**3. Formation of Nanoglasses**

As was shown, both heat treatment and deformation can result in the formation of a heterogeneous amorphous structure. In recent years, the term "nanoglasses" has been used to describe a heterogeneous amorphous structure. Although, as stated above, a heterogeneous amorphous structure was observed in a number of systems by a lot of researchers, the term "nanoglass" was introduced in the work by Gleiter [38] quite recently. Today nanoglasses are the particular focus in world scientific literature. These materials, which di ffer from homogeneous amorphous, and nanocrystalline materials in the structure, can display unique functional properties: mechanical, magnetic, catalytic, and others.

For the first time nanoglasses were synthesized as a bulk sample by consolidation of amorphous powders under pressure. However, the samples obtained by this method had small geometric sizes, and under their obtaining it was extremely di fficult to avoid oxidation of the surface of amorphous nanoparticles and formation of micropores embrittling the samples. Action on an amorphous alloy obtained by melt quenching in the form of a ribbon, can be used as an alternative method of obtaining nanoglasses. The method of obtaining metallic glasses by melt quenching onto a moving substrate is one of the main methods of obtaining. Thereby ribbons with a thickness of 10–50 μm, width of several millimeters to tens of centimeters, and length of several meters are formed. Such alloys have significantly large sizes for both investigation of the structure and properties and use in industry. As stated above, heterogeneity regions resulting in a nanoglass state can be formed in the amorphous phase under different actions: heat treatment of different types, deformation (rolling, hydrostatic, and

quasi-hydrostatic compression, high-pressure torsion, and others) at different temperatures, and irradiation. The separation of the amorphous phase can occur at different scales. Thus, for example, the heating of metallic Zr40Ti10Cu50 glass before the onset of crystallization results in the separation of an amorphous matrix at the scale of about several nanometers [39], which subsequently results in the formation of a composite amorphous–nanocrystalline structure with an extremely small grain size (2–5 nm). At the same time, the separation scale is significantly larger in a number of other systems. It was found that, in the Fe90Zr10 alloy [9], long-term low-temperature annealing (at a temperature of 100 ◦C) leads to the decomposition of the amorphous phase with the size of the heterogeneity regions of about 25 nm. In the annealed Ni70Mo10 P20 alloy [17], heterogeneities sized of about 30 nm are formed below the glass transition temperature; and in Ni70Mo10B20 metallic glass [40] after annealing above the glass transition temperature, regions of an amorphous phase of different compositions up to 50 nm in size are formed.

The irradiation of amorphous alloys, for example, by fast neutrons, can also lead to the formation of an inhomogeneous amorphous structure consisting of regions and di fferent short-range orders. When the amorphous Pd80Si20 alloy was irradiated with fast neutrons with a dose of 5 × 10<sup>20</sup> neutrons/cm2, the formation of structural inhomogeneities was observed, which are clusters with increased electron density, surrounded by boundary regions with a reduced electron density. The diameter of the clusters together with the shell was 10–20 Å [41]. Irradiation can lead not only to the formation of inhomogeneities, but also to their reduction. For example, amorphous alloys (Mo60Ru40)82 B18 and Fe40Ni40 P14 B6 became brittle after isothermal annealing, which is associated with the formation of inhomogeneities. After irradiation, the plasticity returned to its original value.

The peculiarities of structure and properties of nanoglasses are being investigated in alloys of di fferent compositions [42–47].

### **4. Processes of Crystallization of Amorphous Alloys**

Both homogeneous amorphous alloys and nanocrystals crystallize at an increase in the temperature, with the parameters of the formed structure (morphology, phase composition, sizes of structural components, etc.) depending on both heat treatment conditions and sample history.

Crystallization of amorphous (Ni70Mo30)90B10 alloy [48] is an example of the typical transformation from an amorphous to a nanocrystalline state. Figure 1 illustrates bright-field (a) and dark-field (b) images of the microstructure of (Ni70Mo30)90B10 sample which was annealed for 144 h at 873 K. One can see that the nanocrystals nucleate uniformly over the sample and are randomly distributed over an amorphous matrix. An average nanocrystal size is 16 nm. Figure 2 shows a high-resolution image of the structure of (Ni65Mo35)90 B10 alloy sample annealed at 873 K for 72 h. A characteristic feature of the nanocrystal arrangemen<sup>t</sup> in an amorphous matrix is that the nanocrystals do not have regions contacting with each other. The crystals are isolated from each other by the regions of an amorphous matrix. The minimum thickness of a layer of the amorphous phase between the nanocrystals is ~2 nm.

**Figure 1.** (**a**) Bright-field and (**b**) dark-field electron microscope images of the microstructure of (Ni70Mo30)90B10 sample after annealing at 873 K for 144 h.

**Figure 2.** High-resolution electron microscope image of the microstructure of (Ni70Mo30)90B10 sample after annealing at 873 K for 72 h.

The fraction of the crystalline phase increases gradually during heat treatment. The formed nanocrystals are a solid solution of Mo in Ni. A nanocrystal size is several nanometers. As annealing duration increases, the grain size increases insignificantly, and then almost does not change. The composition of regions of the amorphous matrix changes at that; it gets enriched with components which are insoluble or have limited solubility in nanocrystals (Mo, B). A composition change of the amorphous phase leads to a change in its crystallization temperature, which results in the completion of nanocrystal growth. Such component redistribution resulting in a composition change in intercrystalline regions of the amorphous phase was observed in alloys of other compositions (Fe-Zr-B, Fe-Si-B-Cu-Nb) [49,50]. In some cases, the dependence of nanocrystal size on the distance from the surface was observed under the crystallization of the ribbons of amorphous alloys [51]. Thus, for example, in the subsurface regions of the above mentioned (Ni70Mo30)90B10 alloy annealed for 72 h at 873 K, an average size of the nanocrystalline phase grains is 20 nm and decreases to 17 nm at a depth of about 8 μm. The revealed difference in the size of nanocrystals (and their lattice parameter) along the sample depth is related to different chemical compositions of the subsurface and deep regions of the alloy.

A nanocrystalline alloy is two-phase; the structure consists of an amorphous matrix and nanocrystals uniformly distributed over the matrix, which have no direct contact with each other. The detailed studies of crystallization processes of amorphous alloys of Ni-Mo-B system [52,53] showed that the lattice parameters of nanocrystals with the fcc lattice in this system change depending on the isothermal annealing duration. The analysis of changes occurring demonstrated that, under primary crystallization, the composition of formed crystals differs from matrix composition, the composition of the remaining amorphous phase changes as crystallization proceeds. In the case of the alloys under study, the amorphous phase gets enriched with a refractory component. On the other hand, the composition of the crystals precipitated during annealing also undergoes some changes. Changes in the nanocrystal composition lead to changes in the lattice parameters. The dissolution of Mo in Ni leads to an increase in the lattice parameter of a solid solution, and dissolution of B leads to its decrease. As is well known [54], at 873 K the equilibrium concentration of Mo in Ni is 17 at.%, and that of B is less than 1 at.%. At the moment of formation, nanocrystals are a supersaturated solid solution of Mo and B in Ni. Under isothermal exposure, codiffusion of Mo and B from nanocrystals occurs as grains grow. Due to different chemical compositions of initial alloys at the initial moment, the compositions of nanocrystals in the alloys also turn to be different. Different values of supersaturation of solid

solution nanocrystals with Mo and B result in di fferent values of the lattice parameter, starting with exposure for 5 h. The maximum value of the lattice parameter is observed in the case of the highest content of Mo in (Ni65Mo35)90B10 alloy and the minimum content of B in (Ni70Mo30)95B5 alloy. At an increase in the concentration of B in (Ni70Mo30)90 B10 alloy as compared with (Ni70Mo30)95 B5 alloy or a decrease in the concentration of Mo in (Ni70Mo30)90B10 alloy as compared with (Ni65Mo35)90B10 alloy, nanocrystals are formed with a low value of the lattice parameter. The codi ffusion of Mo and B into the surrounding matrix occurs under annealing. Loss of the highest amount of Mo (together with B) from the nanocrystals in (Ni65Mo35)90 B10 alloy (as compared with other alloys) leads to an insignificant decrease in the lattice parameter. Since a lower amount of Mo leaves from the nanocrystals in (Ni70Mo30)90B10 alloy, one should expect the lattice parameter to increase due to B di ffusion into the amorphous matrix, occurring at the same time, which is observed in the experiment.

The formed nanocrystalline structure in the above alloys has high thermal stability. The analysis carried out demonstrated that the nanocrystalline structure exists until the nanocrystals are isolated from each other by the amorphous phase. As previously stated, the amorphous matrix is enriched with B and a refractory component (Mo) and has an increased stability as compared with that of the amorphous phase of an initial composition. As soon as the amorphous matrix between the crystals disappears, they begin to grow rapidly. Such a case is shown in Figures 3 and 4. Figure 3 displays bright-field (a) and dark-field (b) images of a large crystal. Figure 4 demonstrates a high-resolution electron microscope image of the crystals contacting with each other. One can see that in this case the boundary is a region with a thickness of several Å, which divides two adjacent crystals. An arrow marks the region of a direct contact of the nanocrystals with each other.

**Figure 3.** Decomposition of the nanocrystalline phase in (Ni65Mo35)90B10 alloy (873 K, 240 h; (**a**) bright-field and (**b**) dark-field images are in the reflection shown in the electron di ffraction pattern).

**Figure 4.** High-resolution image of the structure of annealed (Ni65Mo35)90B10 sample under the decomposition of the nanocrystalline structure. An arrow marks the region of a direct contact of the nanocrystals with each other.
