Self-blocking of shear bands and the delocalization of plastic flows in amorphous alloys upon megaplastic deformation

Features of the formation of shear bands and nanocrystalline phases upon the megaplastic deformation of amorphous alloys based on iron, nickel, and titanium at room temperature in a Bridgman chamber are analyzed via transmission electron microscopy. It is shown that the transition from strongly localized to quasi-homogeneous plastic deformation occurs at a definite stage of the inhomogeneous plastic flow. Mechanisms based on the self-blocking of propagating shear bands by particles of the nanocrystalline phase that emerge due to a dissipative increase in the temperature along the front of shear bands are proposed for the delocalization of plastic flow.     

Tendency of metallic crystals to amorphization in the process of severe (Megaplastic) deformation

The main features of the transition of crystalline Ni₅₀Ti₃₀Hf₂₀, Ti₅₀Ni₂₅Cu₂₅, Zr₅₀Ni₁₈Ti₁₇Cu₁₅, and Fe₇₈B_8.5Si₉P_4.5 alloys with various tendencies to amorphization into an amorphous state upon melt quenching and in the course of severe deformation in Bridgman anvils have been considered. The crystalline state of these alloys has been produced using various methods of annealing. In the iron-based alloy, single-phase and two-phase crystalline states have been studied. The nickel- and titanium-based alloys after annealing were in a single-phase crystalline state; the zirconium-based alloy, in a two-phase state. It is shown that at the same degree of deformation the rates of amorphization of crystalline alloys differ substantially; namely, the single-phase crystalline titanium- and iron-based alloys amorphize easily, whereas the Zr-based alloy amorphizes only poorly, just like the two-phase iron-based alloy. It can be assumed that the tendency to deformation-induced amorphization of crystalline alloys and the corresponding crystalline phases is mainly determined by three factors: mechanical, thermodynamic, and concentration-related.     

An observation of amorphous-crystalline phase transitions at severe plastic deformation of the Ti<Subscript>50</Subscript>Ni<Subscript>25</Subscript>Cu<Subscript>25</Subscript> alloy

The features of structural and phase transitions during severe plastic deformation (in Bridgman anvils) of the amorphous Ti₅₀Ni₂₅Cu₂₅ alloy have been studied by X-ray diffraction and transmission electron microscopy. Application of successively increasing deformation has revealed three cycles of successive phase transitions from amorphous to crystalline state and vice versa. The results obtained are explained in terms of the superposition of the different channels of elastic energy dissipation, which are activated during severe plastic deformation.     

Structural Aspects of Deformational Amorphization of Ti<Subscript>50</Subscript>Ni<Subscript>25</Subscript>Cu<Subscript>25</Subscript> Crystalline Alloy under High Pressure Torsion

The evolution of the structure of the Ti50Ni25Cu25 crystalline alloy during high-pressure torsion at room temperature has been studied. The torsional moment variation curve as a function of the strain value was fixed in situ, which allowed directly observing the transition of the material from the crystalline state to the amorphous state during the HPT. It was found that the amorphization of the material in the course of the HPT begins on the grain boundaries and fragments of the crystalline phase. Amorphized boundaries form a “grain-boundary carcass” in the cells of which the high-defect nanocrystalline phase is formed. Growth of deformation leads to broadening of the “grain-boundary carcass,” loss of stability of the crystalline phase, and, as a consequence, to the phase transition “crystal → amorphous” state.     

Structure evolution of a zirconium-based amorphous alloy upon megaplastic strain in the bridgeman chamber

The structure evolution of the Zr₅₀Ni₁₈Ti₁₇Cu₁₅ massive metal glass upon megaplastic strain (MPS) in the Bridgeman chamber is investigated in a wide interval of strain degrees at room temperature. It is demonstrated that upon MPS with n = 1/2 rotation of the Bridgeman anvil, nanocrystalline phases are precipitated in the alloy. With increasing strain, these phases are dissolved. It is suggested that nucleation centers of nanocrystalline phases (nanoclusters) are formed in the amorphous matrix upon MPS under compression without shear in the Bridgeman chamber. These phases grow in the shear bands transforming into the nanocrystalline phase upon MPS.     

The Possibility of liquid-like and amorphous states existing in nano- and microregions of deformed metallic materials

The possibility of the periodic formation of a liquid-like state in nanoregions of highly nonequilibrium grain boundaries and other regions of defects in metallic materials upon superplastic deformation and intense plastic deformation is considered, based on the results from a thermodynamic analysis of a number of experimental data. Important differences between a liquid-like state and a glass-like (amorphous) state are also considered.     

Effect of the Temperature of Megaplastic Deformation in a Bridgman Chamber on the Formation of Structures and the Physicochemical Properties of Titanium (BT1-0)

The formation of regions corresponding to deformational fragments and dynamically recrystallized grains in the structure of the α-phase after torsion under high hydrostatical pressure at room and cryogenic temperatures is determined by means of transmission electron microscopy. Its applicability to describing defective structures of a two-phase mixture model is shown. In optimizing the results from mechanical and corrosion tests, it is found that two rounds of torsion treatment under hydrostatic pressure is sufficient to obtain the best possible properties.     

On the nature of the “double” yield point in Ti<Subscript>50</Subscript>Ni<Subscript>25</Subscript>Cu<Subscript>25</Subscript> alloy upon high-pressure torsion

The anomalous phenomenon of the “double” yield point attributed with the “crystal ? amorphous state” phase transition during high-pressure torsion of Ti₅₀Ni₂₅Cu₂₅ alloy has been analyzed experimentally and theoretically. Satisfactory correlation between experimental data and theoretical hypothesis has been found.     

Crystallization Features of Rapidly Quenched Ti50Ni20Cu30 Amorphous Alloys after High-Pressure Torsion

Physics of the Solid State - The effect of severe plastic deformation under high-pressure torsion (HPT) on the structural properties of rapidly quenched alloys belonging to the quasi-binary...