Alexander M. Patselov

One of the key challenges in materials science is the search for methods to control the structure and properties of various materials. It was previously established that compression during the ordering of the equiatomic CuAu alloy orients the short c-axes of the crystalline lattice (L10 superstructure) along the direction of the applied force. However, in practice, thin wires are predominantly used, for which only tensile processing is applicable. This work studied wire specimens of the equiatomic CuAu alloy ordered by cooling from 500 °C at a rate of 12 °C/h under a tensile stress of 40 MPa. A comparison was made with specimens ordered in a free state. For the experiments, miniature grips were designed, and all heat treatments were performed in vacuum. X-ray diffraction analysis revealed that the short c-axes of the tetragonal L10 superstructure are predominantly located in the plane of the wire cross-section, perpendicular to the direction of the tensile force. Dilatometric investigation revealed that a specimen ordered under tension subsequently contracts (by approximately ~0.8 %) when heated above the phase transformation temperature, while simultaneously expanding in diameter. This effect is explained by the difference in the volumes of the crystalline lattices of the ordered and disordered phases. It was found that during the heating of a specimen ordered under load, the maxima in the temperature derivatives of electrical resistance change in intensity and shift in temperature. A hypothesis has been put forward that ordering the CuAu alloy under a tensile load increases the disordering temperature and alters the thermal stability of the ordered CuAuI and CuAuII phases. The obtained result is consistent with literature data for the CuAu alloy ordered after compressive deformations.

The authors have studied for the first time the phase composition, microhardness and fine structure of the VT23 (α+β)-titanium alloy, with stable and metastable β-phase, after torsional deformation in a Bridgman chamber under a pressure of 4 GPa at room temperature. It has been found that the alloy microhardness, depending on the true degree of deformation under high hydrostatic pressure, changes along a curve with a maximum. The role of stress-induced βm→α" martensitic transformation in the formation of alloy structure, and microhardness under high-pressure torsion was revealed. The highest microhardness of the alloy with stable β-phase was 395 HV 0.05, and with metastable – 470 HV 0.05. At the same time, the maximum microhardness of metastable alloy, compared to stable alloy, was shifted to the region of lower true strain е=2.6. Using X-ray diffraction analysis, and transmission electron microscopy methods, made it possible to trace the evolution of alloy structure under high-pressure deformation consisting in grinding of α-, and α"-phase plates compared to the quenched state, as well as in the development of deformation βm→α", and α"→βm martensitic transformations. An increase in the degree of deformation by high-pressure torsion to е=7.7...7.9, regardless of the deformation stability of the β-phase, leads to a decrease in the alloy microhardness to a level of 185...205 HV 0.05. This is associated with the development of the dynamic recrystallisation process, and the formation of equiaxed α-phase nanoparticles with a size of 20...50 nm. The differences in the loading-unloading curves revealed by kinetic indentation, corresponded to the nature of the change in the VT23 alloy microhardness, depending on the quenching temperature and the true deformation degree.

Au-Co alloys with limited solubility were synthesized by the high-pressure torsion in boiling nitrogen at various anvil revolutions. Au and Co were initially in the state of a powder mixture in an equiatomic ratio. The obtained alloys were subjected to SEM fractography and XRD analysis in transmission X-ray synchrotron radiation, depending on the amount of strain. It is shown that the morphology of the fracture surfaces of the synthesized alloy depends significantly on strain. It is revealed that the mutual mixing of the components increases with strain. The images of the fracture surfaces of the Au-Co alloys testify that, as the strain and the number of anvil revolutions increase, a transition from ductile fracture, with inclusions of brittle intergranular fracture, to uniformly ductile fracture is observed over the entire thickness of the sample. A further increase in the strain and the number of anvil revolutions corresponds to the transition from the ductile type of the fracture surface to the brittle one. In addition, the fractography of the Au-Co alloys has revealed that the relief of the fracture surface becomes more homogeneous and that the size of the structural elements of the fracture surface decreases with increasing strain.

Measurements of the shear stress were carried out during mechanical alloying by high-pressure torsion of gold-cobalt powder mixing under cryogenic temperature at various pressures for the first time. The pressures were about 5, 8, 10 and 12 GPa. The Au–Co system is characterized by restricted solubility and difference in properties of the constituents. It was found that, in compari-son with the data obtained at room torsion temperature, the values of the shear stress are higher both at the stage of the initial intensive growth and subsequently, upon reaching saturation. With an in-crease in pressure of low-temperature mechanical alloying, a corresponding increase in the resistance to shear strain is observed at all stages of processing. In this case, the shape of the curves at each new pressure changes, as does the staging of the strain dependence. This reflects the extreme heter-ogeneity of the structure formation and phase composition processes of the non-equilibrium alloy. The most intense reflections in the X-ray diffractogram correspond to supersaturated fcc solid solu-tion of cobalt based on gold-base matrix. Also as a result of mechanical alloying, the components of the system are evenly distributed on the fracture surface of the alloy.

The paper studies the interface relief of copper–tantalum, copper–titanium and copper–copper joints. It demonstrates that interfaces could be considered as a fractal object. Fractal dimensions are calculated for these joints. A rule is established for the behavior of the fractal dimension during the transition from flat interfaces to wavy ones; this rule holds true of all joint pairs. Some behavior specifics are found for the fractal dimension of wavy interfaces of the copper–copper joint (which was substituted with Cu-Cu80Ni20 joint for convenience). The study shows that the fractal dimension in general decreases when welding modes are intensified. However, in the case of welding modes that are close in terms of energy input, certain peculiarities or deviations from such behavior can be observed. Graphical Abstract

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The paper studies the interface behavior of multi-layered Cu-Ta composites successfully produced by means of single-shot explosive welding technique. It is found that the transition zone may have three states at the same time: smooth interface, flat interface (consisting of individual cusps) and wavy interface also. The study shows the role of mutual solubility of initial elements in the formation of a welded joint structure. Based on electron microscopic studies, multi-layered composite is compared with two-layered Cu-Ta composites. Similarities of the interface microstructure elements and hardening behavior as well as reasons for the differences between them are discussed.

Self-organization processes that determine the response of a system to strong external influences are investigated. Explosion welding and torsion under pressure are selected as such...