A. Korneva

Phase transitions induced by severe plastic deformation: steady-state and equifinality

Abstract During severe plastic deformation (SPD), a steady-state is usually reached after a certain value of strain (i. e. number of passes during equal-channel pressing or number of rotations during high pressure torsion). The structure and properties of a material in a steady state (including composition of phases) do not depend on those in the starting state before SPD. In other words they are equifinal, and the production of lattice defects is in dynamic equilibrium with defect elimination. Moreover, the SPD-treatment at ambient temperature TSPD = 300 K is frequently equivalent to the heat treatment at a certain elevated temperature Teff > 300 K. For example, the composition of phases in Cu–Ni, Co–Cu and Nd–Fe–B-based alloys after high pressure torsion corresponds to the states at 200, 890 and 1 170 °C, respectively, and is rather insensitive to the high pressure torsion rate (between 0.2 and 2 rpm) and pressure (between 3 and 8 GPa).

Transformation of Hume-Rothery phases under the action of high pressure torsion

It has been revealed experimentally that high-pressure torsion induces phase transformations of certain Hume-Rothery phases (electron compounds) to others. High-pressure torsion induces the ξ → δ + ɛ reaction in copper-tin alloys with the appearance of the δ + ɛ phase mixture as after long-term annealing in the temperature range Teff = 350–589°C. The mass transfer rate driven by high-pressure torsion is 14–18 orders of magnitude higher than the rate of conventional thermal diffusion at the processing temperature THPT. This phenomenon can be explained by an increased concentration of defects (in particular, vacancies) in the steady state under high-pressure torsion, which is equivalent to an increase in the temperature.

Orientation Mapping Study on the Inhomogeneous Microstructure Evolution during Annealing of 6013 Aluminum Alloy

Orientation mapping in transmission electron microscope was successfully applied to study microstructural changes at the initial stage of recrystallization in the aluminum alloy with a bimodal second-phase particle distribution. The alloy samples were reversibly cold rolled resulting in the formation of laminar structure with zones of localized strain around large second-phase particles. Orientation mapping and in-situ investigations carry information about the processes which are active in the deformation zones during annealing.

Microstructure and properties of hard magnetic FeCr30Co8 alloy subjected to plastic deformation by complex loading

Abstract The microstructural, mechanical and magnetic properties of FeCr30Co8 alloy in the a state were examined after upsetting, followed by torsion. The temperatures (750, 800, 850 and 900°C) and deformation rates corresponded to the superplasticity conditions of the Fe–Cr–Co alloys. Due to the torsional deformation, which was applied only to the bottom of the samples, a gradient microstructure was formed in the sections that were parallel to the upsetting direction. In the temperature range from 750 to 850°C, the deformation led to intermetallic r-phase precipitation. The maximum refinement of the microstructure and the maximum r-phase precipitation were observed in the material that was deformed at 800°C. Measurement of the magnetic properties showed a decreasing coercive force in the deformed alloy, in contrast to the mechanical properties of the samples after the magnetic treatment.

Microstructure and some properties of FeCr25Co15 alloy subjected to plastic deformation by complex load

Abstract The microstructural, mechanical and magnetic properties of FeCr25Co15 alloy were examined after upset followed by torsion. Deformation was carried out at temperatures that corresponded to the appearance of α + γ phases. During the process, the initial lamellar microstructure of the material was transformed into a globular one. In the sample sections parallel to the upsetting direction a gradient microstructure, with a layer of submicron grains in the highest deformation zone, was formed. The thickness of the zone did not significantly depend on the deformation temperature. Measurements of the mechanical properties after magnetic treatment showed that the plasticity and strength of the deformed samples increased in comparison with the high coercive state.

Gradient Microstructure of FeCr30Co8 Hard Magnetic Alloy Subjected to Plastic Deformation by Complex Loading

Magnetically hard Fe-Cr-Co-based alloys are distinguished by their good ductility, excellent magnetic properties and low cost. Their superior magnetic properties are obtained by magnetic treatment and multistage tempering, which results in spinodal decomposition of the solid solution into the isomorphous α1 and α2 phases. However, the α1+α2 microstructure causes a reduction in the plasticity and strength of the material. It can often be advantageous for permanent magnets to maintain fine magnetic properties throughout their volume along while retaining good mechanical properties only in the subsurface layer. To improve the mechanical properties of the latter, FeCr30Co8 samples were deformed in tension combined with torsion. Loading was applied at 750°C, which ensured that the conditions for superplastic deformation were fulfilled. Here, we present the results of microstructure investigations of the samples treated in the aforementioned manner. Observations of the longitudinal section of the samples showed the formation of a gradient microstructure with the maximum grain refinement in the surface layer and the characteristic rotation of the elongated α phase grains from positions nearly perpendicular to the tension axis at the surface to positions tilted at approximately 45º to the tension axis inside the material. Deformation at superplastic conditions also activated precipitation of the σ intermetallic phase, particularly in the areas of highest deformation. The refinement of the microstructure and precipitation of the σ-phase resulted in a significant increase in hardness at the surface of the FeCr30Co8 samples.

Microstructure of Hard Magnetic FeCr22Co15 Alloy Subjected to Tension Combined with Torsion at High Temperatures

The paper presents the results of microstructure evolution studies of hard magnetic FeCr22Co15 alloy, destructed by tension and torsion at 800 and 850°C. The temperatures and deformation rates corresponded to the condition of superplasticity of Fe-Cr-Co alloys. Observations of longitudinal section of deformed samples in scanning electron microscope showed a formation of weak gradient microstructure with highest grain refinement in the surface layer of material. Precipitation of intermetallic σ-phase was also observed, with its maximum amount in zones of the highest deformation.

Continuous and Discontinuous Recrystallization of 6013 Aluminum Alloy

In situ orientation mapping using TEM and calorimetric measurements were carried out to investigate the annealing behavior of cold-rolled 6013 aluminum alloy. The recrystallization of the material can be considered to be a number of processes that correspond to two separate stored energy release peaks. In the temperature range of the peak 1, the deformation zones around the large second-phase particles acted as sites for particle-stimulated nucleation. In the matrix, at the same time, some elongation of grains occurred. The elongated matrix grains appeared because of the reduction of the dislocation density and the annihilation of some low-angle grain boundaries between chains of subgrains lying in layers parallel to the sheet plane. The matrix processes in this temperatures range can be considered forms of continuous recrystallization. The matrix high-angle grain boundaries started to migrate at the temperature range of the peak 2. They moved mostly in the direction normal to the sheet plane. Heating of the sample for an appropriate time at those temperatures resulted in the complete discontinuous recrystallization of the material. The recrystallized microstructure was dominated now by elongated grains, which were a few times thicker than those obtained by annealing at the temperatures of the peak 1.

Gradient Microstructure of FeCr30Co8 Hard Magnetic Alloy Subjected to Plastic Deformation by Tension Combined with Torsion at 700 and 720°C

The article presents the results of microstructure evolution studies of the FeCr30Co8 hard magnetic alloy, subjected to deformation by tension and torsion at 700 and 720 °C. The observations in the longitudinal section of the samples in the scanning electron microscope (SEM) showed a formation of gradient microstructure with the maximum grain refinement in the surface layer of the material. The EBSD examination confirmed the refinement of structure in the surface layer and the presence of sub-grained structure of the material. A little larger refinement of α phase grains was observed at temperature of deformation 700 °C than at 720 °C. However, the deformation was inhomogeneous along the whole longitudinal section of the sample. The highest deformation degree resulted from the torsion.

Microstructural changes in Cu-5.8 at.% In alloy caused by high pressure torsion

Abstract Cu-5.8 at.% In alloy annealed at 380 °C for 400 h was subjected to high pressure torsion deformation at room temperature. Detailed microstructural studies were performed using scanning and transmission electron microscopy as well as X-ray diffraction. High pressure torsion resulted in strong grain refinement of the Cu matrix (to the nm range), while there was no refinement or dissolution of δ-phase (Cu7In3). The volume fraction of δ-phase after straining remained almost unchanged, thus indicating its strong stability under high pressure torsion.