S. V. Dobatkin

Severe Plastic Deformation of Amorphous Alloys: I. Structure and Mechanical Properties

The basic regularities of variation in the structure and mechanical properties of amorphous Ni44Fe29Co15Si2B10 alloy at severe plastic deformation (SPD) in a Bridgman cell at different temperatures are considered. It is shown that SPD is accompanied by homogeneous nanocrystallization, which is caused by the plastic flow mode. The transition from inhomogeneous mode of plastic flow to a qualitatively different one has been detected. The SPD structural model of deformational “dissolving” of crystals is proposed to explain why nanocrystals no more than 10 nm in size are observed during SPD. It is found that thermally activated nanocrystallization may occur at very low temperatures (77 K) under very high stress and with a high concentration of excess free volume.

Nanocrystallization of an amorphous Fe80B20 alloy during severe plastic deformation

The structural evolution of an amorphous Fe80B20 alloy subjected to severe plastic deformation at room temperature or at 200°C was studied. Deformation leads to the formation of α-Fe nanocrystals in an amorphous phase. After room-temperature deformation, nanocrystals are localized in shear bands. After deformation at 200°C, the nanocrystal distribution over the alloy is more uniform. Possible causes of the crystallization of the amorphous phase during severe plastic deformation are discussed.

Effective Temperature of High Pressure Torsion in Zr-Nb Alloys

Abstract Severe plastic deformation by the high pressure torsion (HPT) leads to the phase transitions and strong grain refinement. The starting α Zr-phase in Zr alloyed by 2.5 and 8 mass% Nb transforms into β + ω mixture. This β + ω phase mixture can be found in the equilibrium phase diagram at higher (effective) temperature (Teff = 620°C for Zr-2.5 mass% Nb and (Teff = 550°C for Zr-8 mass% Nb). The published papers on phase transitions during HPT are analysed and the values of effective temperature are estimated. Contrary to the increasing temperature, the increasing pressure slows down the diffusion and grain boundary migration. Therefore, the forced atomic movement during HPT produces the states equivalent to higher temperature, but not to the higher pressure.

Influence of equal-channel angular pressing on the structure and mechanical properties of low-carbon steel 10G2FT

Results are presented of the investigation of mechanical properties, microstructure, and phase composition of low-carbon steel 10G2FT (Fe-1.12Mn-0.08V-0.07Ti-0.1C) before and after equal-channel angular pressing (ECAP). It has been established that the ECAP of steel 10G2FT at T = 200°C in the case of the ferritic-pearlitic state and at T = 400°C in the case of the martensitic state leads to the formation of a predominantly submicrocrystalline structure with an average size of structural elements of approximately 0.3 μm, causes an increase in the strength properties, a decrease in the plasticity, and the localization of plastic flow. It has been experimentally shown that the initially martensitic structure after ECAP causes higher strength properties in comparison with the ferritic-pearlitic structure.

Structure and Mechanical Properties of Submicrocrystalline Copper Produced by ECAP to Very High Strains

The present work is aimed at linking the microstuctutral features obtained after severe plastic deformation via ECAP to the tensile behavior and thermal stability of pure (99.98%) copper processed by routes A and Bc to different number of passes. The main conclusion one can draw unambiguously from the currently available results is that the strain path exerts relatively little effect on the resultant tensile properties when the number of pressing is sufficiently large, although there have been some marked differences in crystallographic textures and distribution of grain-boundaries. The effect of the number of pressings on the tensile ductility is considerable.

Structure and fatigue properties of 08Kh18N10T steel after equal-channel angular pressing and heating

The structure of corrosion-resistant austenitic 08Kh18N10T steel is studied after equal-channel angular pressing (ECAP), heating, and subsequent cyclic tests. After ECAP, an oriented mainly subgrain structure with a structural element size of 100–250 nm and a high fraction of deformation twins forms in the austenite of the steel, and 42 vol % of lath martensite appears. Dynamic twinning, martensitic transformation, dynamic recovery, and even recrystallization take place in the 08Kh18N10T steel during cyclic deformation in the course of fatigue tests according to the scheme of repeated tension. The fatigue strength increases after ECAP due to the refinement and twinning of an austenite structure and the appearance of martensite. The fatigue limit is maximal after ECAP and heating at 550°C for 20 h due to a high annealing twin density in a predominantly austenitic recrystallized matrix, intense dynamic twinning, and martensitic transformation during cyclic deformation.

Effect of the initial state of a low-carbon steel on nanostructure formation during high-pressure torsion at high strains and pressures

The formation of nanocrystalline (d < 100 nm) and submicrocrystalline (100 nm < d < 1000 nm) structures in 09G2S and 10G2FT steels has been studied during cold and warm severe plastic deformation (SPD) by torsion at a hydrostatic pressure and subsequent heating. The steels were subjected to SPD in two initial structural states: ferritic-pearlitic and martensitic (bainitic) states. SPD at room temperature results in the formation of a cellular oriented nanostructure with individual equiaxed nanograins. As the deformation temperature increases to 500°C, the fraction of a grain structure and the average grain size increase. In the case of the initial ferritic-pearlitic state, the average grain size in the 10G2FT steel after SPD is larger (95 nm at 20°C and 120 nm at 500°C) than in the case of the initial martensitic structure (65 nm at 20°C and 85 nm at 500°C). The grain size in the 09G2S steel is slightly smaller than in the 10G2FT steel after SPD in both initial states. The coherent-domain sizes determined by X-ray diffraction agree well with the structural-element sizes determined by electron microscopy in the samples subjected to SPD. In the temperature range 20–500°C, torsional SPD results in the formation of a partly nanocrystalline structure and significant hardening of the metal. In both steels, the microhardness increases significantly with respect to the initial state: 2.5–3 times in the case of the initial ferritic-pearlitic state and 1.5–2 times in the case of the martensitic state.

Functional Properties of Nanostructured Ti-50.0 at % Ni Alloys

Ti-50 at % Ni alloy wire is subjected to cold-rolling (true strain e=0.3-1.9) and post-deformation annealing (200–700°C range). For all levels of cold work, the maxima of recovery strain and stress are obtained after annealing in the 350–400°C range. For the moderately-to-high cold-worked material (e=0.3-0.88), this annealing leads to polygonization, while for the severely cold-worked one (e=1.9), to the material nanocrystallization (grains of 50–80 nm in size). Nanocrystallized alloy generates 30 % higher recovery stresses (up to 1450 MPa) and 10% higher completely recoverable strains (more than 8 %) as compared to the polygonized alloy, while having comparable mechanical properties in tension.

Resistance of alloy Zr – 2.5% Nb with ultrafine-grain structure to stress corrosion cracking

The effect of severe plastic deformation of commercial alloy Zr – 2.5% Nb by the method of equal-channel angular pressing on the mechanisms and kinetics of stress corrosion cracking in a 1% “iodine – methanol” medium is studied. Quantitative comparative data on corrosion resistance of the alloy with ultrafine-grain structure after equal-channel angular pressing and with coarse-grain structure after the traditional treatment are obtained.

Stability of Ultrafine-Grained Microstructure in Fcc Metals Processed by Severe Plastic Deformation

The thermal stability of ultrafine-grained (UFG) microstructure in face centered cubic metals processed by severe plastic deformation (SPD) was studied. The influence of the SPD procedure on the stability was investigated for Cu samples processed by Equal-Channel Angular Pressing (ECAP), High-Pressure Torsion (HPT), Multi-Directional Forging and Twist Extrusion at room temperature (RT). It is found that HPT results in the lowest thermal stability due to the very high dislocation density. Furthermore, the effect of the low stacking fault energy of Ag on the stability is also investigated. It is revealed that the UFG microstructure produced in Ag by ECAP is recovered and recrystallized during storage at room temperature. The driving force for this unusual recovery and recrystallization is the high dislocation density developed during ECAP due to the high degree of dislocation dissociation caused by the very low stacking fault energy of Ag.