Papers by Author: Sergey V. Dobatkin

Abstract: By application of thermomechanical controlled rolling and accelerated cooling, the carbon steel grain refinement is limited to levels of ~ 5 μm in steels. The strain assisted or strain induced transformation could be considered for the refining process. The present work, likewise, deals with grain refinement of medium carbon steel containing 0.45 wt pct carbon having different initial microstructure modified by either thermal and/or thermomechanical treatment (TM) prior severe plastic deformation. In case of TM treated steel, structure refinement was conducted in two steps. Preliminary structure refinement has been achieved due to multistep open die forging process which provided total strain of 3. Uniform and fine recrystallized ferrite structure with grain size of the order of 2-5 μm and with nest-like pearlite colonies was obtained. The further grain refinement of steel samples having different initial structure was accomplished during warm Equal Channel Angular Pressing (ECAP) at 400°C. The steel samples of different initial structure were then subjected to six ECAP pressing passes through die channel angle of 120°. The microstructure development was analyzed in dependence of effective strain introduced (εef ~ 2.5 - 4). Employment of this processing route resulted in extensive deformation of ferrite grains where mixture of subgrains and ultrafine grain was found regardless the preliminary treatment of steel. As straining increases the dynamic polygonization and recrystallization became active to form mixture of polygonized subgrains and submicrocrystalline grains having high angle boundaries. The straining and moderate ECAP temperature caused the partial cementite lamellae fragmentation and spheroidization as straining increased. The lamellae cementite spheroidization was more extensive in TM treated steel samples. The tensile behavior was characterized by strength increase for both structural steel states; however the work hardening behavior was modified in steel where preliminary TM treatment was introduced to modified coarse ferrite-pearlite structure.

Abstract: In the present study, ultra fine-grained low carbon steel samples were processed by equal channel angular pressing (ECAP). Mechanical properties of the specimens annealed statically at several temperatures were evaluated by tensile and hardness test. In addition, grain sizes of the specimens were measured by SEM-electron back scattering pattern (SEM-EBSP) and X-ray diffraction analysis. Differential scanning calorimetry (DSC) measurement also evaluated thermal reactions in anneal process of the specimen. As a result, the grain size was changed at the temperature between 550oC and 600oC drastically and the tensile strength also became lower at the same temperature. The relation between yield stress and averaged grain diameter of specimens obeyed the Hall-Petch relation except the normalized specimen. Behavior of grain growth and recovery in structural observation by EBSP corresponded to reaction signal of the DSC curve.

Abstract: Equal channel angular pressing (ECAP) was used for grain refinement and texture modification in the initial pressed Mg-Al-Zn alloy to study the possibility to enhance the low-temperature deformability of the material. The effect of different ECAP regimes by routes A, C, and BC on the submicrocrystalline grain formation, texture evolution, and plasticity of the alloy have been investigated. The ECAP of the alloy results in the formation of ultrafine grained structure with a grain size of 0.8-3.5 µm independent of pressing routes and regimes. The ECAP also drastically changes the axial texture by splitting the initial texture characterized by a sharp basal component to several more scattered orientations. The degree of the orientation scattering depends on the ECAP regime and route. It is proposed to estimate the effect of the texture on the yield strength and plasticity of the alloy after ECAP through generalized Schmid factors. The comparable calculated and experimental results are obtained only for yield strength.

Abstract: The effect of equal channel angular pressing (ECAP) on the structure and mechanical properties of Al-4% Mg-1.5% Mn-0.4% Zr and Al-4% Mg-1.5% Mn-0.4% Zr-0.4% Sc alloys in the initial as-cast state was studied. The ECAP processing was shown to lead to the formation of predominantly submicrocrystalline structure with an average grain size of 850 nm in the Al-Mg-Mn-Zr-Sc alloy and 1060 nm in the Al-Mg-Mn-Zr alloy. It is remarkable that both strength and ductility of the two alloys were enhanced by ECAP. The highest strength was observed in the Al–Mg–Mn–Zr–Sc alloy (UTS = 425MPa), in combination with elongation to failure of EL=17 %.

Abstract: The main functional properties (FP) of Ti-Ni Shape Memory Alloys (SMA) are their critical temperatures of martensitic transformations, their maximum completely recoverable strain (er,1 max) and maximum recovery stress (sr max). Control of the Ti-Ni-based SMA FP develops by forming well-developed dislocation substructures or ultrafine-grained structures using various modes of thermomechanical treatment (TMT), including severe plastic deformation (SPD). The present work shows that TMT, including SPD, under conditions of high pressure torsion (HPT), equal-channel angular pressing (ECAP) or severe cold rolling followed by post-deformation annealing (PDA), which creates nanocrystalline or submicrocrystalline structures, is more beneficial from SMA FP point of view than does traditional TMT creating well-developed dislocation substructure. ECAP and low-temperature TMT by cold rolling followed by PDA allows formation of submicrocrystalline or nanocrystalline structures with grain size from 20 to 300 nm in bulk, and long-size samples of Ti-50.0; 50.6; 50.7%Ni and Ti-47%Ni-3%Fe alloys. The best combination of FP: sr max =1400 MPa and er,1 max=8%, is reached in Ti-Ni SMA after LTMT with e=1.9 followed by annealing at 400°C which results in nanocrystalline (grain size of 50 to 80 nm) structure formation. Application of ultrafine-grained SMA results in decrease in metal consumption for various medical implants and devices based on shape memory and superelastiсity effects.

Abstract: High-purity compounds R2Fe14B (R = Y, Gd, Tb, Dy, Ho and Er) were prepared by arc melting using rare-earth metals purified by vacuum distillation-sublimation. The compounds R2Fe14B are single-phase and have well-defined directional structure. Nanocrystalline structure was formed by severe plastic deformation of the samples by means of torsion for 5 turns on the Bridgeman anvil under the pressure of 4 GPa at room temperature. The performed investigations of magnetic properties of these compounds allowed us to obtain reliable quantitative data on the intrinsic magnetic parameters, such as saturation magnetization, Curie temperature, the remanent magnetization and coercive force. The enhancement of the remanent magnetization was observed for R2Fe14B in nanocrystalline states compared with the crystalline samples due to the intercrystalline exchange interactions.

Abstract: The deformation and fracture of submicrocrystalline aluminum Al-6%Mg and Al- 6.1%Mg-0.3%Sc-0.1%Zr alloys after severe plastic deformation (SPD) by equal channel angular pressing (ECAP) as well as the same convenient alloys were investigated by acoustic emission (AE) method. ECAP resulted in predominantly submicrocrystalline structure with high angle grain boundaries and grain sizes ~ 100-400 nm in Al-6.1%Mg-0.3%Sc-0.1%Zr alloy and ~ 300-700 nm in Al-6%Mg alloy. The AE measurements carried out during material tension tests give new information regarding the processes deformation and fracture in materials and, together with the methods of microstructure, phase and fractography analysis.

Abstract: Ultrafine grained low carbon steel processed by high pressure torsion (HPT) has been investigated. Depending on initial state (ferritic-pearlitic state after normalization at 950°C, or martensitic ones after quenching from 950°C and 1180°C), the evolution of the microstructure and the mechanical properties was investigated after HPT and annealing at 400-600°C using transmission electron microscopy and X-ray analysis. It has been shown that HPT of martensitic low carbon steel provides a finer structure then that for ferritic-pearlitic initial state, and the initial martensitic morphology and phase composition is strongly dependent on the temperature of quenching. The initial structure was refined by HPT to 95nm in ferritic-pearlitic state and up to 65 and 50 nm in martensitic ones (after quenching from 950°C and 1180°C, respectively). Such ultrafine grained structures demonstrate substantial mechanical properties and possess a high thermal stability up to 500°C in all investigated states. Annealing for 1 h at 500°C results in grain growth up to 860nm for ferritic-pearlitic initial state and 150-450 nm for martensitic ones.

Abstract: The structure, mechanical and functional properties of ultrafine-grained low-carbon steels have been studied after severe plastic deformation (SPD) by high pressure torsion (HPT) and equalchannel angular pressing (ECAP). It is revealed that HPT of low carbon steels at a temperature below 0.3 Tm leads to the formation of nanocrystalline structure with a grain size of <100 nm or a mixture of oriented substructure and nanograins. ECAP under similar conditions leads to the formation of submicrocrystalline structure with a grain size of 200-300 nm. The initial martensitic state compared with the initial ferritic-pearlitic state of the low-carbon steels results in formation of finer structure after SPD and less intense grain growth upon heating, i.e., results in a higher thermal stability. Low-carbon low-alloy steels after ECAP are characterized by high strength (UTS > 1000 MPa) and plasticity (EL = 10-15%). The high-strength state after ECAP is retained upon tensile test testing up to a temperature of 500°C. The submicrocrystalline low-carbon steels after ECAP processing and subsequent heating is characterized by an increased impact toughness at test temperatures down to -40°C.

Abstract: Severe plastic deformation of a Mg-Al-Ca alloy resulted in different types of grain structure. High pressure torsion (HPT) was shown to lead to the formation of a nanocrystalline structure with a grain size of 100-200 nm, while equal channel angular pressing (ECAP) produced ultrafine grained (UFG) or submicrocrystalline (SMC) structures, depending on the ECAP temperature. An UFG structure with a grain size of 2-5 -m was formed at 300°C, as distinct from a finer SMC structure with a grain size of 300-800 nm formed at a lower temperature (220°C). The possibility of increasing the strength of the alloy in the UFG condition by a factor of 1.5-2, combined with a reasonable level of ductility and enhanced functional properties was thus demonstrated. ECAP of annealed Mg-Al-Ca with the formation of UFG structure was shown to lead to increased yield strength (by a factor of 2) and enhanced tensile ductility (by a factor of 3).