**1. Introduction**

Metallic alloys in an amorphous state (amorphous alloys or metallic glasses) were obtained for the first time by rapid melt quenching in 1960 [1]. The new material was of grea<sup>t</sup> interest, since it was determined at once that the properties of alloys in an amorphous state di ffer from those of crystalline materials of the same composition (for example, see [2]). Although many years have passed, interest in amorphous alloys persists. So far, there have not been any reliable models of an amorphous structure. There are a number of questions related to evolution of its structure and properties, as well as the possibility of obtaining nanocrystalline materials by thermal and deformation action on amorphous alloys. The properties of nanocrystalline materials and composite amorphous-nanocrystalline materials also di ffer from those of amorphous and traditional crystalline materials. Depending on chemical composition, nanocrystalline metallic materials are characterized by good plasticity and high viscosity, high strength and hardness, low moduli of elasticity, higher di ffusion coe fficients, as well as larger values of thermal expansion coe fficient and better magnetic properties as compared with traditional crystalline materials [3–6]. Partially crystalline alloys containing nanocrystals have hysteresis magnetic properties at the level of the best crystalline and amorphous alloys; at the same time, they have high saturation induction comparable to that of the best high-silicon electrical steel. Light Al-based amorphous-nanocrystalline alloys have high strength, with the values of yield strength able to reach 1.6 GPa [7] at good plasticity.

This review analyzes changes in the structure of the amorphous phase from preparation to the initial stages of crystallization. The transformation of a homogeneous amorphous phase into a heterogeneous phase, the dependence of the scale of inhomogeneities on the component composition, and the conditions of external influences are considered. The e ffect of changes in the amorphous

structure on the forming crystalline structure is shown. The crystallization processes of the amorphous phase, such as the homogeneous and heterogeneous nucleation of crystals, are considered. Particular attention is paid to an important feature of the processes of crystal formation in the amorphous phase, namely, compensation for volume mismatch. In the study of crystallization of metal glasses, this issue is usually not given su fficient attention. Meanwhile, the way of compensating for the volume mismatch has a decisive influence on the forming crystalline structure. When crystals form in the liquid phase (melt solidification), the di fference in the specific volume of the liquid and solid phases is quickly compensated due to the high mobility of atoms at high temperatures. In the amorphous phase, such a process is hampered due to significantly lower di ffusion coe fficients at room temperature or crystallization temperature, which does not exceed 500 ◦C for most amorphous alloys, that is noticeably lower than the melting temperature. As will be shown below, in this case, volume mismatch can be compensated by changing the sequence of formation of crystalline phases, changing the morphology of the crystals formed, the formation of nanocrystals with subsequent recrystallization, or the formation of pores. Data on the influence of thermal or deformation e ffects on the parameters of the crystal structure are presented. The e ffect of plastic deformation on the formation of nanocrystals and the features of the mechanical properties of the amorphous phase near the zones of plastic deformation localization are shown. In conclusion, there is evidence of the possibility of restoring the amorphous structure using the cryogenic cycling method.

Studies of the structure and crystallization processes of metal glasses were carried out on a lot of systems. The main attention in this review is focused on the alloys of such compositions in which the above features are most clearly manifested.

### **2. Structural Changes Occurring during Heating and Deformation**

An amorphous state is unstable, and the amorphous phase decays with the formation of the crystalline phases under heat treatment. In a number of cases, transformation of the structure is more complex, and the structure changes of the amorphous phase are observed before the crystallization onset: separation to regions di ffering in the chemical composition and short-range order. At the same time, a heterogeneous, amorphous structure is formed. The formed amorphous regions generally do not have a sharp interface, the transition may exhibit peculiarities of spinodal decomposition [8,9]. Structure changes in metallic glasses before the crystallization onset were discovered in a large number of alloys [10–18]. A lot of works [19–26] are devoted to the study of the separation of the amorphous phase and heterogeneous amorphous structure formation.

A heterogeneous amorphous structure can be observed both in an initial state immediately after melt quenching and after various external actions: heat treatment, deformation, irradiation, and others. For example, as-prepared amorphous Pd-Au-Si alloy is uniform, but regions di ffering in the chemical composition were formed in the sample after annealing at 400 ◦C [10]. In amorphous (Mo0.6Ru0.4)100-xBx such regions were revealed before the crystallization onset [15], and in amorphous Fe67Co18 B14Si1 alloy having a uniform amorphous structure after the quenching, heating to 400–600 K resulted in the formation of regions enriched with boron and regions with Fe–Co composition [16]. Of course, regions with di fferent chemical compositions are characterized by di fferent radii of the first coordination sphere (di fferent shortest distances between the atoms) and, correspondingly, di fferent short-range order. Thus, short-range order in the amorphous phase, of course, depends on the chemical composition, and it changes with the composition change. An amorphous structure can change significantly with the change in the component concentration; for example, the structure of amorphous (Zr0.667Ni0.333)1-xBx alloy changes with boron concentration change from 0 to 25 at.% [12]. It was shown that short range order type changes when x ≈ 0.05. In Fe-Zr system, the phase diagram of which contains two eutectic regions (at 10 and 76 at.% of Zr), the formation of the amorphous phases is possible near each of the eutectic points. Naturally, the amorphous Fe-based phase and amorphous Zr-based phase will di ffer from each other in the radius of the first coordination sphere and short-range order.

The amorphous phase structure can change significantly not only under heating, but under deformation. Structure changes under deformation action were discovered at the beginning of the investigation of amorphous alloys. In [27], it was found out that under rolling of amorphous Pd80Si20 alloys, a shift of the first peak of the structure factor towards smaller angles occurs, which indicates a change in the radius of the first coordination sphere. The studies showed also that the plastic deformation of metallic glasses at low temperatures and moderate loads is strongly localized, occurring in narrow regions, with shear bands, almost not a ffecting the main part of the amorphous phase. In these bands the structure, of course, should change.

Notably, in another group of works [28–30], the authors of which showed by the example of amorphous Zr-based alloys that in the absence of plastic deformation, tension leads to the change of a distance between the atoms, with these changes depending on the applied stress orientation. Amorphous Zr-based alloys become brittle and hardly undergo plastic deformation, that is why the mentioned works considered the region of elastic deformation only. Later the investigations of structure changes under plastic deformation were carried out for Pd40Ni40P20 alloy. The authors of [31] found out that after rolling deformation the amorphous alloy structure becomes anisotropic: after deformation, the distance between the atoms along the rolling direction increases, while that in a perpendicular direction almost does not change. The observed e ffect decreases with time. The structure change is accompanied by a decrease in the transverse sound velocity in the deformed samples compared with the initial samples. All the obtained results [28–31] are an evidence of the elliptical nature of the first coordination "sphere" in the deformed samples. Based on the studies carried out using X-ray radiation with di fferent wavelengths [31], it was also discovered that the structure changes are more pronounced in a subsurface region of the sample where deformation is maximum. Thus, the structure of the deformed samples may di ffer in cross-section. Structure changes during deformation were observed also under the use of small-angle X-ray scattering. Thus, for example, in Pd80Si20 alloy, deformation of 20% resulted in a significant increase in the intensity of small-angle scattering [32], and in Fe40Ni40P14B6 alloy [33], the existing inhomogeneities with a size of 32 Å increased up to 35 Å after 2-h annealing at 100 ◦C. Changes in the amorphous phase structure bring changes in the physical properties: the Curie temperature increase [34] or there is an emergence of two Curie temperature values (in the case of separation of the amorphous phase and formation of areas di ffering in the chemical composition and type of short-range order [35]), embrittlement [36,37], etc.
