M. V. Degtyarev

Evolution of the structure and hardness of nickel upon cold and low-temperature deformation under pressure

The effect of the deformation temperature ensuring different mobility of dislocations on the change of stages of the structural state of single-crystal nickel has been studied. It has been shown that the deformation temperature affects the type of arising boundaries and the degree of deformation that corresponds to the transition of the material to a new structural state. The formation of microtwins and deformation bands in the structure at the liquid-nitrogen temperature not only retards the formation of a homogeneous submicrocrystalline structure but also leads to a lesser strain hardening.

Deformation and dynamic recrystallization in copper at different deformation rates in Bridgman anvils

An analysis of structural changes taking place in copper upon deformation by shear under pressure has been performed using the Zener-Hollomon formalism. This analysis has shown that the temperature—strain-rate limits of the stages of structural states are independent of the rate of the anvil rotation. It has been demonstrated that the size of structural elements depends on the temperature-compensated rate of deformation. The use of a low rate of the anvil rotation has allowed detecting a cyclic character of dynamic recrystallization upon shear under pressure. A leading growth of some grains occurs during postdynamic recrystallization if the deformation is stopped at the stage of partial dynamic recrystallization or at the stage of complete dynamic recrystallization when the recrystallization cycle is not complete after the end of deformation.

Recrystallization of the ultradispersed structure of pure iron formed at different stages of the deformation-induced strain hardening

Grain growth has been investigated during heating of pure single-phase iron with an ultra-dispersed structure of different types under conditions which exclude retardation by impurity atoms or by dispersed particles. The rate of growth mainly depends on the type of boundaries formed in this structure during a preliminary treatment, which determines the different kinetics of primary recrystallization. The thermal stability of submicrocrystalline (SMC) structure increases with increasing degree of deformation. In contrast to the materials with an impurity and carbide retardation, in pure iron an intermediate annealing of the SMC structure, which usually leads during low-temperature recrystallization to the formation of a honeycomb structure, does not influence thermal stability. The formation of thermally activated centers of recrystallization has been found to occur during heating of iron with a uniform isotropic SMC structure at the temperature of the onset of the recrystallization of moderately deformed iron. The formation of a honeycomb structure does not lead to a considerable reduction in the accumulated strain energy, and, during further heating, thermally activated nuclei of recrystallization are formed.

Structure evolution of pure iron upon low-temperature deformation under high pressure

Structure evolution of iron (99.97% purity) deformed by shear under pressure at 80 K in a medium of liquid nitrogen has been investigated. It has been found that, along with dislocation slip, twinning and development of deformation microbands become operative mechanisms of low-temperature deformation. This led to specific type of inhomogeneity of the structure in which, up to ultimately attained degrees of deformation, low-angle misorientations are retained and, unlike room-temperature deformation, no homogeneous submicrocrystalline (SMC) structure is formed. Twinning contributes to the refinement of structure elements that are more than 1 μm in size; the further refinement occurs by the dislocation-disclination mechanism and goes to the steady-state stage.

Effect of Annealing Temperature on the Recrystallization of Nickel with Different Ultradisperse Structures

Various structures (cellular, mixed, and submicrocrystaline) were realized in samples of single-crystal nickel (99.98 wt % purity) using shear deformation under a pressure at room temperature. The presence of microcrystallites in the nickel structure after deformation was shown to lead to the development of recrystallization during annealing in the temperature range of 250–350°C via both continuous and discontinuous mechanisms. In the case of the continuous mechanism, the microcrystallites formed during deformation are recrystallization centers; in the case of the discontinuous mechanism, the recrystallization centers are the thermoactivated nuclei formed during annealing. A nonmonotonous dependence of the average recrystallized-grain size on the heating temperature was found and causes for this dependence are discussed.

Recrystallization of nickel upon heating below the temperature of thermoactivated nucleation

The main mechanisms of grain growth upon low-temperature recrystallization of pure nickel (99.98%) with structures of various types formed upon deformation in Bridgman anvils have been studied. A decrease in the amount of the stored energy of deformation at the stage of submicrocrystalline (SMC) structure has been revealed using the method of differential scanning calorimetry. The isothermal annealings with durations of up to 64 h made it possible to show that the low-temperature recrystallization in both the mixed and SMC structures is developed via the growth of separate centers that are formed during deformation. As a result, no homogeneous submicrograin structure is formed in nickel upon low-temperature recrystallization.

Correlation between the copper structure and temperature-rate parameters of pressure-induced shear deformation

Interest in materials with nanoand submicrocrystalline structures stimulates the development of methods of intense deformation impact. Due to its high plasticity, copper is a model material for testing these techniques, and the evolution of the copper structure under large plastic deformation was studied in many works [1–5]. Change in the structure and properties of copper under large deformation differs from that in iron alloys. Indeed, the start temperature of copper recrystallization varies nonmonotonically upon an increase in the number of cycles of equal-channel angular pressing [1], a few large recrystallized grains are present in the structure immediately after equal-channel angular pressing [2], and the recrystallization process upon further heating is anomalous [1]. The same anomalous growth of a grain is observed upon heating copper undergoing pressure-induced shear deformation [3]. These effects can be attributed to the development of dynamic recrystallization whose structural indications are found upon both pressure-induced shear deformation of copper with e > 5 [4] and torsion of cylindrical copper specimens at room temperature [5]. Nevertheless, many authors disregard dynamic recrystallization and analyze the evolution of the copper structure similarly to the case of cold deformation [1, 3]. Dynamic recrystallization shows that deformation under these conditions may be treated as hot deformation. In this case, the material structure is determined by the temperature and rate of deformation rather than by its degree. The joint effect of the temperature and rate of deformation is represented by the Zener–Hollomon parameter [6]. The maps of structure states presenting the processes of structure formation for various Zener– Hollomon parameters upon hot deformation of fcc single crystals were published in [7]. Thus, this work is

Effect of temperature of HPT deformation and the initial orientation on the structural evolution in single-crystal niobium

The structural evolution and hardness of sing-crystal niobium with various initial orientations are investigated after its deformation in Bridgman anvils at room (290 K) and cryogenic (80 K) temperatures. It is shown that no twinning occurs upon cryogenic deformation; thin prolonged bands dividing the matrix into weakly misoriented regions are formed. The uniform-in-size structure of a nanoscale level (dav = 40 nm) is formed during cryogenic deformation after the maximum achieved true strain. The average microcrystallite size observed after room-temperature deformation is 120 nm.

Recrystallization of submicrocrystalline niobium upon heating above and below the temperature of thermally activated nucleation

The recrystallization of a niobium submicrocrystalline structure created by high-pressure torsion at room temperature has been investigated. It has been shown that continuous recrystallization begins at just 300°С. It has been characterized by the nonuniform growth of microcrystallites, which prevents the formation of a uniform submicrograin structure. The formation of thermally activated recrystallization nuclei at 900°C increases the nonuniformity of grain size and somehow refines recrystallized grains.

Mechanisms of the formation of a bulk superconducting phase MgB2 at high temperatures

The structure of MgB2 samples synthesized from magnesium flakes and a boron powder at temperatures of 900–1000°C has been investigated. It has been found that the samples have “dendrite-like” and layered structures formed as a result of the dissolution of solid boron in liquid magnesium followed by crystallization. The obtained structures have been analyzed within the theoretical concepts of crystallization from melt.