f}}\right)=ln\left({\displaystyle \frac{2.65}{0.26}}\right)\cong 2.3$$

where h_i and h_f denote the initial and final thicknesses of the hybrid composite, respectively. The cumulative strains experienced by the individual layers were: ε_cumNiTi = 3.74 and ε_cumNiFeGa = 3.2, respectively.

Figure 1 schematically illustrates the fabrication route, including the initial dimensions of the NiTi and NiFeGa specimens and the successive HSHPT processing steps. The hybrid composite was produced through three consecutive HSHPT operations.

The strain relationships commonly employed for conventional High-Pressure Torsion (HPT) require the specimen kinematics to be fully defined throughout the deformation process. In conventional HPT, this condition is satisfied because the angular displacement of the specimen is prescribed by the rotation of the upper anvil, allowing the imposed strain to be determined from the known number of rotations. Consequently, the torsional shear strain and the corresponding equivalent strain can be calculated using the well-established relationships reported in the HPT literature, as comprehensively reviewed by Zhilyaev and Langdon [1].

The torsional shear strain is expressed as

$$\gamma ={\displaystyle \frac{2\pi Nr}{h}},$$

where (N) is the number of rotations, (r) is the radial position and (h) is the final specimen thickness. The corresponding equivalent von Mises strain is given by

$${\epsilon }_{eq}={\displaystyle \frac{\gamma }{\sqrt{3}}}={\displaystyle \frac{2\pi Nr}{\sqrt{3}h}},$$

whereas, for large plastic deformations, the logarithmic (true) strain accounting for the reduction in specimen thickness may be written as

$$\epsilon =ln\left[{\left(1+\left({{\displaystyle \frac{\phi r}{h}}}^2\right)\right)}^{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}\right]+ln\left({\displaystyle \frac{h_i}{h_f}}\right),$$

which, for

$${\displaystyle \frac{\phi r}{h}}\gg 1,$$

reduces to

$$\epsilon \cong ln\left({\displaystyle \frac{2\pi Nr{h}_i}{h_f^2}}\right),$$

However, the direct application of these relationships to the HSHPT process is not rigorous. Unlike conventional HPT, HSHPT is characterized by extremely high rotational speeds, severe plastic deformation, frictional heating, and partial slippage at the specimen–anvil interface. Under these conditions, the effective angular displacement experienced by the specimen cannot be determined experimentally and does not necessarily coincide with the angular displacement prescribed by the rotation of the upper anvil. Consequently, the effective number of rotations experienced by the material remains unknown. Since the effective angular displacement cannot be determined experimentally, neither the equivalent von Mises strain nor the logarithmic strain expressions that include the torsional contribution can be rigorously evaluated without introducing additional assumptions regarding the actual deformation kinematics of the specimen. In the present work, no such assumptions were introduced because they cannot be validated experimentally. Therefore, the imposed deformation was quantified exclusively by the logarithmic true strain associated with the experimentally measured thickness reduction,

$$\epsilon =ln\left({\displaystyle \frac{h_i}{h_f}}\right),$$

where h_i and h_f denote the initial and final specimen thicknesses, respectively.

It should be emphasized that this expression is not an empirical relationship. Rather, it represents the exact definition of the Hencky (true) strain for uniaxial compression and depends exclusively on experimentally measured quantities. Therefore, the calculated value is rigorously defined within the framework of continuum mechanics. Nevertheless, because the torsional contribution cannot be quantified without knowledge of the effective angular displacement of the specimen, the calculated value should be regarded as an experimentally verifiable lower bound for the total strain imposed during the HSHPT process rather than as the total equivalent strain developed within the material. Accordingly, the adopted methodology avoids introducing experimentally unverifiable assumptions while providing a rigorous, reproducible and experimentally verifiable lower-bound estimate of the imposed deformation.

Due to the specific kinematics of the HSHPT process, the resulting specimens exhibit a truncated cone geometry, characterized by a gradual variation in thickness along the generator. The thickness decreases from the central region towards the periphery as a direct consequence of the non-uniform strain distribution generated by the combined action of compressive loading and torsional deformation. Accordingly, the imposed strain increases radially, reaching its maximum at the specimen periphery. This strain gradient gives rise to three distinct regions, each exhibiting characteristic microstructural features associated with the local deformation level. The central region experiences relatively low deformation, whereas the intermediate and, particularly, the peripheral regions are subjected to substantially higher strain, resulting in more pronounced grain refinement and microstructural evolution. All subsequent investigations presented in this work were performed on specimens extracted from the peripheral region of the truncated-cone samples, where the imposed deformation is greatest. This approach ensures that the analyses were carried out on the most refined and representative microstructure produced under HSHPT conditions, consistent with previous studies on similar systems [50].

Scanning electron microscopy (SEM) as employedusing a ZEISS Gemini 500 microscope (Carl Zeiss, Oberkochen, Germany) to investigate the grain structure and the integrity of the interfacial bonding in the cross-section of the Ni_50.3Ti_49.7/Ni₅₇Fe₁₈Ga₂₅ bi-layer composite. Energy-dispersive X-ray spectroscopy (EDX) was used to evaluate the chemical composition across the composite thickness, with particular emphasis on the interface between the two constituent alloys.

Two independently fabricated hybrid specimens, designated Hybrid_1 and Hybrid_2, were processed under identical HSHPT conditions. Hybrid_1 was used as the reference specimen for comprehensive microstructural, calorimetric, and magnetic characterization, whereas Hybrid_2 was investigated by differential scanning calorimetry (DSC) to assess the reproducibility of the transformation behavior.

Differential scanning calorimetry (DSC) measurements were performed using a Netzsch DSC 204 F1 Phoenix calorimeter (NETZSCH, Selb, Germany) equipped with Proteus software. 2007 Although the DSC measurements were performed over the temperature range of 193–673 K, the martensitic transformation occurred between approximately 300 and 420 K. Therefore, only the temperature interval of 273–473 K, encompassing the complete transformation, is presented and discussed in this work. After completing the transformation cycles, the samples were heated to 673 K and held for 20 min inside the DSC chamber to perform an in situ post-deformation heat treatment. All DSC experiments were conducted at a constant heating/cooling rate of 20 K/min under a protective helium atmosphere. The DSC measurements enabled the determination of the characteristic transformation temperatures and the corresponding transformation enthalpies.

Magnetic measurements were performed using a Quantum Design SQUID magnetometer (San Diego, CA, USA), operating in the Reciprocal Space Option (RSO) mode. The magnetic field was applied along the longitudinal direction of the samples in order to minimize demagnetization effects.

To facilitate the interpretation of the calorimetric and magnetic responses of the hybrid composite, the volume fractions of the two constituent layers were estimated from the specimen geometry based on the well-defined interface separating them. The corresponding mass fractions were then calculated from the measured specimen masses and the experimentally determined densities of the two alloys. These values provide a quantitative basis for interpreting the measured transformation enthalpy and magnetic response of the hybrid composite in terms of the relative contributions of the NiTi and NiFeGa constituents.

The calculated volume and mass fractions for the investigated hybrid composite are summarized in

Table 1. Geometrical, physical, and calculated parameters of the NiTi/NiFeGa hybrid composite used for estimating the relative contributions of the constituent layers to the calorimetric and magnetic responses.

Constituent	Mass (g)	Volume (mm3)	Density (g/cm3)	Volume Fraction (%)	Mass Fraction (%)
Ni50.3Ti49.7	2.17	349.91	6.20	77.06	73.06
Ni57Fe18Ga25	0.80	104.18	7.68	22.94	26.94
Total	2.97	454.09	-	100	100

. These values provide a quantitative framework for interpreting the calorimetric and magnetic responses of the hybrid composite in terms of the relative contributions of the NiTi and NiFeGa constituents.

The calculated densities are in good agreement with literature values reported for NiTi (≈6.45 g/cm³) [51] and NiFeGa Heusler alloys (≈7.68 g g/cm³), depending on composition) [52]. This agreement supports the reliability of the estimated volume and mass fractions.

The authors used an AI-based language model (ChatGPT, version 5.5, OpenAI) to assist in improving the clarity, structure, and linguistic expression of the manuscript. All scientific content, results, and interpretations were developed, verified, and approved by the authors. The authors have reviewed and edited the output and take full responsibility for the content of this publication.

3. Results

Unless otherwise specified, all results presented in the following sections were obtained from specimens extracted from the peripheral region of the HSHPT disc, where the imposed deformation is greatest. This region was selected because it exhibits the most refined microstructure produced during HSHPT processing.

3.1. Scanning Electron Microscopy (SEM)

For clarity and to enable a direct correlation between the interfacial microstructure and the thermal and magnetic properties discussed in this work, selected SEM and EDX results from our previous study [53] are reproduced here under the appropriate license. The microstructural characterization of the NiTi/NiFeGa bilayer composite was performed by SEM coupled with EDX analysis, with the aim of evaluating the interface quality and identifying possible interdiffusion between the constituent layers. The cross-sectional SEM image (Figure 2) reveals a continuous and well-defined interface between the NiTi and NiFeGa layers, without evidence of cracks, delamination, or the formation of an intermediate reaction layer. The interface remains sharp and uniform along the bonding direction, indicating effective metallurgical joining achieved through the HSHPT process. Further insight into the elemental distribution is provided by the EDX maps (Figure 2). The results show a clear spatial separation of the constituent elements: Ti is confined to the NiTi layer, while Fe and Ga are localized within the NiFeGa layer. As expected, Ni is present in both regions due to its presence in both alloys. The absence of significant overlap between Ti, Fe, and Ga distributions across the interface indicates a well-preserved chemical separation between the two materials.

The compositional variation across the bonding interface is further illustrated by the EDX line scan shown in Figure 3, acquired perpendicular to the interface. The elemental profiles reveal an abrupt compositional transition: the Ti signal decreases sharply when crossing from the NiTi layer into the NiFeGa region, whereas the Fe and Ga signals increase correspondingly. No extended mixed-composition region is observed, indicating that chemical interdiffusion is negligible within the spatial resolution of the SEM/EDX technique. Although slight compositional gradients extending over a few micrometres can be observed for some elements, these are most likely attributable to limited atomic mobility during processing and/or the intrinsic spatial resolution of the EDX method, rather than to the formation of a distinct diffusion layer.

The EDX line scan is presented as a qualitative illustration of the elemental partitioning across the interface. During specimen preparation, the samples were ground using SiC abrasive papers and mirror-polished with an alumina suspension. Consequently, weak Si and Al signals may arise from polishing residues or other superficial contamination commonly encountered in SEM/EDX analysis. Their intensities remain close to the background level and are negligible compared with the characteristic signals of Ni, Ti, Fe, and Ga. Furthermore, point EDX analyses performed on both the NiTi and NiFeGa layers did not detect Al or Si as constituent elements of either alloy. These weak signals are therefore attributed to surface contamination and do not affect the interpretation of the elemental distribution across the interface.

The absence of detectable chemical interdiffusion indicates that the HSHPT processing conditions did not promote significant long-range atomic diffusion, despite the severe plastic deformation. Overall, the combined SEM and EDX analyses demonstrate that HSHPT produces a well-bonded NiTi/NiFeGa interface while preserving the chemical integrity of the constituent layers, with no evidence of intermetallic phase formation or significant chemical interdiffusion within the spatial resolution of the SEM/EDX technique.

3.2. Differential Scanning Calorimetry (DSC)

Figure 4 presents the DSC curves of the reference NiTi and NiFeGa alloys, recorded under a protective helium atmosphere at a heating/cooling rate of 20 K/min. The heating and cooling branches are indicated by arrows, while the exothermic heat flow is plotted upward.

Figure 5 presents the DSC response of two independently fabricated hybrid specimens (Hybrid_1 and Hybrid_2), both processed under identical HSHPT conditions. Two consecutive thermal (heating–cooling) cycles were recorded for each specimen to assess both the stability of the calorimetric response upon repeated thermal cycling and its reproducibility between independently prepared specimens. The first heating–cooling scan (cycle 1) corresponds to the strongly non-equilibrium state of the as-processed material, whereas the second scan (cycle 2) reflects the stabilized calorimetric response established after the initial thermal cycle. A pronounced upward shift of the first reverse transformation peak is observed in the first DSC cycle, whereas the subsequent cycles recover the normal transformation temperatures. This behavior is characteristic of the martensite stabilization effect [54,55,56,57,58,59,60] and is discussed in detail in the following section.

As shown in Figure 5, the hybrid composites exhibit a remarkable increase in transformation enthalpy compared with the reference materials, reaching values approximately twice those of the NiTi alloy. A substantial increase in the thermal hysteresis (Af − Ms) is also observed, suggesting that interfacial constraints and deformation-induced defects introduced during HSHPT processing contribute to the transformation behavior.

This state undergoes partial relaxation during the first thermal cycle, leading to the significant asymmetry observed between Q_A–M and Q_M–A. The broader and less uniform endothermic peaks observed during the first heating suggest a distribution of transformation barriers arising from residual stresses, defects, and interfacial constraints. Following the initial cycle, this relaxation results in modifications not only of the characteristic transformation temperatures, but also of the thermal hysteresis and transformation enthalpy.

Table 2. Ms, Mf, Mp denote martensite start, finish and peak temperatures; As, Af, Ap denote austenite start, finish and peak temperatures; (Af − Ms) defines the thermal hysterezis; Q_A–M corresponds to the enthalpy of the forward (austenite → martensite) transformation, while Q_M–A corresponds to the reverse transformation, Qm denotes the average transformation enthalpy.

Sample	Ms//Mf//Mp (K)	As//Af//Ap (K)	Af-Ms (K)	QA–M (J/g)	QM–A (J/g)	Qm (J/g)	Hc (Oe)	Ms (Am2/Kg)
Ni50.3Ti49.7	308//215//244	251//325//282.6	17	14.32	10.41	12.365
Ni57Fe18Ga25	399//360//382	376//420//398	21	0.685	0.512	0.6	128	54
Hybrid_1 cycle 1	348//323//340	358//408//390	60	24.68	19.4	22.04
Hybrid_1 cycle 2	348//323//337	333//378//373	30	24.9	24.23	24.56	260	1.4
Hybrid_2 cycle 1	349//325//336	371//420//400	81	23.6	18.8	21.2
Hybrid_2 cycle 2	346//315//335	335//377//371	31	23.5	22.4	22.95
HybridT400 cycle 1	351//300//334	357//415//374	64	14.63	16.2	15.42
HybridT400 cycle 2	351//300//333	341//377//371	26	15.1	15.27	15.185

summarizes the martensitic and austenitic transformation temperatures, thermal hysteresis, and transformation enthalpies for the reference and hybrid samples.

The correlation between the enhanced thermal hysteresis and the difference between Q_A–M and Q_M–A highlights the strongly irreversible character of the martensitic transformation in the hybrid system, governed by interfacial constraints and defect-mediated energy dissipation. The increase in transformation enthalpy is likely associated with additional elastic and interfacial energy stored during HSHPT processing, which is subsequently released during the transformation. These effects are therefore attributed to microstructural and interfacial constraints rather than to chemical interdiffusion.

The increased peak intensity observed in the hybrid composites is consistent with the higher transformation enthalpy values summarized in Table 2, supporting the interpretation that additional elastic and interfacial energy contributions are involved in the transformation. Figure 5b presents the DSC response of a second independently fabricated hybrid specimen (Hybrid_2), processed under identical conditions as Hybrid_1. The close agreement between the two specimens confirms the reproducibility of the observed transformation behavior.

While slight variations in peak shape are observed—likely reflecting the non-equilibrium processes inherent to HSHPT—the martensitic transformation temperatures and transformation enthalpy (Q) remain consistent, confirming the robustness of the observed behavior.

Post-deformation heat treatment partially relaxes the non-equilibrium microstructure induced by HSHPT, leading to a reduction in both thermal hysteresis and transformation enthalpy (Figure 5c). Hereafter, Hybrid_T400 denotes the hybrid specimen subjected to a post-deformation heat treatment at 673 K for 20 min. The asymmetry between the austenite-to-martensite Q_A–M and martensite-to-austenite Q_M–A transformation enthalpies is also reduced, providing further evidence of microstructural relaxation following the thermal treatment.

Although the transformation enthalpy decreases after the post-deformation heat treatment, it remains higher than that of the reference NiTi alloy. The broader and less uniform endothermic response observed during the first heating suggests that annealing at 673 K induces only partial relaxation of the HSHPT-generated microstructure. The origin of the remaining irreversible changes is discussed in the following section.

To further investigate the microstructural changes associated with the DSC response after post-deformation annealing, additional SEM and EDX analyses were performed on the hybrid specimen heat-treated at 673 K for 20 min.

3.3. Microstructural Characterization After Annealing

Representative cross-sectional SEM images together with the corresponding EDS elemental maps of the hybrid specimen after the DSC thermal treatment at 673 K for 20 min (Hybrid_T400) are presented in Figure 6. The bilayer architecture is retained, with a sharp and well-defined interface between the NiTi and NiFeGa layers. No continuous reaction layer or extended diffusion zone is observed, indicating that the thermal exposure during the DSC experiment did not promote significant interfacial chemical homogenization. The elemental maps further confirm that the elemental partitioning remains largely unchanged, with Ti confined to the NiTi layer, whereas Fe and Ga remain predominantly localized within the NiFeGa region. A weak oxygen signal indicates only limited surface oxidation following the thermal treatment.

Several localized contrast features are visible within the NiTi layer, particularly in the vicinity of the interface. Although these heterogeneities may be consistent with the formation or redistribution of fine precipitates during processing and subsequent thermal exposure, their crystallographic nature cannot be established from the present SEM/EDS observations. Accordingly, no specific precipitate phase is assigned. Despite slight local variations in elemental intensity, the overall elemental partitioning between the two constituent layers remains unchanged. Overall, these observations indicate that annealing at 673 K for 20 min primarily promotes partial microstructural relaxation while preserving the chemical integrity of the NiTi/NiFeGa hybrid architecture.

Higher-magnification SEM observations (Figure 7) reveal several discrete contrast features within the NiTi layer, particularly in the vicinity of the interface. Rather than forming a continuous interfacial reaction zone, these features remain localized, suggesting localized microstructural evolution during the DSC thermal treatment. Although they may be consistent with precipitation-related phenomena, their crystallographic nature cannot be established from the present SEM/EDS observations, and therefore no specific precipitate phase is assigned. Their localized distribution further indicates that the thermal treatment promoted local structural evolution without inducing extensive interfacial reactions or significant chemical homogenization across the NiTi/NiFeGa interface.

Figure 8 presents cross-sectional SEM micrographs together with the corresponding EDS elemental maps of the hybrid specimen after the DSC thermal treatment at 673 K for 20 min (Hybrid_T400). The bilayer architecture is retained, with a sharp and well-defined interface between the NiTi and NiFeGa layers. The elemental maps confirm that the chemical partitioning of the constituent elements remains largely unchanged, with Ti predominantly confined to the NiTi layer, whereas Fe and Ga remain localized within the NiFeGa layer. No continuous interfacial reaction layer or extended diffusion zone is observed, indicating that the thermal exposure during the DSC experiment did not promote significant chemical homogenization across the interface. A weak oxygen signal indicates only limited oxidation following the thermal treatment.

Overall, the microstructural observations are consistent with the DSC response, suggesting that the post-deformation heat treatment at 673 K promotes partial relaxation of the HSHPT-induced non-equilibrium microstructure, accompanied by localized microstructural evolution rather than extensive chemical homogenization across the interface.

3.4. Magnetometry

3.4.1. Thermo-Magnetic Characterization over the Temperature Range 5–300 K

Temperature dependence of magnetization recorded in 200 Oe applied magnetic field (Figure 9a) reveals that the initial Ni₅₇Fe₁₈Ga₂₅ alloy exhibits a classical ferromagnetic M(T) dependence, with a clear decrease toward Tc (~250–270 K). The martensitic transformation occurs at temperatures above Tc and therefore takes place outside the ferromagnetic regime. The behavior of the hybrid is different, as shown in Figure 9b. It exhibits a much slower decrease in magnetization with temperature, while the magnetization values are significantly lower (~0.8 A·m²/kg vs. ~8 A·m²/kg at 200 K). Despite this magnetic dilution, the hybrid material also shows a magnetic response associated with the martensitic transformation. Therefore, the hybrid composite can be described as weakly ferromagnetic.

3.4.2. Magnetic Hysteresis Curves

The hysteresis curves recorded at 5 K for the NiFeGa alloy (Figure 10a) exhibit a low coercive field (Hc = 128 Oe), with magnetization saturation above 1 T, characteristic of a soft magnetic material. In contrast, the hybrid sample (Figure 10b) shows a markedly reduced magnetization and an approximately twofold increase in coercive field (Hc = 260 Oe), reflecting the modified magnetic response induced by severe plastic deformation. In addition, the more rounded shape of the hysteresis loop observed for the hybrid sample suggests a gradual approach to saturation, indicative of a broadened distribution of local magnetic environments. This behavior can be attributed to structural disorder, interfacial effects, and deformation-induced defects induced during HSHPT processing.

The magnetic response of the hybrid composite is significantly reduced compared to the parent NiFeGa alloy, due to the presence of the weakly magnetic NiTi phase. Nevertheless, the temperature-dependent magnetization retains a clear signature of the martensitic transformation, indicating that the magnetic response is coupled to the structural transition. The increased coercivity observed in the hybrid sample reflects enhanced pinning effects arising from the high defect density and interfacial structure induced via processing route.

4. Discussion

The microstructural analysis provides essential insight into the origin of the modified transformation and magnetic behavior observed in the NiTi/NiFeGa hybrid composite. SEM (Figure 1) reveals a continuous, well-defined interface between the two constituent layers, while EDX elemental mapping and line-scan analyses (Figure 2) confirm a sharp compositional transition across the interface. Titanium remains confined to the NiTi layer, whereas Fe and Ga are localized within the NiFeGa layer, with Ni present in both alloys, as expected. No continuous diffusion zone or intermetallic reaction layer is detected within the spatial resolution of the SEM/EDX technique, demonstrating that the HSHPT processing conditions preserve the chemical integrity of the constituent layers despite the severe plastic deformation. Consequently, bonding between the two dissimilar alloys is achieved primarily through intimate interfacial contact under high pressure rather than through extensive thermally activated interdiffusion.

Particularly noteworthy is that two chemically distinct alloys, whose martensitic transformations are separated by approximately 135 K in the initial state, exhibit a single dominant calorimetric transformation after HSHPT processing while retaining a chemically sharp interface. This observation strongly suggests that the transformation behavior is governed predominantly by mechanical coupling between the constituent layers rather than by chemical homogenization.

The upward shift of the first reverse transformation peak observed in the first DSC cycle (Figure 5) is characteristic of the martensite stabilization effect (the phenomenon in which previously deformed martensite transforms back to austenite at higher temperatures than undeformed martensite). Consistent with the classical martensite stabilization effect, the upward shift is observed only during the first reverse transformation, whereas subsequent DSC cycles recover the normal transformation temperatures. Initially, this effect was interpreted in terms of the additional elastic and irreversible energies stored during deformation, which increase the thermodynamic driving force required for reverse transformation [54]. More recent studies have shown that the effect cannot be fully explained solely by plastic deformation. Instead, martensite reorientation, detwinning, and the associated damage or loss of coherency of martensite interfaces have been proposed as the major contributing mechanisms responsible for reducing interface mobility and increasing the overheating required for the first reverse transformation. Furthermore, the magnitude of the stabilization effect has been shown to increase with the volume fraction of martensite, consistent with an increased probability of martensite–martensite interface interactions [55,56,57].

The present HSHPT-processed hybrid exhibits a similar upward shift of the first reverse transformation peak. However, unlike conventionally deformed monolithic NiTi alloys, the present material contains a highly defected ultrafine-grained microstructure together with mechanically constrained NiTi/NiFeGa interfaces. Therefore, the observed thermal response is likely governed by the combined effects of classical martensite stabilization and the additional interfacial constraints introduced by the HSHPT process. Although most mechanistic investigations have been performed on NiTi alloys, martensite stabilization has also been reported in several other shape memory alloy systems, including TiNiHf [58], CoNiAlFe [59], and CoNiGa [60], indicating that it represents a general feature of thermoelastic martensitic transformations rather than a phenomenon specific to NiTi.

The calorimetric results reveal a substantial increase in transformation enthalpy in the hybrid composite compared with the reference NiTi alloy, accompanied by broader transformation peaks and a significant increase in thermal hysteresis (Af − Ms). Together with the emergence of a single dominant transformation despite the initially well-separated transformation temperatures of the constituent alloys, these features indicate that the martensitic transformation is strongly influenced by the mechanical interaction between the two layers. The increased hysteresis is consistent with the presence of additional energy barriers arising from interfacial constraints and the high density of defects introduced during severe plastic deformation. Furthermore, the difference between the forward (Q_A–M) and reverse (Q_M–A) transformation enthalpies reflects the non-equilibrium and dissipative character of the transformation cycle. This behavior can be attributed to irreversible processes such as interface friction, dislocation motion, and defect rearrangement, which contribute to energy dissipation during the transformation.

Considering the mass fractions of the two constituent layers, namely 73.1 wt.% NiTi and 26.9 wt.% NiFeGa, the transformation enthalpy expected for a bilayer in which the two alloys transform independently can be estimated using a simple rule of mixtures. Based on the transformation enthalpies measured for the individual constituents (Table 1), the expected value is approximately 9 J/g. In contrast, the experimentally measured transformation enthalpy of the hybrid reaches approximately 20 J/g, more than twice the value predicted by the rule of mixtures. This pronounced enhancement demonstrates that the calorimetric response of the hybrid cannot be interpreted as a simple superposition of the independent transformations of the NiTi and NiFeGa layers. Instead, it provides strong evidence for a cooperative transformation process promoted by the mechanical coupling established at the interface together with the highly deformed microstructure generated during HSHPT processing.

The non-equilibrium microstructure generated by HSHPT gives rise to a distribution of local transformation barriers, which is reflected in the broad and non-uniform DSC peaks observed during the first heating cycle. Following the initial thermal cycle, partial structural relaxation occurs, leading to a reduction in the asymmetry between Q_A–M and Q_M–A together with modifications of the transformation temperatures and thermal hysteresis. The reduced difference between the forward and reverse transformation enthalpies further supports a decrease in the dissipative contribution associated with irreversible processes such as defect rearrangement and interface friction.

Although post-deformation heat treatment at 673 K reduces both the transformation enthalpy and the thermal hysteresis, the transformation enthalpy remains significantly higher than that of the reference NiTi alloy, indicating that the effects of interfacial mechanical coupling and the HSHPT-induced microstructure are only partially relaxed. SEM observations after annealing reveal localized microstructural heterogeneities within the NiTi layer that may be consistent with precipitation-related phenomena, while the EDS elemental maps indicate that the chemical partitioning between the constituent layers remains essentially unchanged and oxidation is limited. Taken together, these observations suggest that annealing at 673 K primarily promotes partial microstructural relaxation accompanied by localized structural evolution, rather than extensive interfacial reactions or chemical homogenization.

The heat treatment at 673 K promotes partial relaxation of the highly deformed HSHPT microstructure, as evidenced by the stabilization of the DSC response. However, the observed reduction in transformation enthalpy is unlikely to be governed solely by defect relaxation. Higher-magnification SEM observations reveal localized heterogeneities within the NiTi layer that may be consistent with precipitation-related phenomena commonly reported for Ni-rich NiTi alloys, while the oxygen maps indicate only limited oxidation following the DSC thermal treatment. Although the present SEM/EDS observations do not permit an unambiguous identification of the underlying phases, the absence of a continuous interfacial reaction layer and the preservation of the elemental partitioning indicate that the thermal exposure did not promote extensive chemical homogenization across the NiTi/NiFeGa interface. Instead, the combined effects of partial structural relaxation and localized microstructural evolution are likely to contribute to the irreversible reduction in the transformable austenite fraction and the corresponding decrease in transformation enthalpy after thermal exposure.

This behavior suggests that the microstructural features introduced during HSHPT, including interfacial mechanical coupling and deformation-induced defects, are only partially relaxed and continue to influence the transformation energetics. The broader and less uniform endothermic response observed during the first heating further supports partial relaxation of the HSHPT-generated non-equilibrium microstructure during annealing at 673 K. Combined with the SEM/EDS observations, these results indicate that the heat treatment promotes partial microstructural relaxation accompanied by localized microstructural evolution, while preserving the chemical partitioning between the NiTi and NiFeGa layers without extensive chemical homogenization.

The reproducibility of the calorimetric response is confirmed by the DSC measurements performed on a second independently fabricated hybrid specimen (Hybrid_2), which exhibits comparable transformation temperatures and transformation enthalpies to those of Hybrid_1. Minor differences in peak shape are observed, most likely reflecting the inherent variability associated with the non-equilibrium microstructure generated during the HSHPT process.

The magnetic measurements provide independent evidence that the functional response of the hybrid composite is strongly modified by HSHPT processing. Part of the reduction in the measured saturation magnetization is naturally associated with the dilution effect arising from the NiTi layer, which accounts for approximately 73 wt.% of the hybrid composite. However, this compositional effect alone cannot account for the nearly forty-fold decrease in saturation magnetization relative to the reference NiFeGa alloy. The results therefore indicate that HSHPT processing also substantially reduces the intrinsic specific magnetization of the NiFeGa constituent. This reduction is most likely associated with the high density of deformation-induced defects generated during HSHPT, which alter the local atomic environment of the magnetic atoms and consequently weaken the exchange interactions, leading to a lower net magnetic moment. Remarkably, the magnetic anomaly occurs within the same temperature interval as the cooperative martensitic transformation, despite the strong magnetic dilution and the fact that the NiFeGa constituent is not a ferromagnetic shape memory alloy.

Field-dependent magnetization measurements at low temperature reveal an increase in coercivity in the hybrid composite compared with the parent NiFeGa alloy. This behavior is consistent with enhanced domain-wall pinning arising from the high density of deformation-induced defects, the refined microstructure, and the mechanically constrained interface generated during HSHPT processing. Furthermore, the more rounded hysteresis loop observed for the hybrid composite indicates a more gradual approach to magnetic saturation, suggesting a broader distribution of local magnetic environments associated with structural disorder and interfacial constraints.

Taken together, these results demonstrate that the primary consequence of HSHPT processing in the present system is not chemical homogenization, but the formation of a defect-rich, mechanically coupled bilayer architecture. This microstructural state introduces additional elastic and interfacial energy contributions that enhance the transformation enthalpy and increase the thermal hysteresis while simultaneously modifying the magnetic response. The persistence of a well-defined magnetic anomaly within the temperature interval of the cooperative martensitic transformation further supports the close interplay between the structural and magnetic responses in the hybrid composite.

Although NiTi and NiFeGa undergo martensitic transformations in distinct temperature ranges, the HSHPT-processed bilayer behaves as a mechanically coupled system. Interfacial mechanical coupling, together with the defect-rich microstructure generated during HSHPT, promotes a cooperative transformation response in which each constituent influences the transformation behavior of the other, resulting in an integrated thermomagnetic response.

One of the most remarkable observations of the present study is that the hybrid composite exhibits a single dominant DSC transformation peak despite being composed of two chemically distinct alloys whose characteristic transformation temperatures differ by approximately 135 K. In the hybrid, this common transformation occurs approximately 95 K above the transformation of the reference NiTi alloy and approximately 40 K below that of the reference NiFeGa alloy.

Moreover, the thermomagnetic measurements indicate that the magnetic response associated with the NiFeGa constituent evolves within the same temperature interval as the cooperative calorimetric transformation. Together, these observations suggest that the two constituent alloys no longer transform independently after HSHPT processing. Instead, they exhibit a cooperative transformation response mediated by interfacial mechanical coupling and the defect-rich microstructure generated during severe plastic deformation.

Although the present results provide strong evidence for a cooperative transformation mediated by mechanical coupling following HSHPT processing, the underlying microscopic mechanisms remain to be fully elucidated. Previous TEM studies on HSHPT-processed NiTi/NiFeGa hybrids have demonstrated the formation of ultrafine-grained, defect-rich microstructures together with high-quality interfaces, without evidence of interfacial reaction layers [53]. Nevertheless, dedicated TEM investigations of the hybrid materials examined in the present work, combined with systematic studies on bilayers composed of alloys exhibiting different transformation temperatures, will be required to determine whether the observed synchronized transformation is a general consequence of HSHPT-induced mechanical coupling or a phenomenon specific to the present alloy combination. Such studies would provide valuable insight into the extent to which mechanically mediated cooperative transformations can be generalized to other hybrid shape-memory systems.

Taken together, the DSC and SEM/EDS results indicate that annealing at 673 K primarily promotes partial relaxation of the HSHPT-induced non-equilibrium microstructure, accompanied by localized microstructural evolution that may be consistent with precipitation-related phenomena, while preserving the chemical partitioning of the constituent layers without extensive interfacial chemical homogenization.

5. Conclusions

A NiTi/NiFeGa hybrid composite was successfully fabricated by high-speed high-pressure torsion (HSHPT), resulting in a well-defined bilayer structure with a sharp interface and no detectable interdiffusion, as confirmed by SEM and EDX analyses.

Despite the lack of chemical mixing, the hybrid composite exhibits a strongly modified transformation behavior compared to the reference alloys. The calorimetric results reveal an approximately twofold increase in transformation enthalpy compared to the NiTi reference alloy, accompanied by an increase in thermal hysteresis and a broadening of the transformation peaks, effects associated with the defect-rich microstructure and mechanically constrained interface generated by HSHPT.

Post-deformation heat treatment at 673 K promotes partial relaxation of the HSHPT-induced non-equilibrium microstructure, accompanied by localized microstructural evolution without extensive interfacial chemical homogenization. Although the transformation enthalpy decreases after annealing, it remains significantly higher than that of the reference NiTi alloy, indicating that the effects of interfacial mechanical coupling and the defect-rich microstructure are only partially relaxed.

Magnetic measurements provide complementary evidence that HSHPT profoundly modifies the functional response of the hybrid composite. Although the presence of the NiTi layer contributes to the reduction in the overall saturation magnetization, the observed decrease greatly exceeds that expected from magnetic dilution alone, indicating that severe plastic deformation also substantially alters the intrinsic magnetic response of the NiFeGa constituent. In addition, the increased coercivity is consistent with enhanced domain-wall pinning associated with the refined, defect-rich microstructure and interfacial mechanical constraints.

The most remarkable finding of the present work is that two chemically distinct alloys, whose martensitic transformations are initially separated by approximately 135 K, exhibit a single dominant calorimetric transformation after HSHPT processing while retaining a chemically sharp interface. This behavior strongly suggests that the hybrid transforms as a mechanically coupled system in which interfacial mechanical coupling and the defect-rich HSHPT microstructure promote a cooperative transformation response rather than independent transformations of the constituent alloys.

Future investigations combining advanced TEM characterization with hybrid systems composed of alloys exhibiting different transformation temperatures will be required to determine the generality of this cooperative transformation mechanism.

Overall, the results demonstrate that HSHPT processing provides an effective route for tailoring the thermo-magnetic behavior of hybrid functional materials through microstructural and interfacial engineering, without the need for chemical mixing or formation of new phases.