**1. Introduction**

The first metallic glass was produced in 1960 [1]. Since this moment, these materials have been provoking grea<sup>t</sup> interest as both non-crystalline metallic materials and the basis for the creation of composite amorphous-nanocrystalline materials. A special group of metallic glasses is bulk amorphous alloys, many of which have good mechanical properties. Among these alloys are high-strength Zr-based bulk amorphous alloys, which, in particular, can be used for medical applications such as struts for cardiovascular stents [2,3]. Zr-based bulk alloys were studied in a number of works [4–6]. The processes of crystallization of Zr-based bulk amorphous alloys were investigated mainly under heat treatment. At an initial stage of the devitrification of these alloys, the formation of metastable crystalline [7,8] and quasi-crystalline [9,10] phases was observed. Under heating or annealing, after the completion of the first crystallization stage alloys have an amorphous-nanocrystalline structure, with the fraction of the crystalline phase depending on heat treatment conditions. Another method of impact on the structure of bulk metallic glasses is severe plastic deformation. One of its main methods is high-pressure torsion. This action also leads to crystal formation in the amorphous phase, with crystal formation starting in the regions of plastic deformation localization, i.e., shear bands or their vicinity. Crystal formation in the places of plastic deformation localization is caused by an increase in the free volume fraction and, correspondingly, by enhanced values of diffusion coefficients in these regions [11–13]. The fraction of the nanocrystalline phase under plastic deformation also depends on treatment conditions, i.e., the value of applied pressure, the rate, duration, and temperature of deformation. In turn, the properties of a produced material depend on the formed structure and the

fraction of the crystalline phase. Plastic deformation is realized by the formation and propagation of shear bands. At that stage, nanocrystals being present in the structure can have an inhibitory action on the propagation of the bands [14].

In some studies of the processes of metallic glass crystallization, it was shown that the structure formed under crystallization depends significantly on the conditions of production of an amorphous alloy [15], as well as on the conditions of treatments (heat or deformation ones) of the amorphous structure both in the limit of an amorphous state and at early crystallization stages [15,16]. However, despite active studies of the processes of the devitrification of Zr-based alloys, treatment parameters necessary for the formation of an amorphous-nanocrystalline structure remain unknown. Therefore, the present work aims at carrying out a comparative study of devitrification processes of Zr55Cu30Al15Ni5 bulk alloy under heat treatment and deformation.
