Добірка наукової літератури з теми "Strain Induced Boundary Migration (SIBM)"

Examination of the SIBM mechanism based on the dislocation substructure at the interior of the Goss oriented grain was carried out by changing the grain size prior to the temper rolling. The following results were obtained. 1) SIBM significantly increased Goss orientation during the eminent grain growth with the initial grain sizes from 18 to 55μm. 2) When the initial grain sizes were large, i.e. 37μm and 55μm, the rolling with the reduction beneath the critical value could not promote SIBM, even the normal grain growth could also be hindered. Consequently a proposal was made that the nucleation of the recovery appeared among substructure domains containing sluggish strain. There exists an adequate size of the domain which varies with the change both of the rolling reduction and the initial grain size.

Texture formation through strain induced boundary migration (SIBM) was investigated. Temper rolling reduction before final annealing for SIBM was varied between 0 and 26% and grain sizes and textures were measured using EBSD. In the specimen which was temper rolled to 5%, in which grain growth by SIBM occurred most efficiently, a strong Goss component (which was a minor component after rolling), developed during annealing. From the EBSD image quality analysis, it was found that stored energy increased significantly in the Goss component with strain (from 5 to 9 %), whereas it was always relatively small in the D-Cube component ({001}<110>), compared to Goss and g-fibre components. Based on these results, a mechanism of grain growth by SIBM was suggested. Texture evolution during annealing could thus be explained by the hypothesis, speculated from the analysis of orientation stability, that D-Cube grains are associated with more homogeneous dislocations distributions than Goss grains, in which the co-existence of high and low dislocation density zones could favour grain growth by SIBM.

AbstractThe feasibility of establishing thermomechanical conditions to promote {100}//ND fiber texture via strain-induced boundary migration (SIBM) recrystallization mechanism in a non-grain oriented (NGO) electrical steel was investigated. Single-hit uniaxial compression at various temperatures and strains has been applied on Fe-6 wt pct Si to establish the relationship between stored energy and the softening mechanisms. Recovery only and recrystallization by SIBM or by subgrain growth (SGG) have been observed depending on the stored energy level. A strong {100}//ND fiber recrystallization texture, i.e., 45 pct area fraction, was seen in the sample which was deformed to 0.2 strain at 650 °C and then annealed at 1000 °C for 15 minutes, whereas only 13 pct {100}//ND fiber component was observed after 0.4 strain at 500 °C followed by the same annealing treatment. By examining the same microstructural region before and after annealing via EBSD, it has been shown that {100}//ND textured recrystallized grains were formed adjacent to the {100}// ND textured deformed matrix. Low stored energy has been shown to favor the formation of {100}//ND texture recrystallized grains via SIBM recrystallization mechanism attributed to its slow recrystallization nucleation rate. The results from the deformation studies have been used to suggest a processing window map concept to define the recovery, SIBM, and SGG regions for the starting as-cast columnar microstructure.

The present study investigates the occurrence and effectiveness of the dissociation mechanism of Σ3 CSL boundaries into its variants such as Σ9 and Σ27a-b during strain-annealed grain boundary engineering (GBE) of Hastelloy-X. Multiple cold-rolling strain levels and annealing conditions are studied and it is observed that the density of ∑3 boundaries decreases proportionally to the amount of strain induced boundary migration (SIBM) during the GBE process. The dissociation mechanism of Σ3 annealing twins is activated at the onset of SIBM, causing an increase in the density of the Σ3n variants. It is shown that at high annealing times or temperatures, the rate of generation of CSL boundaries through dissociation mechanism is lower than their annihilation rate. It is further suggested that the dissociation mechanism of ∑3 boundaries during GB migration is more efficient when the amount of applied strain prior to annealing is kept low, thus promoting disruption of the random GB network.

In order to get optimal grain boundary character distribution (GBCD) and grain boundary properties, thermomechanical processing (TMP) is usually adopted in grain boundary engineering. However, the mechanism behind the TMP treatments and GBCD optimization is still unclear. The present study has conducted a series experiments involving low-strain TMPs to study the relationship between TMP parameters and the behind microstructural evolution. The experimental results indicate that in the scope of low-strain TMP, strain induced boundary migration (SIBM) is the most effective process for GBCD optimization. Besides, SIBM and grain growth would gradually transfer to recrystallization with the increase of pre-deformation level and annealing temperature. Further quasi in-situ EBSD results infer that SBIM is activated locally in some region with high stored energy, and further gradual initiation of SIBM from one region to another contributes to the gradual increase of special boundaries with annealing time.

Semi-processed nonoriented electrical steels are very attractive products whose magnetic properties are significantly improved through annealing treatment in customers plant. The improvement is due to strong Goss texture formation by strain induced boundary migration (SIBM). In order to the effect of temper rolling reduction on the strengthening of Goss texture, temper rolling reduction was changed in the range of 2% to 8%. The annealing times was changed from 10 minutes to 180 minutes. A mechanism of grain growth during SIBM is suggested from our experimental data. In the specimen temper-rolled by 2%, relatively strong {111}<112> texture develops, whereas in the specimens temper-rolled by 4% through 8%, strong Goss texture develops as a result of SIBM during final annealing. It can be found from observed EBSD data that the Goss grains have the lowest stored energy in all temper-rolled specimens, which is confirmed by average image quality value in EBSD measurements. However, for the Goss grains to grow preferentially, stored energy difference between Goss grains and their neighboring grains may have to be higher than a certain critical value.

In order to elucidate the predominance of Goss grains after SIBM in electrical steel sheets, Goss, D-Cube and {111}<112> grains after temper rolling of 5 and 9% reduction were observed by TEM. In 5% strain the amount of dislocations in Goss grains was the smallest of the three orientations. In 9% strain dislocations in Goss grains were distributed more heterogeneously than the other two types of grains. It is considered that {111}<112> grains have large amounts of dislocations owing to high Taylor factors and the differences of microstructures between Goss and D-Cube grains are due to orientation stabilities. Goss grains are speculated to be easy to recover and therefore they are predominant after SIBM.

Pilot-scale plate rolling experiments and laboratory thermomechanical processing experiments were carried out to understand the mechanism of microstructural banding in low-carbon microalloyed steels. The microstructural banding originates with large elongated austenite grains, which are present at the roughing stage of rolling. The large austenite grains develop when conditions favour abnormal grain growth during reheat and/or strain induced grain boundary migration (SIBM) in the first few rolling passes. Microstructural banding is eliminated by designing TMP schedules to avoid abnormal grain growth and SIBM.

Pilot-scale plate rolling experiments and laboratory thermomechanical processing experiments were carried out to understand the mechanism of microstructural banding in low-carbon microalloyed steels. The microstructural banding originates with large elongated austenite grains, which are present at the roughing stage of rolling. The large austenite grains develop when conditions favour abnormal grain growth during reheat and/or strain induced grain boundary migration (SIBM) in the first few rolling passes. Microstructural banding is eliminated by designing TMP schedules to avoid abnormal grain growth and SIBM.

Friction Stir Processing (FSP) is emerging as one of the most competent Severe Plastic Deformation (SPD) methods for producing bulk ultra-fine grained materials with improved properties. The significant advantage of FSP is that it can be used for localized microstructural modification which is not possible with the other common SPD techniques such as Equal Channel Angular Processing (ECAP), High Pressure Torsion (HPT), and Accumulative Roll Bonding (ARB). The process is derived from the basic principles of Friction Stir Welding (FSW), a solid state welding technique developed for the high strength aluminium alloys used in structural applications. In FSP, a non-consumable rotating tool with a shoulder and a pin is traversed along the region on the work-piece which is to be modified. In the present investigation on FSP, two heat treatable aluminium alloys with different hot deformation behaviour, 2024 (Al-Cu-Mg) and 2219 (Al-Cu) and a strain hardenable alloy 5086 (Al-Mg) has been considered. FSP involves complex thermo-mechanical interactions and hence the optimization of process parameters is an important aspect to be considered for a successful processing. The number of process parameters involved is more in FSP and the three most important parameters are tool rotational speed, tool plunge depth (normal load on the work piece) and the tool traverse speed. These parameters are varied for a fixed tool geometry and tool tilt angle in a custom-built FSW/FSP machine. A parametric study has been carried out in order to have a clearer picture on the relative importance of various parameters by using tools of different pin lengths. The tool plunge depth and tool rotational speed are also varied in the parametric study. It has been observed that tool plunge depth is the most important parameter for the FSP of high strength aluminium alloys and the first parameter to be optimized. As per the inputs obtained from this parametric study, a systematic experimental procedure has been developed (a bottom-up approach) for optimizing the most important process parameters of FSP. The optimal process parameters obtained from the experimental bottom-up approach has helped in achieving bulk tensile strength higher than the starting material strength for the strain hardenable alloy 5086-O. In heat treatable alloys, due to the presence of a weaker heat affected zone the achievable strength in a single pass FSP were 93% and 80-85% of the starting material strength in the alloys 2024 and 2219 respectively. Micro-tensile testing of the samples taken from the nugget zone of the alloy 2024 indicated an ultimate tensile strength of 1.3 times the starting material strength. This strength increase is attributed to the combined effects of grain size strengthening and precipitation hardening. FSP has been perceived as a grain refinement technique and hence the most important region in any processed sample is the nugget zone. Due to the continuous stirring of the tool pin at high rotation rates, it is possible that different regions in the nugget zone can develop varied microstructure and crystallographic texture. The nugget zone of the optimally processed samples are characterized in detail using the advanced characterization techniques such as Scanning Electron Microscopy (SEM), Electron Back-Scattered Diffraction (EBSD), X-Ray Diffraction (XRD) and Electron Probe Micro-Analyzer (EPMA) in order to understand the underlying micro-mechanisms of microstructure and texture evolution. Micro-texture studies on the alloys revealed gradients in textures across the thickness with the dominance of shear texture components. The bulk texture is weaker in all the three alloys. Bulk texture measurements revealed that the texture development during FSP is an alloy independent phenomenon. The dominant texture component observed is different in heat treatable and strain hardenable alloys. The dominant component of texture is identical in both the heat treatable alloys irrespective of the differences in optimal process parameters and the thickness of the plates used. Microstructural evolution during FSP is more of an alloy dependent phenomenon. Particle Stimulated Nucleation (PSN) and Strain Induced Boundary Migration (SIBM) are observed as the dominant nucleation mechanisms of Dynamic Recrystallization (DRX) in the heat treatable and strain hardenable alloys respectively. Normal grain growth through the Burke and Turnbull mechanism is observed with the presence of few larger grains in the microstructure caused by geometrical coalescence. DRX has been observed to occur through separate nucleation and grain growth stage in all the three aluminium alloys and hence indicative of a discontinuous process. Bulk texture development during FSP has been correlated to the microstructure evolution with the mechanisms of PSN and SIBM both weakening the textures in all the alloys. In order to expand the understanding as a commercially viable technique and studying the stability of FSP microstructure and texture, multiple processing routes have been employed. In the Multi-Pass FSP (MP-FSP), the processing is carried out at the same location and the objective is to study the stability of the processed samples under extreme conditions of strain and temperature. In Multi-Track FSP (MT-FSP), an overlap ratio of 0.33 is selected for the successive passes which will allow partial nugget zone penetration. MT-FSP can be used for producing large volume of fine grained materials. It is observed that the microstructure and crystallographic texture is stable under the mild and extreme conditions of strain and temperature. Subsequent heat treatment studies after FSP in the alloy 2024 confirmed that the processed microstructure is stable up to temperatures as high as 723K (450°C). These results are indicative of the advantage of FSP as a successful materials processing technique in which the retained lower strain energies leading to the development of a stable microstructure and texture. Compressive residual stresses are observed at different regions in the nugget zone of all the alloys after FSP. This is attributed to the combined effects of a solid state processing route and the optimal selection of process parameters.