The Effect of a Coating on the Crystallization of Multicomponent Co-Based Amorphous Alloys

Abstract

The effect of protective coatings on the crystallization of amorphous Co-based alloys was studied using the methods of X-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. The crystallization of the amorphous alloys was studied on as-prepared samples, deformed samples, and deformed samples with a protective coating. After heat treatment, the fraction of the crystalline phase in the pre-deformed samples was higher than in the undeformed samples. When using a protective coating, the fraction of nanocrystals formed during heat treatment increased. The size of the crystals formed in deformed samples was smaller, and in the deformed samples with a protective coating, they were slightly larger than in the corresponding initial samples. The reasons for the differences in the formed structure in the amorphous alloys under study are discussed in terms of free volume.

1. Introduction

Composite amorphous–nanocrystalline alloys are a relatively new class of materials. The structure of partially crystalline alloys consists of nanocrystals and a surrounding amorphous matrix [1,2,3]. These materials are usually obtained by the controlled crystallization of amorphous alloys [4,5,6]. Most often, a nanocrystalline structure is obtained using heat treatment, and in recent years, deformation effects on the amorphous phase have also been used for this purpose [7,8,9,10,11,12,13,14,15,16,17,18]. Nanocrystals are usually formed by a primary crystallization reaction; the composition of nanocrystals and the surrounding amorphous matrix turn out to be different. During the formation and growth of nanocrystals, alloy components that are not part of the nanocrystals diffuse from the reaction front deep into the amorphous phase. The amorphous matrix changes its composition, the crystallization temperature of such an amorphous phase turns out to be higher, and an increase in the temperature is required for its crystallization. The presence of an amorphous layer ensures the stability of the amorphous–nanocrystalline structure.

Depending on the crystallization conditions, the crystal structure formed can have significantly different parameters: crystal size, phase composition, relative position or orientation of structural components, and crystalline phase fraction; in this case, the formation of metastable phases is often observed [19,20]. One example of the formation of different crystal structures is the widely studied Fe-B alloy. During its crystallization, eutectic colonies or a grain structure can be formed, and the crystal size can differ by an order of magnitude [21,22]. All these parameters affect the properties of the material. Thus, in Fe-based alloys, the coercivity can change by several times [23], and significant changes in mechanical properties occur in Zr-based alloys [24]. Numerous studies of the properties indicate their significant dependence on the parameters of the crystallized structure (the number, size, and location of nanocrystals, and the presence of orientational relationships between the lattices of the resulting phases). Currently, the search for ways to obtain amorphous–nanocrystalline structures is one of the most important areas of research into nanomaterials.

As shown in [25], the transformation of the amorphous phase into crystalline phases can occur through one of the following reactions: primary crystallization (formation of a crystal with a composition different from the composition of amorphous matrix), eutectic crystallization (simultaneous formation of two crystalline phases) and polymorphic crystallization (composition of the crystal and the amorphous phases are the same). During primary crystallization, a gradient of concentrations of components appears in front of the growing crystal. Since the reaction involves atoms diffusing over significant distances, the rate of crystal growth decreases over time. The main difference between polymorphic and eutectic crystallization is due to the fact that in the first case, the composition of the resulting crystal corresponds to the amorphous phase composition, and the redistribution of components does not occur. During eutectic crystallization, diffusion over long distances must occur, so one can expect that the growth rate will decrease as the distance between the components of the eutectic increases. In addition to crystal growth, crystal nucleation is an important process during crystallization. Crystal nucleation can occur by a homogeneous or heterogeneous mechanism [26,27]. The ratio of nucleation and growth rates is an important question in the formation of a structure. At a low nucleation rate and a high growth rate, a small number of crystals are formed in the amorphous matrix, which can reach significant sizes. In the case of a high nucleation rate and a low growth rate, a very large number of crystals are formed in the amorphous phase, the growth of which (especially according to the primary crystallization mechanism) quickly stops due to the overlap of diffusion fields, as a result of which a fine-crystalline structure will be formed in the material. As a rule, it is by this mechanism that a nanocrystalline structure based on metallic glasses is formed.

The development of methods for producing nanostructures with different structural parameters is based on the study of crystallization of the amorphous phase under various influences. The amorphous–crystalline structure formed under heat treatment usually consists of crystals that are randomly distributed in an amorphous phase [28]. For nanocrystals to be formed, the crystallization process must occur at a high nucleation rate and low crystal growth rate. In a number of systems, these conditions are easy to achieve. However, if the alloy contains rapidly diffusing components, the crystal growth rate is high. Such alloys include, in particular, the group of alloys such as Fe-B, Fe-Si-B [29], Co-Si-B [30] and others [31].

To obtain a nanostructure in these alloys, alloying components are added to their composition to ensure the required conditions are fulfilled: to provide a large number of potential sites for the nucleation of crystals and a low rate of their growth. Since, as noted above, generally, crystallization of the amorphous phase follows the primary mechanism of crystallization in a diffusion manner, components characterized by a low diffusion coefficient are introduced to slow down crystal growth. The first example of producing such an alloy was an Fe-Si-B alloy, to which Nb and Cu were added. The nanocrystalline Fe_73.8Si₁₃B_9.1Cu₁Nb_3.1 alloy (Finemet) obtained by heat treatment [28] was the ancestor of a large group of ferromagnetic nanocrystalline materials. Nanostructures were obtained in a similar way in a number of amorphous alloys [32,33,34]. However, the addition of alloying components that promote the formation of a nanostructure can lead to deterioration in the properties of the material. For example, in the Finemet alloy, the presence of Nb and Cu worsens the soft magnetic properties of Fe-based alloys, reducing the saturation magnetization.

Another method to obtain a nanocrystalline structure from an amorphous phase is plastic deformation. Currently, there are many works on both the features of plastic deformation of the amorphous phase and the conditions for nanocrystal formation under deformation [31,35,36,37,38,39]. Plastic deformation in amorphous alloys has a number of distinctive features. According to Hooke’s law, under elastic deformation, elongation depends linearly on stress. At higher loads, this dependence deviates from the linear law, in particular, for amorphous alloys. If the shape of the sample is not completely restored when the load is removed, a so-called mechanical hysteresis loop appears, indicating inelastic deformation of the material. The energy corresponding to the area of this loop is used to displace atoms in unstable positions. The magnitude of these displacements in amorphous alloys is significantly greater (about an order of magnitude) than in crystalline alloys. It has been established that the inelasticity of amorphous phase is related to the free volume in their structure: as the free volume decreases, the inelastic deformation also decreases [40]. Further plastic deformation of amorphous alloys takes place through the formation and propagation of shear bands. An important feature of plastic deformation of amorphous alloys is an increase in the free volume concentration in the shear bands, i.e., an increase in the average distance between atoms.

Hence, the deformation of amorphous alloys is highly localized and leads to the formation of shear bands where the processes of diffusion mass transfer are facilitated (the density of amorphous phase in the shear bands is lower than that in the surrounding amorphous matrix). The thickness of shear bands is 5–20 nm [41,42,43], their structure is complex, and changes in the amorphous phase structure can spread over long distances from the shear band, up to 200 nm. The amorphous phase in shear bands has a lower density compared to the surrounding amorphous matrix [43], and the difference in density can reach 10% [44]. In these regions, the processes of diffusion mass transfer are naturally facilitated. It was found that the diffusion coefficient in shear bands at room temperature was five to six orders of magnitude higher than in the surrounding amorphous matrix [45]. The degree of changes in the structure in the shear band naturally depends on the deformation conditions. The formation of nanocrystals starts in shear bands and their surroundings [31]. Under deformation, nanocrystals are formed even in alloys where they are not formed under heat treatment. As a result of deformation, the structure of the amorphous alloy becomes inhomogeneous and represents a kind of nanoglass; the amorphous phase consists of regions of different densities: the regions of shear bands (lower density) and the undeformed amorphous phase (higher density).

The studies showed that the state of the amorphous phase before crystallization could significantly affect the parameters of the crystalline structure formed. When studying light Al-based alloys, it was found that the nanocrystals formed during plastic deformation were smaller than those of the nanocrystals formed under thermal treatment; the fraction of nanocrystals in the deformed samples was higher than that in the samples subjected to thermal treatment [46]. It was also found that when using a combined treatment (annealing after preliminary deformation), the size of the nanocrystals turned out to be intermediate.

As noted above, in the regions of shear bands, the concentration of free volume is increased [47], i.e., they are zones of lower density. With an increasing degree of plastic deformation, the concentration of free volume increases [48,49,50,51]. The formation of nanocrystals in these areas occurs more easily. However, under heating or annealing, the free volume concentration decreases. A decrease in the free volume leads to a change in the properties of the material: an increase in Young’s modulus, thermal resistance, embrittlement, and a decrease in the viscosity, internal friction, superconducting transition temperature, etc. [52,53,54]. To retain the high physical properties of the material, one needs to avoid a decrease in the free volume. To prevent free volume from reaching the surface of the sample, the authors of [55] proposed using special protective coatings made of crystalline materials. They showed that sprayed coatings will be effective if the energy of vacancy formation in the coating material is greater than in the amorphous phase. Under such conditions, the release of free volume from the amorphous phase through the crystalline coating material turns out to be thermodynamically unfavorable. This idea was implemented in [56], where it was shown that applying a protective coating to amorphous Al-based alloys actually allowed one to maintain free volume in the amorphous structure; the parameters of the structure formed under crystallization in the uncoated samples and those with a protective coating turned out to be different.

The amorphous Al-based alloys belong to the group of materials in which a nanocrystalline structure is formed during both thermal and deformation treatment. It would be interesting to compare crystallization processes in deformed Co-based alloys (Co-Si-B system), where a nanostructure is formed during heat treatment only with the addition of alloying components. Amorphous Fe- and Co- based alloys on iron and cobalt have high magnetic properties. The introduction of alloying components can have different effects on the magnetic properties [57]. For example, the addition of Fe to amorphous cobalt-based alloys increases the saturation magnetization and decreases the coercive force [58]; with the addition of Pt [59] in amorphous alloys of the Co-Si-B system, a decrease in saturation magnetization is observed. Amorphous Co-based alloys have a lower saturation magnetization than Fe-based alloys but a higher Curie temperature [60], which allows the use of such magnetic materials at elevated temperatures.

This work aimed to study the effect of a protective coating on the crystallization of deformed amorphous Co-based alloys.

2. Materials and Methods

Ingots of the Co₆₇Si₁₂B₉Fe₇Nb₅ and Co₅₆B₂₀Fe₁₆Nb₈ alloys were prepared by arc melting under purified argon from pure (>99.9%) components; they were melted three times to homogenization before melt quenching in a high-purity argon atmosphere. The amorphous alloys were obtained through the rapid quenching of a melt in the form of ribbons. The cooling rate was 10⁶ K/s. The thickness of the as-prepared ribbons was 20–30 μm, and the width was ~1 cm. The composition of the as-prepared ribbons was monitored using local X-ray microanalysis by means of a Zeiss Supra 50VP scanning electron microscope (Zeiss, Jena, Germany). The accuracy of determining the composition for all components, except for boron, was about 0.1%. The thermal analysis of the alloys under study was carried out on a Perkin-Elmer DSC-7 differential scanning calorimeter (Perkin-Elmer, Waltham, MA, USA). The samples were heated at rates of 5, 10 and 20 K/min. The amorphous ribbons were subjected to plastic deformation at room temperature by multiple rolling on a laboratory rolling mill manufactured by VEB Schwermaschinenbau (Brandenburg, Germany). The degree of deformation ε was calculated using the formula

$$\epsilon =\frac{h_0-h_1}{h_0},$$

(1)

where h₀ and h₁ are the thickness before and after deformation, respectively. A protective Ta coating was applied to both sides of the deformed amorphous ribbons. The coating was applied by cathode sputtering. Before coating, the surface of the samples was cleaned of contamination. Cleaning was carried out in an ultrasonic bath for 1 min. Cathode sputtering of tantalum was carried out with argon ions at a voltage of 3 kV for 30 min. The coating thickness was 100–200 nm. The quality and thickness of the coating were assessed using a Zeiss Supra 50VP scanning electron microscope. Figure 1a shows an example of the sample with a deposited Ta layer, and Figure 1b displays the EDS spectrum of the alloy with a Ta coating. Figure 1a is a secondary electron image of the sample surface, and the Ta layer is indicated by the arrows. Figure 1b shows an EDS spectrum of the sample surface indicating the presence of tantalum.

The initial amorphous alloys and those after deformation (with and without a coating) were subjected to simultaneous isothermal annealing in a resistance furnace. The X-ray structural studies were carried out on a Siemens D-500 X-ray diffractometer using Co Kα radiation. Structure monitoring was carried out at each stage of sample processing (quenching, deformation, application of a protective Ta coating). When conducting X-ray diffraction studies, special substrates were used that did not produce their own reflections. To process the spectra, special programs were used to perform smoothing, background correction, and the separation of overlapping maxima. The size of the nanocrystals was estimated from the half-width of the diffraction line using the well-known Selyakov–Scherrer formula [61].

3. Results and Discussion

3.1. Thermal Characteristics of the as-Quenched Alloys

To determine the thermal characteristics and conditions of heat treatment, the method of differential scanning calorimetry was used. Figure 2 and Figure 3 illustrate DSC curves of the amorphous Co₆₇Si₁₂B₉Fe₇Nb₅ and Co₅₆B₂₀Fe₁₆Nb₈ alloys for three different heating rates: 20 K/min (curve 1), 10 K/min (curve 2), and 5 K/min (curve 3). The data are given for the first crystallization stage (there is one peak in the DSC curves) at which nanocrystals are formed.

Table 1. Temperature of the onset of the first crystallization stage of the amorphous Co₆₇Si₁₂B₉Fe₇Nb₅ and Co₅₆B₂₀Fe₁₆Nb₈ alloys.

Heating Rate, β, K/min	Temperature of Crystallization Onset, Tcr, K
	Co67Si12B9Fe7Nb5	Co56B20Fe16Nb8
5	712	743
10	719	752
20	727	763

shows temperatures of the onset of the first stage of crystallization of the amorphous alloys under study for different heating rates. With an increase in the heating rate, the temperature of crystallization onset (as well as the temperature of the peak) shifts to higher temperatures. It can be seen that the temperature of beginning of crystallization onset strongly depends on the alloy composition: the temperature of crystallization start for the amorphous Co₅₆B₂₀Fe₁₆Nb₈ alloy is higher than for Co₆₇Si₁₂B₉Fe₇Nb₅.

By using calorimetry data, the activation energy of the first crystallization stage of the amorphous Co₆₇Si₁₂B₉Fe₇Nb₅ and Co₅₆B₂₀Fe₁₆Nb₈ alloys was calculated. The activation energy of crystallization was determined by the well-known Kissinger method [62] using the equation

$$\ln \left(\frac{T^2}{\beta }\right)=\frac{E}{RT}+const,$$

(2)

where β is the heating rate, R is the gas constant, T is the temperature at which crystallization begins at a certain heating rate, and E is the desired activation energy of crystallization. The crystallization activation energy was calculated from the ln(T_κp²/β) − 1000/T_κp graphs, which give approximately straight lines. The slope of the graphs indicates the value of $\frac{E}{R}$. Figure 4 shows Kissinger plots for the amorphous alloys under study; the numbers indicate the following: 1—the Co₆₇Si₁₂B₉Fe₇Nb₅ sample; 2—the Co₅₆B₂₀Fe₁₆Nb₈ sample.

To confirm the results obtained using the Kissinger method, two other related calculation methods, the Ozawa method [63] and the Augis–Bennett method [64], were used according to the corresponding equations

$$\ln \left(\frac{1}{\beta }\right)=\frac{E}{RT}+const,$$

(3)

$$\ln \left(\frac{T}{\beta }\right)=\frac{E}{RT}+const$$

(4)

In this case, the activation energy was also calculated from the slope of the graphs ln(1/β) − 1000/T_κp and ln(T_κp/β) − 1000/T_κp (Figure 5 and Figure 6). In Figure 5 and Figure 6, the numbers indicate the following: 1—the Co₆₇Si₁₂B₉Fe₇Nb₅ sample; 2—the Co₅₆B₂₀Fe₁₆Nb₈ sample. The values of the activation energy of the first crystallization stage of the amorphous alloys under study are summarized in

Table 2. Activation energy of the first stage of crystallization of the amorphous Co₆₇Si₁₂B₉Fe₇Nb₅ and Co₅₆B₂₀Fe₁₆Nb₈ alloys.

Composition	Activation Energy of the First Stage of Crystallization, Ea, kJ/mol
	Kissinger	Ozawa	Augis–Bennett
Co67Si12B9Fe7Nb5	385	397	391
Co56B20Fe16Nb8	425	438	432

.

It can be seen that the values of the activation energy determined by the Kissinger, Ozawa, and Augis–Bennett methods are slightly different. The activation energy was higher in the case of using the Ozawa method, there was lower activation energy when using the Kissinger method, and the intermediate one was when using the Augis–Bennett method. The observed change in the activation energy with a change in the estimation method used is in good agreement with the literature data [56]. The results obtained showed that the activation energy of the first crystallization stage of the amorphous Co₅₆B₂₀Fe₁₆Nb₈ alloy was higher than that of the amorphous Co₆₇Si₁₂B₉Fe₇Nb₅ alloy (for all three of these methods). The kinetics of crystallization of amorphous alloys is largely determined by the chemical composition of the alloys. The addition of elements with a large atomic radius to amorphous alloys prevents the diffusion of atoms, leading to an increase in the energy barrier to crystallization and, therefore, to higher activation energy of crystallization. In the alloys under study, niobium had the largest atomic radius (Nb—1.429 Å, Co—1.253 Å, Fe—1.241 Å, Si—1.170 Å, and B—0.831 Å). As follows from Table 2, the activation energy of crystallization of the alloy with higher Nb content (Co₅₆B₂₀Fe₁₆Nb₈) is indeed greater than that of Co₆₇Si₁₂B₉Fe₇Nb₅ alloy. This indicates a higher thermal stability of the amorphous alloy with high Nb content.

3.2. Crystallization of Initial Samples after Quenching

Figure 7 illustrates an X-ray diffraction pattern of the initial samples. The X-ray diffraction pattern contains only a wide amorphous halo with no signs of crystalline phases. No signs of crystalline phases were found in the corresponding electron microscopic images.

Figure 8 shows X-ray diffraction patterns of the samples of the Co₆₇Si₁₂B₉Fe₇Nb₅ (1) and Co₅₆B₂₀Fe₁₆Nb₈ (2) alloys after isothermal annealing at 823 K for 1 h. These conditions correspond to the completion of the first crystallization stage of the Co₆₇Si₁₂B₉Fe₇Nb₅ alloy and to the first crystallization stage for the Co₅₆B₂₀Fe₁₆Nb₈ alloy. These annealing conditions were chosen in order to study only the beginning of crystallization of both alloys. An increase in temperature (to complete the first stage of crystallization of the Co₅₆B₂₀Fe₁₆Nb₈ alloy) could lead to the development of the second stage of crystallization in the Co₆₇Si₁₂B₉Fe₇Nb₅ alloy. X-ray diffraction studies showed that a phase with a bcc lattice was formed in both alloys at the first crystallization stage. This phase is atypical for Co-based alloys. Usually, during the crystallization of amorphous Co-based alloys, precipitates of Co or its borides are formed [65,66]. Crystals of Co borides and silicides are often formed at subsequent stages of amorphous phase crystallization [67]; the simultaneous formation of Co crystals of various modifications (α-Co with an hcp lattice and β-Co with an fcc lattice) can also be observed [68]. In the alloys under study, at subsequent crystallization stages, the formation of Co phases with hcp and fcc lattices is also observed; however, the first phase formed during heating is the phase with a bcc lattice.

The formation of a phase with a bcc lattice in Co-based alloys was previously observed only at a high concentration of the alloying element Fe (at a Fe:Co ratio of at least 1:2). Later, it was shown that this phase could be formed at lower Fe concentrations [69], in particular, in a concentration range in which, according to the phase diagram, its formation is impossible. However, it is important to note that the formation of the bcc phase at lower Fe concentrations was observed only if another alloying component with a bcc lattice (Nb) was present in the alloy. It was shown in [70] that the formation of the bcc phase in Co-based alloys alloyed with Fe and Nb was due to the structural similarity (short-range order type) of the crystal lattices of the alloying elements and the formed phase. If in a heterogeneous amorphous phase there are ordered regions, the short-range order of which corresponds to that of the formed crystalline phase, these regions can be sites for the facilitated formation of crystals with the corresponding lattice. In the Co-based alloys under study, these regions may be Nb, Fe, or Nb/Fe clusters.

Figure 9 shows an electron microscopic image of the structure of the Co₆₇Si₁₂B₉Fe₇Nb₅ alloy after the end of the first stage of crystallization. It can be seen that after the completion of the first crystallization stage, the structure contains bcc nanocrystals and the remaining amorphous phase. This structure is typical for the alloys under study and is similar to the structure of other multicomponent alloys with bcc alloying components [70].

In the alloys under study, the lattice parameters of the bcc phases formed are 2.822 Å and 2.837 Ǻ for Co₆₇Si₁₂B₉Fe₇Nb₅ and Co₅₆B₂₀Fe₁₆Nb₈, respectively. Obviously, the difference in the lattice parameters is due to different chemical compositions and is related to the formation of a solid solution of alloy components in cobalt.

Under subsequent heating, crystallization of the remaining amorphous phase and the formation of a multiphase structure occur. Figure 10 illustrates an X-ray diffraction pattern of the Co₆₇Si₁₂B₉Fe₇Nb₅ alloy after isothermal annealing at 873 K. It contains reflections from crystals of two modifications of cobalt (with hcp and fcc lattices) and cobalt silicide (Co₂Si), and part of the amorphous phase is retained. With a further increase in the temperature (or time) of isothermal annealing, crystals of cobalt borides appear, and the sample becomes completely crystalline.

3.3. Crystallization of Deformed Samples

It was previously established that not only lattice parameters but also the size of nanocrystals depended on the concentration of alloying elements with a bcc lattice. It was shown that with an increase in the concentration of alloying elements, the size of crystals decreased, and the fraction of crystals increased. As noted above, the deformation of the amorphous phase can also lead to the formation of smaller nanocrystals. Therefore, it is natural to compare the structure formed during crystallization in the as-prepared and deformed amorphous phases.

To investigate the effect of deformation on the crystallization of amorphous alloys, the samples were subjected to plastic deformation by multiple cold rolling. Under this deformation, shear bands are formed, which are the regions of localization of plastic deformation characterized by an enhanced free volume concentration. Figure 11 shows an image of the surface of the deformed amorphous Co₆₇Si₁₂B₉Fe₇Nb₅ alloy, on which numerous steps are visible; they are the places where shear bands emerge on the sample surface.

After deformation, the samples remained amorphous. The deformed samples after rolling and the initial undeformed samples were subjected to simultaneous isothermal annealing. The annealing conditions were selected based on the differential scanning calorimetry data and corresponded to the onset of the first crystallization stage. The temperature of isothermal annealing was 723 K for the amorphous Co₆₇Si₁₂B₉Fe₇Nb₅ alloy and 758 K for the amorphous Co₅₆B₂₀Fe₁₆Nb₈ alloy; the annealing time was 1 h. At the initial crystallization stages, with a small fraction of the crystalline phase, the main changes in the X-ray diffraction patterns are observed in the region of the first diffuse maximum; therefore, these areas of the X-ray diffraction patterns are shown in the subsequent curves.

Figure 12 depicts X-ray diffraction patterns of the Co₆₇Si₁₂B₉Fe₇Nb₅ samples annealed at 723 K for 1 h. The inset shows the peak of the maximum; the numbers indicate the following: 1—the initial undeformed sample (black curve); 2—the deformed sample with a deformation degree of 1% (blue curve); 3—the deformed sample with a deformation degree of 20% (red curve).

X-ray diffraction patterns of the samples at the first crystallization stage are a superposition of diffraction reflections from the crystalline phase and diffuse scattering from the amorphous phase. To determine the fraction and size of the formed crystals, the overlapping maxima were separated into crystalline and amorphous parts. When separating the overlapping maxima, the parameters (positions and half-widths) of the initial amorphous phases (samples before the treatment) were used. The X-ray diffraction patterns (Figure 12, curves 1–3) with separated maxima are presented in Figure 13. Figure 13a is an X-ray diffraction pattern of the initial undeformed sample (curve 1 in Figure 13); Figure 13b is an X-ray diffraction pattern of the deformed sample with a deformation degree of 1% (curve 2 in Figure 12); and Figure 13c corresponds to an X-ray diffraction pattern of the deformed sample with a deformation degree of 20% (curve 3 in Figure 13). In Figure 13a–c, the numbers indicate the following: 1—an experimental curve (black curve); 2—a total calculated curve (the sum of curves 3 and 4, red curve); 3—a diffuse reflection from the amorphous phase (blue curve); 4—a diffraction peak (110) corresponded to bcc crystals (green curve).

An analysis of the integral reflection intensity showed that the intensity of the (110) diffraction line from bcc crystals (Figure 13a–c, curves 4) in the deformed samples was higher than in the initial sample. With an increase in the deformation, the intensity of reflection from bcc crystals increased, which indicates the formation of a larger number of nanocrystals in the deformed samples. The fraction of the crystalline phase increased with an increasing deformation degree. Based on the data obtained for the integral intensities of reflections from bcc crystals and scattering from the amorphous phase, the fraction of the crystalline phase in the samples under study was estimated. The estimation showed that the proportion of crystals in the initial sample was 14%. In the deformed sample with a deformation degree of 1%, the proportion of crystals slightly increased. In this sample, it was 17%. When analyzing the integral intensities of the maxima in the deformed sample with a higher deformation degree (20%), it turned out that the proportion of crystals increased noticeably. In this case, it was 20%. The crystal size was determined from the half-width of the diffraction line. In the initial sample and the deformed sample with a deformation degree of 1%, the crystal size was the same and was 10 nm. At a higher deformation degree, the size of the formed crystals turned out to be smaller—8 nm. The deformation did not lead to a change in the lattice parameter of the bcc crystals formed during heat treatment. The obtained data are summarized in

Table 3. Size and fraction of bcc crystals in the Co₆₇Si₁₂B₉Fe₇Nb₅ samples after isothermal annealing at 723 K for 1 h.

Deformation Degree, %	Lattice Parameter of bcc Crystals, Å	Size of bcc Crystals, nm	Fraction of bcc Crystals, %
–	2.833	10	14
1	2.833	10	17
20	2.833	8	20

.

The amorphous Co₅₆B₂₀Fe₁₆Nb₈ alloy was subjected to a lower degree of deformation. Figure 14 shows X-ray diffraction patterns of the Co₅₆B₂₀Fe₁₆Nb₈ samples annealed at 758 K for 1 h. The inset shows the peak of the maximum; the numbers indicate the following: 1—the initial undeformed sample; 2—the deformed sample with a deformation degree of 5%.

It can also be seen that the intensity increased in the deformed sample (Figure 14, curve 2), which indicates a slight increase in the fraction of bcc crystals. The estimation showed that the fraction of crystals in the deformed sample was slightly higher than in the initial sample, while the nanocrystals size was the same and was 7 nm. The lattice parameter of bcc crystals in the deformed and undeformed samples was the same.

Hence, the proportion of nanocrystals in the pre-deformed amorphous alloys after heat treatment was higher than in the undeformed alloys, and the nanocrystal size was smaller. The observed effect (a change in the fraction and size of nanocrystals) in the amorphous Co₆₇Si₁₂B₉Fe₇Nb₅ alloy was much greater than in Co₅₆B₂₀Fe₁₆Nb₈, which is obviously associated with different degrees of deformation.

3.4. Crystallization of Samples with a Protective Coating

As shown above, free volume plays a great role in the processes of crystallization of amorphous alloys. The results obtained in [71] indicate the possibility of obtaining a higher fraction of nanocrystals under conditions that would allow maintaining free volume. By analogy with [71], the crystallization of deformed amorphous alloys with a protective coating was studied. The studies were carried out on the amorphous Co₅₆B₂₀Fe₁₆Nb₈ alloy with a deformation degree of 5%. As mentioned in the Materials and Methods, the deformed Co₅₆B₂₀Fe₁₆Nb₈ sample was coated with a protective Ta coating (Figure 1); after the application, the sample remained amorphous. The coated and uncoated samples were subjected to isothermal annealing at 758 K for 1 h. Figure 15 shows X-ray diffraction patterns of the Co₅₆B₂₀Fe₁₆Nb₈ samples after annealing (1—the uncoated sample; 2—the sample with a protective Ta coating). As in previous cases, the experimental curves were divided into components (Figure 16a,b): diffuse scattering from the amorphous phase (curves 3 in Figure 16a,b) and diffraction reflection (110) from the bcc phase (curves 4 in Figure 16a,b).

It is clear from the obtained X-ray diffraction patterns that the peak intensity in the sample with a protective Ta coating is significantly higher than in the uncoated sample.

An analysis of the integral intensities of reflections from the bcc crystals and scatterings from the amorphous phase showed that the fraction of bcc crystals was 26% in the uncoated sample and 30% in the sample with a protective Ta coating. The size of the crystals in the uncoated sample was 7 nm, and in the sample with a protective Ta coating, it was slightly larger—7.5 nm. The lattice parameter of the bcc phase in the samples was the same. The obtained data are summarized in

Table 4. Size and fraction of bcc crystals in the Co₅₆B₂₀Fe₁₆Nb₈ samples after isothermal annealing at 758 K for 1 h.

Coating	Lattice Parameter of bcc Crystals, Å	Size of bcc Crystals, nm	Fraction of bcc Crystals, %
–	2.845	7	26
Ta	2.845	7.5	30

.

The state of the coating is undoubtedly important for the kinetics of crystallization and, in particular, it can play a decisive role in the kinetics of the removal of free volume from the sample to the surface. The authors of [72] observed the formation of a porous structure at the interface between a nickel based superalloy and a high-entropy rare-earth aluminate (Y_0.2Yb_0.2Lu_0.2Eu_0.2Er_0.2)₃Al₅O₁₂ coating. The coating thickness in this case was 5 µm. In our case, the thickness of the coating was significantly smaller (100 nm), and we did not observe the formation of pores in the area between the sample and the coating.

As a result of the studies, we found the following:

-

At the initial stage of crystallization of amorphous alloys of the Co-(Si)-B system alloyed with Fe and Nb, nanocrystals of a metastable phase with a bcc lattice were formed, the parameters of which depended on the composition of the alloy;

-

Preliminary deformation before crystallization contributed to a rise in the proportion of the nanocrystalline phase, while its lattice parameter remained unchanged;

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The proportion of the nanocrystalline phase in samples with a protective coating was higher than in samples without a coating.

The results obtained agree with the known data on the effect of free volume on the processes of crystallization of the amorphous alloys [43,44,71]. As mentioned above, plastic deformation leads to a noticeable increase in free volume and acceleration of diffusion mass transfer processes in the region of shear bands. The characteristics of shear bands (size, number, location, etc.) naturally depend on the method of deformation. When using the method of multiple rolling, the near-surface regions of the samples are subjected to the highest deformation. It was shown in [51] that with this deformation method, a change in the fraction of free volume in the near-surface and deeper layers of samples could differ by a factor of two to three. Since the amount of free volume depends on the deformation degree, crystallization processes in samples with a higher deformation degree should proceed faster, which is observed experimentally.

Under conditions of maintaining free volume, nanocrystallization processes should be facilitated. During heat treatment, the free volume leaves the amorphous phase by diffusion, annihilating on the sample surface. As mentioned above, to maintain the free volume in the amorphous phase, it was proposed in [55] to use protective crystalline coatings with a high energy of vacancy formation compared to that in the amorphous phase. Our estimate of the energy of vacancy formation in the Co₅₆B₂₀Fe₁₆Nb₈ alloy is 1.7 × 10²⁹ eV/m³. Since the energy of vacancy formation in a protective Ta coating is approximately 1.6 times higher than that in the alloy under study (2.7 × 10²⁹ eV/m³ in Ta and 1.7 × 10²⁹ eV/m³ in the alloy under study), this coating should effectively prevent the release of free volume to the surface sample. The results obtained (an increase in the nanocrystal fraction in the coated sample) confirm the effectiveness of using a protective coating to maintain the free volume and increase the fraction of the nanocrystalline component in the structure. This increase in the fraction of the nanocrystalline phase in the samples with a protective coating agrees with the previously obtained data for Al₈₇Ni₈Gd₅ [71] and Fe₇₈Si₁₃B₉ [73].

The possibility of increasing the concentration of free volume in the amorphous phase during plastic deformation and maintaining it using protective coatings provides new ways to control the parameters of the crystal structure being formed, and, as a result, to obtain new materials with record physical and chemical properties.

Thus, the important results of the study are the following:

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The coating of amorphous alloys with a crystalline layer proposed in [55], indeed, allow for maintaining the free volume in the amorphous phase,

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The free volume introduced during the deformation of an amorphous alloy helps to accelerate the process of nanocrystallization.

Knowledge of these features of the amorphous alloy crystallization opens up ways to create structures with the desired structural parameters and, as a consequence, with the desired properties.

4. Conclusions

The effect of protective coatings on the crystallization of the amorphous Co₆₇Si₁₂B₉Fe₇Nb₅ and Co₅₆B₂₀Fe₁₆Nb₈ alloys was studied using the methods of X-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. The crystallization of the amorphous alloys was investigated on as-prepared samples, deformed samples, and deformed samples with a protective coating.

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The activation energies of the first crystallization stage of both alloys were determined. The activation energy of crystallization of Co₅₆B₂₀Fe₁₆Nb₈ alloy is higher than that of Co₆₇Si₁₂B₉Fe₇Nb₅ alloy.

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The nanocrystals of a metastable bcc phase were formed in the amorphous alloy at the first crystallization stage.

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The proportion of the formed nanocrystalline phase depended on the degree of deformation. After the completion of the first crystallization stage, the fraction of the nanocrystalline phase in the pre-deformed Co₆₇Si₁₂B₉Fe₇Nb₅ alloy (deformation degree of 20%) was ~1.4 times higher than in the undeformed alloy.

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The proportion of the crystalline phase in the samples with a protective Ta coating was higher than in the samples without a coating. The formed crystals in the deformed samples were smaller, and in the deformed samples with a protective Ta coating, they were slightly larger than in the corresponding initial alloys.