A magnetically soft yet mechanically strong and ductile Ta free CoFeNi high entropy alloy with Al and Ti additions

Domain: neuroscience
Published: 2026-02-05
Editorial outcome: major_revision

Abstract
Next generation high speed, large torque, rotating electrical machines demand materials with an excellent balance of magnetic and mechanical properties. High entropy alloys (HEAs) can exhibit this unique combination of properties, e.g., CoFeNi based HEAs containing Ta. The high Ta content makes these alloys prohibitively expensive for industrial applications. Hence, we develop Ta free HEA using a thermodynamic approach. The addition of Al and Ti to the base CoFeNi alloy is studied. We investigate two compositions: Ti0.24CoFeNi0.78 and Al0.3Ti0.2CoFeNi. In their solution treated and quenched condition these HEAs exhibit a low coercivity Hc ~1.8–2.4 Oe, high saturation magnetization Ms ~96.5-137.3 emu/g, coupled with a high yield strength, YS ~780-850 MPa, ultimate tensile strength, UTS ~1200 MPa, large tensile elongation, ε ~29-35%, and with desired high electrical resistivity (~269.5-308.3 µΩ·cm), making them attractive for high frequency electrical machines. Importantly, the latter HEA is further strengthened to a very high YS ~1140-1200 MPa with excellent ductility (ε ~18-25%) and electrical resistivity (296.1-459.3 µΩ·cm), but at the cost of higher Hc values ~8.8-48.6 Oe.

Introduction
Soft magnetic materials (SMMs) play a pivotal role in rotating electrical machines, which underpin a wide range of modern energy conversion and mobility technologies. The rising global emphasis on reducing fossil fuel consumption and greenhouse gas emissions has accelerated the adoption of electric machines in electric vehicles, aircraft, and other technological sectors. According to the U.S. Energy Information Administration’s Monthly Energy Review (April 2024)1, nearly 20% of the electrical energy generated in the U.S. power sector is lost before reaching its end use, underscoring the urgent need for improved efficiency in electrical systems. Enhancing efficiency requires materials that combine optimal magnetic, electrical, and mechanical properties, or materials with tunable multifunctional characteristics, to meet the demands of next-generation motors, e.g., for electric vehicles, designed for large torque, high-frequency operation, and more compact architectures2,3.

The essential requirements for SMMs include: (i) low coercivity (Hc) to minimize hysteresis losses during rapid magnetization–demagnetization cycles, (ii) high saturation magnetization (Ms) for maximizing magnetic flux density and torque, and (iii) elevated electrical resistivity (ρ) to reduce eddy current losses4–6. Equally critical are superior mechanical properties, particularly high yield strength to prevent plastic deformation under cyclic stresses and adequate ductility for manufacturing flexibility and damage tolerance7. Meeting this complex set of requirements would substantially expand the application envelope of SMMs and reduce energy losses across electrical machines.

Despite decades of progress, simultaneously achieving this balance of properties remains a formidable challenge. Conventional SMMs—including Fe–Si silicon steels (3–6 wt.% Si), Fe–Ni permalloys, Fe–Ni–Mo supermalloys, and Fe–Co alloys such as Hiperco 50 and Hiperco 50HS—are widely used in electrical machines. These alloys exhibit low Hc (0.01–5.0 Oe) and high Ms (64–235 emu/g)7–9 values, yet they fall short in mechanical strength (maximum ~624–810 MPa in Hiperco 50HS)10–13 and electrical resistivity (27–90 μΩ·cm)11,14. Such limitations are particularly acute in high-torque, high-frequency rotor applications in automotive and aviation use cases, where premature mechanical failure and excessive eddy current losses are prevalent14,15. Therefore, future progress in electrical machine design hinges on engineering single-composition soft magnetic materials that possess the appropriate magnetic, electrical, and mechanical property set.

Recent advances in high-entropy alloys (HEAs) and complex concentrated alloys (CCAs) have created new opportunities for designing alloys with enhanced multifunctional properties16,17. By combining multiple principal magnetic elements, HEAs offer a platform to simultaneously tailor magnetic, structural, and mechanical characteristics18–20. The alloying elements and processing conditions are decisive: they influence phase stability, microstructural evolution, and interfacial characteristics, which in turn, can affect the relevant properties21,22. HEAs containing concentrated amounts of ferromagnetic elements, such as Co, Fe, and Ni, can exhibit promising soft magnetic properties. Further, the addition of atoms such as Al and Ti, which exhibit large negative enthalpies of mixing with these ferromagnetic elements, promotes chemical ordering within face-centered cubic (FCC) solid solutions, often through the formation of ordered intermetallic nanoprecipitates such as the L12 or the B2 phases16,23,24.

The ternary CoFeNi alloy has emerged as a promising base for soft magnetic HEAs. It exhibits excellent magnetic properties25, and the interaction energies of the three binary pairs (Fe–Ni, Ni–Co, Co–Fe) are nearly zero, yielding a near-ideal random solid solution21,26. Equiatomic CoFeNi forms a single-phase FCC structure16,27, which confers excellent ductility but insufficient mechanical strength28. Various strengthening mechanisms have been applied, including solid solution strengthening29, grain refinement30, dislocation strengthening31, and multiphase strengthening32–35. Of these, nanoprecipitation strengthening via coherent L12-type ordered precipitates is especially effective, as established in Ni-based superalloys (γ+γ′ structure)36, Co-based alloys (e.g., Co-Ti-V)37–39, and Fe-based systems40–42.

Building on this principle, Al or Ti additions to CoFeNi can induce coherent L12 nanoprecipitates within the FCC matrix, yielding significant strengthening without severely compromising ductility21,22,29. For instance, Han et al.43 reported a CoFeNi–Ta/Al HEA containing >42 vol.% ordered L12 nanoprecipitates, which achieved a high yield strength (~1202 MPa) and good tensile ductility (~15%), but exhibited relatively high coercivity (~10.7 Oe). Similarly, Xu et al.44 studied (FeCoNi)90–Ti5Al5 alloys, identifying the (FeCoNi)90–Ti5Al5 composition as optimal. This alloy contained <1 nm nanoprecipitates with ~9.8% volume fraction, yielding a balanced property set: YS = 463 MPa, ductility = 50.7%, Ms = 121.1 emu/g, and Hc = 6.5 Oe.

Phase stability in Co- and Ni-based HEAs is highly sensitive to Al and Ti content. Prior work on Al-containing CoCrFeNi CCAs revealed compositional transitions from FCC/L12 to FCC/L12+BCC/B2, and finally to BCC/B2 with increasing Al content24,45. In FCC-based HEAs, Al often stabilizes metastable/stable L12 and/or B2 nanoprecipitates36,46. The simultaneous addition of Ti has proven effective in stabilizing and increasing the mass fraction of the L12 phase. For example, a CoCrFeNi base alloy with 4 at.% Al + 2 at.% Ti achieved strength ≥ 1 GPa and ductility ~17%47, while a CoFeNi base alloy with 8.5 at.% Al + 5.7 at.% Ti reached even higher ductility (~50%) alongside excellent strength48. Prior investigations into an Al0.3CoFeNi HEA underscored the pivotal role of Al in inducing ordering tendencies21,22. This alloy exhibits a combination of long-range ordered (LRO) phases and a disordered FCC solid solution. At 500 °C, L12 nanoprecipitates enhanced hardness and tensile yield stress, while further thermo-mechanical processing yielded a hierarchical four-phase microstructure (FCC+L12+BCC+B2). Depending on the morphology, this complex microstructure delivered exceptional mechanical performance, with the microstructure with multi-scale hierarchical precipitates achieving yield stresses up to ~1490 MPa, ultimate tensile strengths of ~1663 MPa, and ductility of ~12%. However, these mechanical gains were offset by high coercivity (~160 Oe), presenting an opportunity for further microstructure engineering and alloy development.

These findings highlight the central challenge: while introducing ordered nanoprecipitates significantly enhances strength, it often increases coercivity. Achieving a low-coercivity, high-strength microstructure thus requires careful stabilization of nanoscale L12 phases while avoiding large mass fractions of the magnetically deleterious B2 phase or coarse lamellar morphologies.

In this study, we build upon the previous work on CoFeNi and Al0.3CoFeNi alloys by introducing Ti as an additional alloying element to enhance the stability and volume fraction of ordered L12 nanoprecipitates within an FCC matrix. Solution thermodynamic modeling guided our alloy design. Our central hypothesis is that increasing the fraction of fully coherent nanoscale L12 precipitates will simultaneously provide high yield strength and preserve low coercivity. As established in superalloy design, Ti is a potent L12 stabilizer36, and we hypothesized that its addition to CoFeNi and Al0.3CoFeNi-based alloys would yield HEAs with a unique combination of properties: high strength, high ductility, low coercivity, high saturation magnetization, and elevated electrical resistivity. Such alloys would represent a significant advance toward next-generation SMMs for high-frequency, high-torque electrical machines.

Results
Thermodynamic modeling
Pseudo-binary sections of the Ti-Al-CoFeNi and Ti–CoFeNi systems, calculated using Thermo-Calc® with the TCHEA6 database, are shown in Figs. 1a and 1b, respectively. In both cases, alloy compositions were chosen to maximize Ti incorporation while preserving a stable FCC single-phase field at high temperatures: Al0.3Ti0.2CoFeNi (Al8.5Ti5.7Co28.5Fe28.5Ni28.5 in atom %) for the quinary system and Ti0.24CoFeNi0.78 (Ti8Co33Fe33Ni26 in atom %) for the quaternary system. The corresponding phase fraction diagrams (Figs. 1b, d) indicate that both alloys transform from a high-temperature FCC phase into multi-phase fields containing ordered intermetallics upon cooling. Specifically, the Al0.3Ti0.2CoFeNi alloy is predicted to exhibit FCC+L12+B2 stability between 800 and 1150 °C, while the Ti0.24CoFeNi0.78 alloy is expected to form FCC+L12 phases in this temperature range.

To experimentally validate these predictions, homogenized and cold-rolled (90%) samples of both alloys were solutionized in the high-temperature FCC field (1200 °C for Al0.3Ti0.2CoFeNi, 1000 °C for Ti0.24CoFeNi0.78) and water-quenched. Subsequent annealing treatments were performed within the predicted multiphase regions: the Al0.3Ti0.2CoFeNi alloy was annealed at 800 °C for 4 h after solutionizing or, alternatively, annealed directly at 900 °C for 4 h from the cold-rolled state; the Ti0.24CoFeNi0.78 alloy was annealed at 1000 °C for 15 min. These conditions were selected to promote the evolution of ordered L12 and B2 phases from the disordered FCC matrix.

Evolution of mechanical and magnetic properties
Tensile properties. The tensile behavior of Ti0.24CoFeNi0.78 and Al0.3Ti0.2CoFeNi HEAs at room temperature under three different conditions is summarized in Fig. 2a. In the solutionized and water-quenched state (CRSA), the Ti0.24CoFeNi0.78 alloy exhibited a yield strength (YS) of ~850 MPa, ultimate tensile strength (UTS) of ~1200 MPa, and elongation to failure of 29%. Under the same condition, the Al0.3Ti0.2CoFeNi alloy showed slightly lower strength (YS ~780 MPa, UTS ~1110 MPa) but superior ductility (35%). Subsequent isothermal annealing of the solutionized Al0.3Ti0.2CoFeNi alloy at 800 °C for 4 h (CRSA800) markedly enhanced strength, with YS ~1200 MPa and UTS ~1420 MPa, while maintaining 25% ductility. In the cold-rolled (90%) plus directly annealed (900 °C/4 h) condition (CR900), the alloy displayed YS ~1140 MPa, UTS ~1500 MPa, and 18% elongation.

For context, the tensile curve of a solutionized CoFeNi alloy is also plotted in Fig. 2a. The comparison highlights the substantial strengthening imparted by Al and Ti additions. Relative to solutionized CoFeNi, the Al0.3Ti0.2CoFeNi HEA in the CRSA state exhibits nearly a threefold increase in YS and a twofold increase in UTS. Remarkably, this strengthening was achieved without significant loss of ductility, pointing to a modified deformation mechanism that mitigates the typical strength–ductility trade-off. Prior studies have shown that while CoFeNi deforms through homogeneous dislocation-mediated plasticity, the incorporation of Al and Ti in 3d transition-metal HEAs promotes planar slip, enabling sustained work hardening and enhanced ductility49.

The corresponding work-hardening responses for CoFeNi, Ti0.24CoFeNi0.78, and the three conditions of Al0.3Ti0.2CoFeNi (CRSA, CRSA800, CR900) are presented in Fig. 2b, further illustrating the role of compositional and processing modifications in tailoring the strength–ductility balance.

Magnetic properties and electrical resistivity measurements. Figure 2c presents the room temperature magnetization (M) vs applied magnetic field (H) curves for the base CoFeNi alloy, the Al0.3Ti0.2CoFeNi HEA under three processing conditions (CRSA, CRSA800, CR900), and the Ti0.24CoFeNi0.78 alloy in the CRSA condition. All samples display typical soft-ferromagnetic behavior. To highlight coercivity evolution, a magnified view of the curves near the origin (H=0, M=0) is shown as an inset. Ms and Hc values are extracted from the curves and summarized in Table 1.

Relative to CoFeNi, the Al0.3Ti0.2CoFeNi HEA exhibits a markedly reduced Ms of ~100 emu/g across all conditions, representing ~70% lower magnetization. While this value is significantly below conventional soft magnetic alloys like Fe–Si or Fe–Co (>180 emu/g), it is comparable to the values of Ni–Fe-based soft magnets (particularly when compared with commercial Ni-20 wt.% Fe permalloy (82 emu/g) and Ni-15 wt.% Fe-5 wt.% Mo supermalloy (~70 emu/g))9,50 and in line with other high-strength HEAs7,43,51,52. Importantly, the newly developed Ti0.24CoFeNi0.78 alloy demonstrates a superior property balance, achieving Ms = 137.3 emu/g together with Hc = 1.8 Oe, thereby narrowing the gap toward conventional soft magnetic materials.

The coercivity of Al0.3Ti0.2CoFeNi varies strongly with processing conditions. In the CRSA state, the alloy shows a promising Hc ~2.4 Oe, close to the 2.0 Oe of base CoFeNi. Upon annealing at 800 °C (CRSA800), Hc increases substantially to ~48.6 Oe. The CR900 condition yields an intermediate Hc of ~8.8 Oe. Thus, the Al0.3Ti0.2CoFeNi offers better soft magnetic properties in the CRSA condition rather than in the CRSA800 condition.

Figure 2d presents the corresponding electrical resistivity values. All alloy states—Al0.3Ti0.2CoFeNi (CRSA, CRSA800, CR900) and Ti0.24CoFeNi0.78 (CRSA)—exhibit markedly higher resistivities compared to conventional soft magnetic alloys7,9, a highly desirable feature for minimizing eddy current losses. Notably, alloying with Al and Ti increases resistivity by several fold relative to the base CoFeNi alloy (24.0 µΩ·cm)53. These properties are favorable compared to the recently developed Fe34.51Co30.32Ni30.23Ta4.94 alloy, which, despite exhibiting a lower coercivity of 4.52 Oe, possesses a significantly lower resistivity of 62.5 µΩ·cm51,52,54. This combination of high resistivity and low-to-moderate coercivity provides a compelling pathway for developing HEA-based soft magnets that can simultaneously address eddy current suppression and the ability to experience higher stresses in service.

Microstructural characterization
Al0.3Ti0.2CoFeNi alloy
Solutionized and quenched sample. The nanoscale features of the Al0.3Ti0.2CoFeNi alloy in the CRSA condition were examined using TEM and APT. Figure 3a and b present selected area electron diffraction patterns (SAEDPs), recorded along the <001> and <112>FCC zone axes, respectively. In addition to the fundamental FCC reflections, extra spots were observed at {001} positions, providing clear evidence of L12-type ordering. A corresponding dark-field image (Fig. 3c) reveals a homogeneous distribution of ~10 nm L12 precipitates embedded within the FCC matrix. High-resolution TEM (Fig. 3d) further confirms the atomic-scale structure of the FCC matrix and the ordered L12 phase.

These observations establish that the alloy exhibits a two-phase FCC+L12 microstructure even after solution annealing and quenching. This is consistent with the thermodynamic predictions and suggests that L12 nanoprecipitates form during cooling, most likely driven by the large negative enthalpies of mixing of Al and Ti with Ni, Co, and Fe.

Atom probe tomography (APT) analysis of the Al0.3Ti0.2CoFeNi alloy in the CRSA condition further confirmed the presence of nanoscale L12 precipitates within the FCC matrix. A Ni iso-concentration surface (green color surface in Fig. 3e) constructed at a 35 at.% threshold (Fig. 3e) delineates the precipitates, revealing their nearly spherical morphology and uniform distribution within the Co–Fe-rich FCC matrix. For clarity, a 10 nm thick slice of the reconstructed volume is shown.

The compositional partitioning across the FCC/L12 interface is highlighted in the proximity histogram profiles presented in Fig. 3f. The FCC matrix exhibits an average composition of 8.2Al–4.3Ti–30.4Co–33.8Fe–23.3Ni (at.%), whereas the L12 precipitates are enriched in Ni and Ti, with a representative composition of 9.6Al–13.9Ti–26.2Co–12Fe–38.3Ni (at.%). This strong Ni–Ti enrichment supports their identification as ordered L12 precipitates.

Quantitative analysis yields an average spherical-equivalent precipitate diameter of 11.5±2.2 nm and a number density of 1.24×1023/m3. The size measurement, derived from the enclosed precipitate volume, may include minor uncertainties associated with iso-concentration threshold selection. Similarly, number density estimates were corrected by assigning half-weight to precipitates intersecting the dataset boundaries.

Together with TEM, these APT observations establish that the solution-annealed and quenched Al0.3Ti0.2CoFeNi alloy retains a finely dispersed FCC+L12 microstructure.

CRSA800 condition. A representative SEM backscattered electron (BSE) image of the Al0.3Ti0.2CoFeNi CRSA800 sample is shown in Fig. 4a, while the corresponding electron backscatter diffraction (EBSD) phase and inverse pole figure (IPF) maps are presented in Figs. 4c and 4e, respectively. Based on EBSD analysis, the average FCC grain size of this sample was determined to be ~73 µm. A higher-magnification BSE image (Fig. 4b) reveals the presence of BCC/B2 precipitates, preferentially located at FCC grain boundaries and triple junctions. The associated EBSD phase and IPF maps in Fig. 4d and f confirm these observations.

Further microstructural details are revealed in Fig. 4g, where a high-resolution SEM image from within an FCC grain clearly shows fine nanometer-scale precipitates. The presence of L12 ordering within the FCC matrix is confirmed by the selected area diffraction pattern in the inset in Fig. 4g, recorded along the [011]FCC zone axis, which exhibits distinct (100)-type superlattice reflections. APT results from the CRSA800 condition are shown in Figs. 4h, i. Here, a 10-nm slice of the reconstructed APT volume is shown, with a Ni iso-concentration surface (green color surface in Fig. 4h) constructed at 28.5 at.% to delineate the Ni-rich L12 precipitates within the Co–Fe-rich FCC matrix. Comparison of Fig. 4h with Fig. 3e indicates that annealing at 800 °C for 4 h promoted significant growth and coarsening of the ordered L12 precipitates.

The chemical partitioning across the FCC/L12 interface was quantified using proximity histogram analysis, shown in Fig. 4i. The average compositions of the FCC matrix and L12 precipitates were measured as 8.1Al–2.9Ti–31Co–38.9Fe–19.1Ni (at.%) and 10.1Al–14.5Ti–23.7Co–9.8Fe–41.9Ni (at.%), respectively. The average spherical-equivalent diameter and number density of the L12 precipitates were determined as 27±5.1 nm and 5.76×1021/m3, respectively.

Interestingly, while the compositions of the L12 precipitates and the FCC matrix remain largely unchanged compared to the solution-annealed and quenched condition, the precipitate size more than doubles, accompanied by a reduction in number density by nearly two orders of magnitude. Importantly, this significant change in precipitate size and distribution is expected to strongly influence the mechanical response of the HEA, particularly its Hc behavior, as will be discussed in a later section of this paper.

CR900 condition. To further tailor the microstructure, a different thermo-mechanical processing strategy was applied to the Al0.3Ti0.2CoFeNi HEA. Following homogenization at 1200 °C, the alloy was cold-rolled to 90% thickness reduction and subsequently annealed directly at 900 °C for 4 h (hereafter referred to as CR900). According to phase stability calculations (Fig. 1), this composition is predicted to form a stable three-phase microstructure consisting of FCC, B2, and L12 phases at 900 °C. However, as established in prior studies on FCC-based HEAs, the nucleation of the B2 phase is hindered owing to its high nucleation barrier, whereas the ordered L12 phase nucleates more readily due to a much lower barrier, due to the low FCC/L12 interfacial energy. Consequently, B2 precipitates typically form only heterogeneously at defects such as grain boundaries and triple junctions, unlike the more uniformly distributed L12 precipitates.

The microstructure of the CR900 condition is presented in Fig. 5. A low-magnification SEM backscattered electron image (Fig. 5a), together with EBSD phase and IPF maps (Fig. 5b, c) reveal a refined FCC grain size of ~5 µm. This represents a substantial reduction compared to the cold-rolled + homogenized + quenched condition, where the FCC grains coarsened to ~73 µm. In addition to the refined FCC grains, a secondary population of BCC/B2 precipitates with an average size of ~1 µm was observed preferentially decorating FCC grain boundaries and triple junctions. The corresponding area fraction of these precipitates was ~5%.

Higher-magnification SEM-EBSD maps (Fig. 5d–f) confirmed that these boundary-associated precipitates were indexed as BCC in EBSD, consistent with the crystallographic symmetry of B2 ordering. Notably, the backscattered electron image (Fig. 5d) also revealed a finer dispersion of intragranular precipitates, which indexed as FCC in EBSD but are likely L12 ordered precipitates, since EBSD cannot differentiate between chemically ordered and disordered FCC structures.

The presence of B2 precipitates was further verified by TEM (Fig. 6). A representative bright-field TEM micrograph (Fig. 6a) shows a precipitate circled for reference, with the associated [001] BCC diffraction pattern (Fig. 6b) exhibiting (010) superlattice reflections, confirming B2 ordering. Dark-field imaging using the (100) superlattice reflection (Fig. 6c) highlighted the ordered domains. A HAADF-STEM image (Fig. 6d) and elemental EDS maps (Fig. 6e–h) revealed strong partitioning of Al to the B2 phase, accompanied by enrichment of Ti and depletion of Fe. Interestingly, the Ti map also showed enrichment in numerous smaller intragranular precipitates within the FCC grains. These features were simultaneously Ni-rich and Fe-depleted, consistent with L12 precipitates.

Further evidence for intragranular L12 ordering was obtained by TEM and APT. A selected area diffraction pattern recorded along the [112]FCC zone axis (Fig. 7a) displayed distinct (110)-type superlattice reflections, indicative of L12 ordering. Complementary APT analysis provided three-dimensional compositional mapping of the CR900 microstructure (Fig. 7b). Ni-rich L12 precipitates were identified by constructing 28.5 at.% Ni iso-concentration surfaces (green color surface), which clearly delineated the precipitates from the Fe–Co-rich FCC matrix. The number density of the L12 precipitates is evaluated as 3.4×1021/m3. The contribution of the precipitates in calculating the number density is considered as half, as none of the precipitates are found to be totally confined within the reconstructed volume. Further, proximity histogram analysis across the FCC/L12 interface (Fig. 7c) quantified the compositional partitioning: the FCC matrix contained ~7.1Al–3.7Ti–31.6Co–37.4Fe–20.1Ni (at.%), while the L12 precipitates were enriched in Ni and Ti (~8.7Al–15.7Ti–24.6Co–9.9Fe–41.1Ni, at.%).

These results establish that direct annealing at 900 °C after 90% cold rolling produces a refined FCC matrix with ~5 µm grains, decorated by B2 precipitates at boundaries and a uniform dispersion of Ni- and Ti-rich L12 precipitates within grains. The dual presence of boundary-associated B2 and intragranular L12 phases highlights the distinct nucleation pathways of the two ordered structures: heterogeneous for B2 and homogeneous for L12. This microstructural configuration is expected to significantly influence the mechanical response of the alloy by combining grain refinement with multi-scale precipitation strengthening.

Ti0.24CoFeNi0.78 alloy
Figure 8 presents the microstructural characterization of the Ti0.24CoFeNi0.78 alloy in its solution-annealed condition. EBSD analysis (Fig. 8a) revealed an equiaxed FCC grain structure with an average grain size of 100±25.6 µm. High-resolution SEM imaging in the backscattered mode (Fig. 8b) showed the presence of nanoscale precipitates, which were further confirmed by dark-field TEM (Fig. 8c). SAEDPs recorded along the <011> BCC zone axis (Fig. 8d) displayed additional {001} and {011} reflections, unambiguously establishing the presence of L12 ordering within the FCC matrix.

Interestingly, despite undergoing solution annealing and subsequent quenching, the alloy retained a dual-phase FCC+L12 microstructure. This indicates that L12 precipitates homogeneously nucleated during cooling, most likely facilitated by the large negative enthalpy of mixing between Ti and Ni/Co/Fe, which promotes ordering. APT provided further insight into the chemical partitioning behavior. Ti atom maps from a 10-nm slice of the reconstructed volume (Fig. 8e) highlighted pronounced Ti partitioning, with L12 precipitates clearly delineated through a Ti iso-concentration surface (black color surface) at a threshold of 15 at.%.

Compositional analysis across the FCC/L12 interface, obtained from proximity histogram profiles (Fig. 8f), revealed distinct partitioning of alloying elements. The average composition of the FCC matrix was 2.5Ti-34.0Co-44.4Fe-19.0Ni (at.%), whereas the L12 precipitates were significantly enriched in Ti and Ni, with an average composition of 23.6Ti-27.6Co-7.3Fe-41.5Ni (at.%). Quantitative analysis of the precipitate population showed an average spherical-equivalent diameter of 4.0±0.8 nm and a number density of 6.29×1023/m3.

These results confirm that, even under solution-annealed and quenched conditions, the Ti0.24CoFeNi0.78 alloy exhibits a stable two-phase FCC+L12 microstructure, driven by strong chemical partitioning and ordering tendencies.

Methods
Alloy preparation and thermo-mechanical processing
Al0.3Ti0.2CoFeNi and Ti0.24CoFeNi0.78 alloys, with nominal compositions 8.5Al–5.7Ti–28.5Co–28.5Fe–28.5Ni (at.%) and 8Ti–33Co–33Fe–26Ni (at.%), respectively, were procured from ACI Alloys Inc. The alloys were vacuum arc remelted (VAR) five times under an Ar atmosphere, with intermediate flipping, to ensure chemical homogeneity. The actual bulk compositions, measured using wavelength dispersive X-ray fluorescence spectroscopy (WDS-XRF, Rigaku Primus IV), were 6.6Ti–7.1Al–29.1Co–28.8Fe–28.4Ni (at.%) and 32.9 Co–33.4Fe–25.8Ni–7.9Ti (at.%), as shown in Supplementary file (Supplementary Table 2). An XRD experiment was carried out to investigate the crystal structure of the alloys (see Supplementary Fig. 1 and Supplementary Table 3) using a tabletop Bruker D6 Phaser instrument with Cu Kα radiation (λ=1.54 Å).

The cast alloys were homogenized at 1200 °C for 1 h, followed by 90% reduction in thickness through cold cross-rolling. The rolled plates were then solution annealed at 1200 °C for 5 min (encapsulated in stainless steel foil) and water-quenched to yield a single-phase FCC solid solution with an average grain size of ~190 μm. This state is referred to as the CRSA condition. A subset of Al0.3Ti0.2CoFeNi samples was subsequently annealed at 800 °C for 4 h (CRSA800). Another set of homogenized and cold-rolled samples was annealed at 900 °C for 4 h (CR900).

Microstructural analysis
Microstructural characterization was carried out using a FEI Nova NanoSEM 230™ scanning electron microscope (SEM) and a FEI Tecnai G2 TF20™ transmission electron microscope (TEM) operated at 200 kV. SEM samples were prepared by grinding and polishing to 4000 grits, followed by final polishing in 1 µm colloidal silica using a Vibromet system. Electron backscatter diffraction (EBSD) measurements were performed on a Thermo Fisher Apreo-2S scanning electron microscope (SEM) equipped with an EDAX Hikari Super ® EBSD detector.

Maps were collected at an accelerating voltage of 20 kV, probe current of 1.6 nA, working distance of ~15 mm, and with the specimen tilted at 70°. The step size was selected based on feature size: 0.2 µm for bulk grains and 50 nm for finer grains. Kikuchi patterns were acquired at a resolution of 480×480 pixels with 4×4 binning.

Indexing was performed using the EDAX OIM Analysis® software suite (version 9.0.1.277), employing the Hough transform method with the FCC and BCC structures (space groups Fm-3m and Im-3m, respectively). Post-processing included the removal of points with CI<0.1. No artificial pattern scaling, filtering, or pattern modification was applied beyond standard background correction provided by the acquisition software.

TEM lift-outs were fabricated using an FEI Nova 200 dual-beam focused ion beam (FIB). A ~15×2×5 μm3 lamella was extracted with an Omni probe micromanipulator, then thinned to ~50–100 nm by sequentially lowering Ga-ion beam voltage and current. Atom probe tomography (APT) was performed on a Cameca LEAP 5000XS in laser mode (30±0.5 K, 250 kHz pulse rate, 0.005–0.01 ion/pulse detection rate, 50 pJ laser energy). Data reconstruction and analysis were conducted using Cameca AP Suite 6.1 with parameters: evaporation field 33 V/nm, field factor 3.3, and image compression factor 1.65.

Mechanical response measurements
Miniature tensile tests were conducted at a strain rate of 0.001 s-1 using a custom-built machine equipped with an LVDT extensometer. Dog-bone specimens with gauge dimensions of ~5×1×1 mm3 were prepared by electric discharge machining. For each condition, three specimens were tested to ensure reproducibility.

Magnetic response measurements
Room-temperature magnetization (M–H) curves of the base CoFeNi alloy, Ti-modified Ti0.24CoFeNi0.78 CRSA, and Al, Ti-modified variants (CRSA, CRSA800, CR900) were measured using a Lakeshore 7404 vibrating sample magnetometer (VSM). For each measurement, cubic specimens of ~2×2×2 mm3 were used. Further, each experiment was repeated thrice to check repeatability.

Resistivity measurements
Electrical resistivity was determined by the standard four-point probe method using specimens of ~2×1×5 mm3. Resistivity values (ρ) were calculated as

ρ = (R×A)/l (2)

where R is the measured resistance, A the cross-sectional area, and l the distance between voltage contacts. For each sample, the reported resistance was averaged over four independent measurements.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper presents a Ta-free Al0.3Ti0.2CoFeNi soft magnetic high entropy alloy (HEA) for rotating electrical machines. By using a thermodynamic approach, the composition of the CoFeNi base alloy is adjusted through the addition of Al and Ti to form ordered L12 nanoprecipitates, aiming to enhance the mechanical properties of the alloy while maintaining low coercivity. The study employs thermodynamic modeling to predict the phase stability of the alloy, prepares samples through different heat treatment processes, and carries out tensile property, magnetic property tests, and microstructural characterization. The results show that the alloy in the solution-treated and quenched state (CRSA) exhibits excellent comprehensive properties, with a yield strength of approximately 780 MPa, an ultimate tensile strength of approximately 1110 MPa, an elongation of approximately 35%, and a coercivity of approximately 2.4 Oe. After specific annealing treatments, the mechanical properties can be further improved while maintaining good ductility and low coercivity.\n1. The introduction could benefit from a more detailed explanation of the challenges faced by traditional soft magnetic materials in rotating electrical machines. At present, what is the most widely used soft magnetic alloy for industrial rotating electrical machines? What is the typical performance? What performance indicators can meet the needs of future development?\n2. Although the authors give the thermodynamic modeling, it is not clear whether the theory is consistent with the experiment. The experiment only selects a limited range of heat treatment temperatures and times, lacking exploration of more process parameters, which may prevent a comprehensive understanding of the alloy's property variation patterns. It is suggested to increase the experiments of heat treatment process parameters such as different annealing temperatures, different annealing times, and different cooling rates, so as to more comprehensively study the property change law of the alloy.\n3. Although the relationship between the microstructure and properties is analyzed, the formation mechanism of the L12 precipitates and their influence mechanism on the properties are not elaborated in depth, lacking theoretical model support.\n4. The analysis of the reasons for the coercivity changes under different heat treatment conditions is not detailed enough. Only the influence of the precipitates is mentioned, without considering other possible factors. There is also a lack of corresponding experimental evidence.\n5. Although an Al0.3Ti0.2CoFeNi alloy composition is proposed, compared with existing relevant studies, the degree of innovation is not prominent enough. The unique advantages of this alloy need to be further emphasized.\n6. In addition to the alloys in table S1, there are many HEAS with high strength and low coercivity, such as FeCoVNbW, which also have good comprehensive properties. The authors should make a more comprehensive comparison to reflect their own advantages.\n7. The discussion section could benefit from a more detailed explanation of how the improved mechanical properties of the Al0.3Ti0.2CoFeNi HEA can enhance the performance of rotating electrical machines.\n8. AlTiCoFeNi HEA is not a NEW alloy system. Please delete all “new”.\n9. Titanium (Ti) is more easily oxidized than Tantalum (Ta). The actual composition of the alloy is different from the nominal composition. Is there an oxygen element? Does it mean that the alloy containing Ti and Al has poor processability?", "reviewer_2": "• What are the noteworthy results?\nTailoring nanoscale L12 precipitates in TiAlCoFeNi HEA to achieve high yield strength (780MPa or higher) and low coercivity (as low as 2.4Oe). Detailed characterization work (TEM, atom probe) on the L12 precipitates in different processing states. Manipulating process parameter in CR900 to achieve a different microstructure (fine grain size, grain boundary B2, low fraction of L12 precipitates), leading to lower coercivity and similar strength to CRSA800.\n• Will the work be of significance to the field and related fields? How does it compare to the established literature? If the work is not original, please provide relevant references.\nI don’t have doubt that this work is original. However, the significance to the field is not high enough to justify being published in this high impact journal. The work is sound to be published at a different journal.\nL12 and B2 precipitate strengthened CoFeNi HEA is not new for mechanical properties and structural application. Controlling precipitate size to prevent pinning magnetic domain walls is critical to achieve low coercivity. The approach of solutioning and quenching to suppress precipitation is also not new for structural alloys.\nIt is inappropriate to only compare mechanical and magnetic properties to the emerging HEAs in similar composition range. Magnetic properties of the cited HEA reference are all quite poor, and they don’t represent state-of-the-art. Benchmarking properties with commercial materials should be included in the manuscript. In fact, I found the impact/significance of the work is not as high as it is claimed due to the magnetic property deficiency, because most commercial SMM (Fe-Si and FeCo) have coercivity lower than 2.4Oe and magnetization of TiAlCoFeNi HEA (~100emu/g) is significantly lower than Fe-Si. Additionally, AC core loss is a key metric for rotor lamination. Resistivity is also critical for eddy current loss. The paper does not include any AC core loss data. Thickness of the cold rolled sheet is also not described (ability to roll sheet down to a typical lamination thickness of 10-30 mil without cracking is important).\nRotors in electric motor/generator are often designed up to yield strength and stators have very low mechanical requirements. Rotor material cannot be too brittle, but ductility is not a key metric. Commercial material Hiperco50HS with ductility below 10% is being used in some electric machines.\nComposition of the TiAlCoFeNi HEA has significant amount of Co and Ni, indicating much higher cost than conventional Fe-Si electrical steel. This could be a show stopper for cost-driven electric motor/generator applications, such as ground EV.\n• Does the work support the conclusions and claims, or is additional evidence needed?\nThe wrong baseline (emerging HEA properties) was chosen, leading to the wrong conclusion for magnetic properties. The resulting yield strength is attractive to rotor application in electric machines, but magnetic saturation and coercivity are inferior to Fe-Si and FeCo commercial material. Additional evidence of AC core loss shall be provided, which is a key metric for motor/generator application.\n• Are there any flaws in the data analysis, interpretation and conclusions? Do these prohibit publication or require revision?\nThe conclusion of 2.4Oe, 8.8Oe coercivity, and 100emu/g Ms being attractive is completely misleading. These properties are inferior to most commercial SMM (such as Fe-Si electrical steel).\nLine 167: Fe-Ni commercial material can have Ms 150 – 200 emu/g, much higher than TiAlCoFeNi HEAs.\nIt seems number density of L12 precipitates in CRSA800 and CR900 shall be quantified to support the lower coercivity mainly due to L12 precipitates, rather than describing qualitatively.\nFigure 10 is not very meaning to practical application. As I mentioned earlier, yield strength is a key metric to rotor in electric machine, not UTS. Ductility is not so important (if >5%). Yield strength rather than UTS normalized by density is a more appropriate metric to compare. Commercial FeCo alloys (soft magnetic versions and high strength versions) are missing in Figure 10. Fig. 10a can be deleted, and replaced with Ms vs. Hc. Fig. 10b can be modified to YS vs. Hc.\n• Is the methodology sound? Does the work meet the expected standards in your field?\nTensile testing and microstructure characterization methods are sound. However, the paper is missing AC core loss data which is essential for electric machine lamination application.\n• Is there enough detail provided in the methods for the work to be reproduced?\nMelting method for the ingot should be specified (VAR, VIM?). EDS measurement of the actual composition is not appropriate because EDS doesn’t have high accuracy to provide quantitative results. Actual composition (major elements) shall be measured by ICP. The reduction, number of path, any intermediate annealing, and finished thickness for the cold rolling process should be specified. Environment of the steel foil encapsulation (whether it was vacuum or inert gas or air) should be described. SEM sample preparation method and EBSD conditions are missing.\nOther comments:\nLine 46: “Traditional SMM materials lack good mechanical properties and typically exhibit low strength and/or poor tensile ductility.” This statement is a bit misleading, without specifying the values of properties.\nRotors in an electric motor/generator only require mechanical strength to meet the stress requirement at its max rotating speed. Fe-Si laminate in low silicon (Si <3%) has relatively low yield strength (~50ksi) and good ductility, but it meets the need of most industrial motor/generator. JFE Steel has Fe-6.5Si with higher yield strength and low coercivity. FeCo laminate has two categories: (1) the magnetic soft, relatively low strength, high ductility version for stator application (such as Hiperco50A); (2) the high yield strength (100ksi) but relatively low ductility and high coercivity version for rotor application (such as Hiperco50HS). Note that rotor material cannot be too brittle, but ductility is not a key metric, because rotors are often designed up to yield strength and stators have very low mechanical requirements. Authors shall add commercial SMM properties to Table 1 for comparison.\nLine 171: CR900 coercivity of 8.8 Oe is too high to be useful. I disagree with the claim of “attractive”. Even CRSA 2.4 Oe is higher than most SMM (around or below 1 Oe).", "reviewer_3": "This study is focussed on the enhancement of the mechanical properties of the soft magnetic CoFeNi base HEA, by preserving a low coercivity and reasonable saturation magnetization. The study titled “A magnetically soft yet mechanically strong and ductile, Ta free Al0.3Ti0.2CoFeNi high entropy alloy” is an interesting work but some problems need to addressed.\n1. It would be beneficial if the author will include XRD pattern for the clear insights of the structure and change in mechanical property and coercivity.\n2. Write the caption of Fig. 4 by mentioning about (a-f).\n3. How the precipitation of small B2 phase is conformed for CR900 and CRSA800 (mentioned in page no.22).\n4. In page no.8 author mentioned that isothermal magnetization, what does it means? There is no meaning of mentioning isothermal magnetization as it has recorded at RT."}, "author_response": "The authors thank the reviewers for their insightful comments and constructive criticism of the manuscript. The authors have put in their best efforts to address all the comments raised by the reviewers. While addressing these comments, the quality of the manuscript has improved significantly. The authors believe that the editor and the reviewers will find the revised manuscript suitable for publication in this esteemed journal. The reviewers’ comments are reproduced below in black and the responses to these are in blue. Any text added to the revised manuscript and reproduced here is highlighted in yellow.\n[... Author responses to Reviewer 1, 2, and 3 ...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all my concerns. This submission can be accepted.", "reviewer_3": "The authors have addressed the reviewer’s queries and provided explanations for the points raised. The revised manuscript reflects improved clarity and a better justification of the results. However, a few aspects still require attention to further strengthen the work. Addressing these issues will improve the overall quality of the manuscript.\nThe authors have claimed the formation of both FCC and L1₂ phases in the Al₀.₃Ti₀.₂CoFeNi (CR900) and Ti₀.₂₄CoFeNi₀.₇₈ (CRSA) HEAs. However, the confirmation of dual-phase formation requires further justification. Although the presence of a superlattice reflection is noted, there is no observable splitting in the fundamental FCC peaks, and the reported lattice parameters for the FCC and L1₂ phases are identical. This raises uncertainty about the distinct existence of separate FCC and L1₂ structures.\nFor Al0.3Ti0.2CoFeNi CRSA 800 and Al0.3Ti0.2CoFeNi CR900 HEAs. Nevertheless, for Al₀.₃Ti₀.₂CoFeNi CRSA 800 and CR900 HEAs, the observed splitting in diffraction peaks supports the possible coexistence of both FCC and L1₂ phases."}, "author_response": "Response to Reviewers’ Comments\nThe authors thank the reviewers for their constructive comments regarding the revised manuscript. The reviewers’ comments are reproduced below in black and the responses to these are in blue. Any text added to the revised manuscript and reproduced here is highlighted in yellow.\n[... Author responses to Reviewer 1 and 3 ...]"}, {"round": 3, "reviewer_comments": {"reviewer_3": "The authors have addressed all of my concerns raised in the previous review. The revisions are satisfactory, and I recommend acceptance of this manuscript."}, "author_response": ""}]

Decision letter