**10. Amorphous Structure Rejuvenation**

As already mentioned, amorphous alloys have a number of good properties [129–134]. Some of them (for example, magnetic properties) can be increased significantly by low-temperature annealing, annealing in a magnetic field, etc. Such treatments, however, often result in the embrittlement of these materials, which limits dramatically the possibilities of their practical application. High-pressure torsion was used to recover plasticity [135–137]. It was shown in [137] that the pair distribution function of atoms changes after deformation. This indicates a noticeable redistribution of atoms, leading to structure change. The authors of [137] considered that this structure variation is caused by local heating because of the deformation. A rising degree of structure disordering after deformation was found also in [138].

Last years, a new method for recovery of plasticity was found; it is cycling in a temperature range between room or elevated temperature and the temperature of liquid nitrogen [139]. This method is called rejuvenation. The main idea of cryogenic thermal cycling is a variation of the structure under the action of stresses caused by a non-uniform change in the thermal expansion coe fficient (TEC) in a sample. A heterogeneous amorphous phase contains regions with di fferent chemical composition, density, short-range order type, etc. Since such non-uniform regions are characterized by di fferent value of TEC, an abrupt change in the temperature under thermal cycling will induce stresses which cause irreversible local atomic restructurings [140,141]. The researchers succeeded in recovering plasticity in the Zr55Cu30Al10Ni5 alloy by this method [140]. The authors of this work, however, did not reveal any significant changes in the structure. The method of cryogenic cycling aroused a big interest [142–144]. The idea of induction of stresses in the alloy with the non-uniform distribution of the thermal expansion coe fficients turned to be very promising. The use of this method allowed for the observation of a structural change in inhomogeneous amorphous Zr46Cu38Al8Ag8 alloy [145], as well as completely recovering the amorphous phase in partially crystalline Al88Ni6Y6 and Al87Ni8Gd5 alloys [146,147]. The authors of the latter work used the idea of increasing the di fference in the thermal expansion coe fficients due to the formation of a small number of nanocrystals in the amorphous phase and the subsequent cryogenic thermal cycling of a structure consisting of the amorphous phase and Al nanocrystals uniformly distributed over it. Figure 19 illustrates the initial parts of the X-ray di ffraction patterns of Al88Ni6Y6 alloy before (a) and after (b) the cryogenic thermal cycling (60 cycles). One can see that the intensity of the reflection corresponding to nanocrystals (curve 5) decreases significantly during cryogenic cycling. The authors of [146,147] did not observe any signs of the crystalline phases with an increase in the treatment duration. So, it was shown that the method of cryogenic rejuvenation actually enables recovering an amorphous structure in partially crystalline alloys, thus increasing the plasticity of a material.

**Figure 19.** An initial part of the X-ray di ffraction pattern of Al88Ni6Y6 sample after deformation (**a**) and after deformation plus cryogenic cycling (**b**): (1) experimental spectrum, (3,4) diffuse reflections corresponding to 2 amorphous phases, (5) (111) Al nanocrystal reflection, (2) sum of 3–6 lines [reproduced from Mater. Let. 2019, 240, 159 with permission from Elsevier, 2020] [146].

### **11. Possibility of Controlling the Structure of Fully or Partially Crystallized Samples**

An important consequence of the investigation results described above is the possibility of controlling a structure formed under crystallization. As shown above, both the phase composition [148] and parameters of the formed structure [106–108] depend on the history of a sample and the crystallization conditions. Also, it was found out that, for example, in amorphous alloys of Ni–Mo–P and Ni–Mo–B systems, quite di fferent crystalline structures arise under the crystallization above or below the glass transition temperature [149–151]. This di fference is caused by di fferent structures of the amorphous phase in these temperature ranges. When carrying out crystallization under the conditions when it is preceded by separation of the amorphous phase, one can form a nanocrystalline structure which will not be formed under conventional annealing (for example, in Ni-Mo-B system [40]). Another example is ferromagnetic Fe72Al5P10Ga2C6B4Si1 alloy with good magnetic properties, where a nanocrystalline structure is formed above the glass transition temperature only [152,153]. These structures also include the so-called SS-phase which was discovered under the crystallization of a number of Fe-, Co-, Ni-, and Pd-based amorphous alloys in 1976 [154], when there was no term "nanostructures". Another example is [155], where the possibility of obtaining an amorphous sample with the crystalline surface (Figure 20) or a crystalline sample with the amorphous surface (Figure 21) was demonstrated by an amorphous alloy of Fe-B-P system. The materials obtained by this approach will be characterized by di fferent physical properties. The latter example shows how the knowledge of processes occurring within an amorphous state and under the crystallization of amorphous alloys allows obtaining di fferent types of structures.

**Figure 20.** Microstructure of Fe83B10P7 alloy sample partially crystallized in-situ [reproduced from Physics of The Solid State 1991, 33, 2527 with permission from Pleiadis Publishing, 2020 ] [155].

**Figure 21.** Microstructure of pre-annealed Fe83B10P7 sample partially crystallized in situ [reproduced from Physics of The Solid State 1991, 33, 2527 with permission from Pleiadis Publishing, 2020] [155].
