1 Introduction

The AlxCoCrFeNi high-entropy alloy system has attracted much attention due to its structure and properties, which can be adjusted by the Al content [1,2,3,4,5]. In addition to the strength increasing with the phase transition caused by the rise of Al content, the magnetic properties will also vary with the microstructure evolution [6,7,8].

For nanocrystalline high-entropy alloys composite (nc-HEAC, usually one nanoscale material mixed into a matrix material with the coarse grain), the interface diffusion between the two phases plays a pivotal role in the microstructure and properties evolution of the composite [9,10,11,12,13]. The preparation method of high-entropy alloy composites is similar to that of high-entropy alloys, including powder metallurgy, induction melting, and additive manufacturing [14,15,16], which needs high-temperature forming, resulting in the growth of two-phase grains, lack of sufficient grain boundary, and diffusion driving force to cause the diffusion only occurs in the shallower interface of the two phases. The further diffusion process and mechanisms have not been thoroughly clear. At present, the effect of nanoscale thermal diffusion-induced phase transition on properties of nano dual-phase HEAC has not been systematically studied.

In this work, the dual-phase Al/CoCrFeNi nanocrystalline high-entropy alloy composite was prepared using laser source inert gas condensation equipment (laser-IGC). By controlling the diffusion behavior between the nanostructured Al and CoCrFeNi HEA, we can continuously regulate the phase structure and magnetic properties of AlCoCrFeNi HEA. In situ high-energy synchrotron X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) were used to study the relationship between the magnetic properties and the microstructure evolution of Al/CoCrFeNi nc-HEAC during annealing.

2 Experimental

The laser-IGC equipment and the detailed preparation process of nc-HEA were reported earlier [17]. The Al plate and Co25Cr25Fe25Ni25 HEA plate were used as the targets and evaporated by laser beam simultaneously. The Al plate (99.999%) was purchased directly from Zhongnuo New Materials Co., Ltd (Beijing). The Co25Cr25Fe25Ni25 HEA was prepared by arc-melting a mixture of the constituent elements (> 99.9%) in a Ti-gettered high-purity argon atmosphere. The molten alloy was suction-cast into a 10 mm (width) × 60 mm (length) × 2 mm (thickness) copper mold, and named as as-cast sample. The as-prepared Al/CoCrFeNi nc-HEAC sample is 10 mm (diameter) × 0.3 mm (thickness) in size. The Al/CoCrFeNi nc-HEACs with 0 at%, 16 at% and 40 at% Al contents were prepared by adjusting the evaporation time of the laser beam on the targets and named as laser-IGC CoCrFeNi, Al-16% and Al-40%, respectively. In the current work, we also labeled the 40%-Al/CoCrFeNi nc-HEAC at room temperature as IGC-25 °C. The Al-40% nc-HEAC samples were used for annealing at a temperature of 200–900 °C for 1 h.

The microstructure of the laser-IGC Al/CoCrFeNi nc-HEAC was examined using high-resolution transmission electron microscopy (HRTEM, Talos F200S G2 200 kV) equipped with energy dispersive X-ray spectrometry (EDS) and high-energy synchrotron X-ray diffraction (XRD) performed at beamline 11-ID-C of the advanced photon source (APS), Argonne National Laboratory. High-energy monochromatic X-rays with a wavelength of 0.01173 nm were used for data collection. Magnetization hysteresis loops was measured using a vibrating sample magnetometer at room temperature with applied magnetic fields up to 7.0 T, and temperature dependence of magnetization was measured in the temperature ranges of 4–400 K using a physical property measurement system (Quantum Design Dynacool-9).

3 Results and discussion

3.1 Microstructure

The microstructure and phase composition of the as-prepared 40%-Al/CoCrFeNi nc-HEAC are shown in Fig. 1. High-angle annular dark-field scanning transmission (HAADF-STEM) image is mainly of uniformly mixed bright colors CoCrFeNi HEA and dark Al nanoparticles. The specific composition of CoCrFeNi HEA (Area a1) and Al (Area a2) is shown in Table 1, and the average grain size of CoCrFeNi HEA electron microscopy (HAADF-STEM) and EDS mapping analysis (Fig. 1a) shows that the nc-HEAC is composed of nanoparticle that evaluated by TEM (Fig. 1b) with size of ~ 26 nm. In addition, it is interesting that there are not only FCC-Al (Fig. 1c1) and FCC-CoCrFeNi (Fig. 1c2) phases in the nc-HEAC (where Z represents the zone axis), but also a nano-phase with HCP structure (Fig. 1c3). The fast Fourier transform (FFT) analysis shows that the lattice constants of FCC-CoCrFeNi HEA and FCC-Al are aHEA = (0.3523 ± 0.0003) nm and aAl = (0.4082 ± 0.0005) nm, respectively, which are well consistent with those of the CoCrFeNi HEA and Al, and the lattice constants of the HCP phase are a = (0.2544 ± 0.0003) nm and c = (0.4023 ± 0.0002) nm.

Fig.1
figure 1

Microstructure of as-prepared 40%-Al/CoCrFeNi nc-HEAC: a HAADF-STEM image and corresponding EDS mapping (Areas a1, CoCrFeNi HEA particle; Area a2, Al particle); b bright-field TEM image; c HRTEM image and corresponding FFT patterns of c1 FCC-Al phase, c2 FCC-CoCrFeNi HEA phase and c3 HCP phase collected from nc-HEAC

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Table 1 Summary of EDS results showing structure and chemical compositions of phases formed in Al-40% nc-HEAC sample at room temperature (RT) and after annealing at 425 and 500 °C (at%)
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3.2 Magnetic properties

Figure 2 shows magnetic hysteresis loops (M–H) and the temperature dependence of the magnetization (M–T) curves of the as-cast CoCrFeNi HEA and Al-40% nc-HEAC after annealing. The as-cast CoCrFeNi HEA sample exhibits a paramagnetic property consistent with the results reported in Refs. [18,19,20], while the laser-IGC nc-HEAC shows ferromagnetic behavior (Fig. 2a). Moreover, the saturation magnetization (Ms) and Curie temperature (Tc) of the nc-HEAC can be controlled by heat treatment. The results show that the Ms of the laser-IGC nc-HEAC significantly increases from 7.7 to 35.9 A.m2·kg−1 after annealing at 500 °C, and then decreases with annealing temperatures increasing up to 900 °C. Figure 2b shows the normalized magnetization as a function of temperature for several samples annealed at different temperatures. The Tc of the as-cast CoCrFeNi HEA sample (98 K) is very close to the value of 104 K reported in Ref. [18]. For the laser-IGC nc-HEAC, the Tc increases ~ 1.5 times compared with that for the as-cast CoCrFeNi sample. After the annealing temperature is higher than 400 °C, Tc increases with the increase in temperature. The Ms and Tc of the as-cast CoCrFeNi HEA and the laser-IGC nc-HEAC after annealing at different temperatures are summarized in Table 2.

Fig. 2
figure 2

Magnetic properties of as-cast CoCrFeNi HEA and Al-40% nc-HEAC annealed at different temperatures: a M–H curves measured at a room temperature; b M–T curves at temperature range of 4–400 K

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Table 2 Summary of saturation magnetization (Ms) and Curie temperatures (Tc) of as-cast CoCrFeNi HEA and laser-IGC Al-40% nc-HEAC samples after annealing
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3.3 Microstructure evolution during annealing

To explore the origin of the ferromagnetic properties in the initial Al-40% nc-HEAC, the synchrotron XRD experiments was conducted to investigate the phase composition of the as-cast CoCrFeNi HEA, laser-IGC CoCrFeNi nc-HEA, Al-16% nc-HEAC and Al-40% nc-HEAC (Fig. 3). Figure 3a shows that the as-cast CoCrFeNi sample has a single FCC structure, which is consistent with the reported literature [21,22,23,24,25]. By contrast, the solid diamond marks an additional diffraction peak in the laser-IGC CoCrFeNi nc-HEA and Al-16% nc-HEAC corresponding to an HCP-CoNi phase identified by Rietveld refinement. The lattice constants of the HCP-CoNi phase are a = 0.2504 nm, c = 0.4065 nm, consistent with those of HCP phase in Fig. 1. However, due to the high content of Al in Al-40% nc-HEAC, the (111) crystal plane peak of the CoCrFeNi HEA overlaps with the (110) crystal plane peak of Al, which masks the diffraction peak of the HCP-CoNi phase. Moreover, the magnetic properties of laser-IGC CoCrFeNi nc-HEA show that it has ferromagnetic behavior (Fig. 3b), indicating that the presence of the CoNi phase dominates the ferromagnetic properties of the Al-40% nc-HEAC. We also performed high-energy synchrotron XRD of Al-16% nc-HEAC with 30–1000 °C in situ heat treatment. As shown in Fig. 3c, the CoNi phase fraction increases with annealing temperature increasing and reaches a peak value at 300 °C, then decreases and disappears entirely above 580 °C.

Fig. 3
figure 3

a High-energy synchrotron XRD patterns of as-cast CoCrFeNi HEA, laser-IGC CoCrFeNi nc-HEA, Al-16% and Al-40% nc-HEACs; b M–H curves measured of laser-IGC CoCrFeNi nc-HEA and as-cast CoCrFeNi HEA at room temperature; c phase fraction of CoNi at 25–600 °C; d in situ synchrotron XRD patterns of Al-40% nc-HEAC at 30–1000 °C; e phase fraction of FCC-Al, BCC and FCC phase at 30–800 °C; f intensity of diffraction peaks of B2 and B2/BCC phases with temperature

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To reveal the correlation adjustable magnetic property and microstructure of the Al-40% nc-HEAC during heat treatment, we studied the microstructures evolution in detail by in situ synchrotron XRD. Figure 3d shows a series of in situ synchrotron XRD patterns (30–1000 °C), where the nc-HEAC sample mainly consists of the FCC-CoCrFeNi (marks by the solid rectangle) and FCC-Al (marks by solid circles) phases. As the annealing temperature increases, it can be seen that the content of the FCC-Al phase gradually decreases and disappears entirely at about 370 °C (Fig. 3e). After that, some additional peaks marked with diamond and triangle emerge, which correspond to AlNi B2 and B2/BCC phases identified by Rietveld refinement. At temperatures above 400 °C, the content of the BCC phase increases rapidly, while the content of the FCC phase decreases rapidly. At 500 °C, FCC phase of the Al-40% nc-HEAC sample is almost completely transformed into BCC phase (96%). The phase fractions of the BCC and FCC phases at different temperatures are shown in Fig. 3e, consistent with the variation trend in magnetic properties. The evolution of XRD patterns indicates that FCC-Al with a lower melting point diffuses into the CoCrFeNi HEA with temperature increasing and alloying with the HEA to form a new FCC-AlCoCrFeNi HEA (200–370 °C). With the increase in the Al content and the intensification of element diffusion in the HEA grains, the AlCoCrFeNi high-entropy alloy gradually transforms from FCC to BCC structure (370–500 °C), which is similar to the microstructure evolution process of AlxCoCrFeNi HEA with the increase in Al content [4, 26]. With the further increase in the annealing temperature, the structure of the nc-HEAC did not change significantly.

Moreover, the phase fraction trend of B2 and B2/BCC phases during the annealing process was also analyzed, and the diffraction intensity of the B2/BCC phase tends to be saturated at 500 °C (Fig. 3f). In comparison, the diffraction peak intensity of the B2 phase is not saturated until 1000 °C, indicating that the B2 phase content increases with the further increase in annealing temperature, which is consistent with previously reported result [27, 28].

The microstructure evolution after annealing of the Al-40% nc-HEAC is investigated using HRTEM to identify the diffusion behavior (Fig. 4). TEM image of the sample annealed at 425 °C (Fig. 4a) shows that the HEA grains still maintain spherical particle and the grain size did not increase significantly compared with the initial samples (Fig. 1). FFT patterns in Fig. 4b1 (obtained from the blue rectangular area in Fig. 4b) and b2 (obtained from the orange rectangular area in Fig. 4b) clearly show the morphology of the new BCC phase formed in the FCC-HEA matrix. It can be seen from Fig. 4b1, b2 that the FCC-HEA matrix and the BCC phases have a crystallographic orientation relationship of [011]FCC//[111]BCC and (\(\overline{1}11\))FCC//(\(\overline{1}01\))BCC and the specific composition of Areas b1, b2 are shown in Table 1, in which the BCC phase is enriched with Al and Ni, corresponding to the B2 phase [4, 26]. TEM results show that Al diffused from the grain edge into the interior of the CoCrFeNi HEA to form the FCC-AlCoCrFeNi HEA. With the increase in annealing temperature, element diffusion intensifies, and the AlCoCrFeNi HEA grain edge region with more Al preferentially transforms into BCC structure. In contrast, the internal structure of the AlCoCrFeNi HEA still maintains FCC structure. This confirms the results of XRD analysis in Fig. 3.

Fig. 4
figure 4

Microstructure of Al-40% nc-HEAC sample after annealing at 425 °C: a bright-field TEM image; b HRTEM image and corresponding FFT patterns of b1 FCC phase and b2 BCC phase collected from grain interior

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Figure 5 shows microstructure and chemical element composition and distribution of the Al-40% nc-HEAC after annealing at 500 °C. As shown in Fig. 5a, the morphology of the sample annealed at 500 °C has grown from the initial spherical grains to equiaxed with an average size of ~ 41 nm. The diffraction ring of the inset shows that its microstructure is a single BCC structure without an obvious FCC structure. The superlattice reflections (marked by the red circle) in the FFT pattern (Fig. 5b1) obtained from the red rectangular position in the micrograph in Fig. 5b clearly shows the ordered BCC (B2) phase. The FFT pattern shown in Fig. 5b2 obtained from the green rectangular position confirms the presence of the disordered BCC phase. The B2 phase is highly coherent with the disordered BCC phase [29, 30]. The chemical compositions of Areas b1 and b2 are shown in Table 1, which highlights that the B2 phase is richer in Al and Ni, and the disordered BCC phase is richer in Fe and Cr. The elemental distribution in the annealed sample was further analyzed by EDS mapping (Fig. 5c). It can be observed that Al and Ni tend to segregate into B2 phase, and Fe and Cr tend to segregate into disordered BCC phase. This result is consistent with TEM observations and the results of the XRD analysis.

Fig. 5
figure 5

Microstructure of Al-40% nc-HEAC sample after annealing at 500 °C: a bright-field TEM image; b HRTEM image of ordered BCC(B2) and disordered BCC phase, b1, b2 corresponding FFT patterns collected from grain interior and EDS chemical analysis; c STEM image and EDS elemental mappings

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3.4 Discussion

The microstructure of the initial nc-HEAC is composed of FCC-CoCrFeNi, FCC-Al, and HCP-CoNi phases (Fig. 1). During annealing, the magnetic properties change with the microstructure evolution.

3.4.1 Formation of HCP-CoNi phase

We have detected ferromagnetic properties in the Al/CoCrFeNi nc-HEAC, which is origin from the laser-IGC CoCrFeNi nc-HEA. Unlike the CoCrFeNi HEA with a single FCC structure prepared by casting and other methods, the laser-IGC CoCrFeNi nc-HEA owns an additional CoNi HCP phase, similar to the laser-IGC CoCrFeNiMn nc-HEA [31], which is formed during the process of laser-evaporating CoCrFeNi target. The formation mechanism is that during the laser pulse, the plume is emitted from the CoCrFeNi HEA. The shock wave generated by the collision between the evaporated atoms and the inert gas atoms restricts the space of the atom plume and causes it to be supersaturated. Then, the target material atoms form a condensed phase and homogeneous nucleation to form nanoparticles. The existence of thermal effects and nano-size effects cause these nanoparticles to produce other nano-precipitated phases during their formation. These processes have been verified in previous researches [32,33,34,35,36,37].

3.4.2 Magnetic mechanisms in Al-40% nc-HEAC during heat treatment

When the annealing temperature is at 25–370 °C, the ferromagnetic behavior of Al-40% nc-HEAC is mainly contributed by the CoNi phase. The content of the CoNi precipitation phase increases with the annealing temperature increasing (Fig. 3c). It can be observed from Fig. 2a that the ferromagnetic properties of the nc-HEACs have been enhanced with the increase in the CoNi phase. The presence of magnetic elements is of great significance to the magnetic property of the alloy. Both Co and Ni are typical magnetic elements. The alloys composed of these two elements have strong ferromagnetic characteristics [38,39,40].

At 370–500 °C, the enhancement of magnetic properties is mainly due to the phase transition from FCC to BCC. In the early stage of annealing (200–370 °C), the Al atoms enter the FCC-CoCrFeNi HEA to form FCC-AlCoCrFeNi HEA. As the temperature rises, further diffusion happens in the composite, Ni atoms preferentially escape from the FCC-phase region and alloying with Al to form the Al, Ni-rich B2 phase. After that, the region with Ni atoms is severely lost, forming Fe, Cr-rich disordered BCC phase. The AlCoCrFeNi HEA with the FCC structure transforms into BCC-AlCoCrFeNi HEA. This possible phase separation can be explained based on the binary mixing enthalpy of the elements in the present alloy. In this AlCoCrFeNi HEA, Al and Ni have the highest negative mixing enthalpy (− 22 kJ·mol−1), which promotes the formation of the B2 phase. Previous studies reported that the FCC structure and the BCC structure in AlCoCrFeNi HEAs possess significantly different magnetic performances. The appearance of the BCC phase with the increase in Al content leads to the transition from the paramagnetic of the CoCrFeNi HEA to the ferromagnetic of AlCoCrFeNi HEA at room temperature [8, 20, 41].

When the temperature is above 500 °C, with the further increase in the annealing temperature, the proportion of the B2 phase increases (Fig. 3f). Kao et al. [42] explored the magnetic properties of as-cast, homogenized, and deformed AlCoCrFeNi HEA at different temperatures. The results show that the Al, Ni-rich B2 phase has lower ferromagnetic properties compared with the Fe, Cr-rich disordered BCC phase. Therefore, the decrease in ferromagnetic is due to the increase in the proportion of B2 phase in the AlCoCrFeNi HEA (Fig. 3f).

4 Conclusion

In this work, the Al/CoCrFeNi nc-HEAC consisting of FCC-Al, FCC-CoCrFeNi, and HCP-CoNi phases was prepared by laser-IGC technique. The magnetic property of the Al/CoCrFeNi nc-HEAC could be regulated by heat treatment. The saturation magnetization of the Al-40% nc-HEAC increases with the annealing temperature and reaches a peak value of 35.9 A·m2·kg−1 at 500 °C. Systematic microstructural-magnetic evolution investigations of the nc-HEAC were presented. Precipitated phase and diffusion-induced phase transition in the nc-HEAC during annealing were revealed in detail. In the early stage of annealing, the ferromagnetism is contributed by the CoNi precipitation phase. Meanwhile, the Al diffuses into the CoCrFeNi HEA nanoparticles to form FCC-AlCoCrFeNi HEA. With the increase in temperature, the diffusion-induced phase transition from FCC to BCC structure of the AlCoCrFeNi HEA further enhances the magnetic properties.