Density Of Low-angle Boundaries Research Articles

Neural Network Modeling of Microstructure Formation in an AlMg6/10% SiC Metal Matrix Composite and Identification of Its Softening Mechanisms under High-Temperature Deformation

The paper investigates the rheological behavior and microstructuring of an AlMg6/10% SiC metal matrix composite (MMC). The rheological behavior and microstructuring of the AlMg6/10% SiC composite is studied for strain rates ranging between 0.1 and 4 s−1 and temperatures ranging from 300 to 500 °C. The microstructure formation is studied using EBSD analysis, as well as finite element simulation and neural network models. The paper proposes a new method of adding data to a training sample, which allows neural networks to correctly predict the behavior of microstructure parameters, such as the average grain diameter, and the fraction and density of low-angle boundaries with scanty initial experimental data. The use of neural networks has made it possible to relate the thermomechanical parameters of deformation to the microstructure parameters formed under these conditions. These dependences allow us to establish that, at strain rates ranging from 0.1 to 4 s−1 and temperatures between 300 to 500 °C, the main softening processes in the AlMg6/10% SiC MMC are dynamic recovery and continuous dynamic recrystallization accompanied, under certain strain and strain rate conditions at 300 and 350 °C, by geometric recrystallization.

Pearlite Spheroidisation and Microstructure Refinement Through Heavy Warm Deformation of Hot Rolled 55VNb Microalloyed Steel

The microstructure evolution of 55VNb microalloyed steel during warm deformation via single pass uniaxial compression was researched, and the effect of deformation conditions on dynamic spheroidisation of cementite lamellae and ferrite conditioning for a range of deformation temperatures (600 °C to 700 °C) and strain rates (1 to 10 s−1) analysed. Cementite lamellae appear to subdivide irrespective of deformation temperature with the ferrite phase penetrating the pattern formed by the cementite crystallites, in turn confirming that the dissolution of this phase during deformation is an important mechanism leading to the break-up of plates and subsequent globulisation. EBSD measurements allowed orientation gradients leading to the final subdivision of the cementite to be determined. Ferrite softening during heavy warm deformation is attributed to dynamic recovery and continuous dynamic recrystallisation, although the evolution of this phase depends, to a great extent, on the region subject to study, as confirmed by local EBSD studies. Misorientation profiles obtained in different regions of ferrite and pearlite enabled the different stages of the microstructural evolution to be monitored for each phase, this being developed via a variety of mechanisms under the same deformation conditions. Finally, the increase in low angle boundary density correlates with the Zenner–Hollomon parameter, and a linear relation between the density of low angle boundaries and steady-state stress estimated for pearlite and ferrite was found, indicating that new boundaries would have been formed dynamically during deformation. High angle boundary density also increases with deformation, although this is almost irrespective of the temperature and strain rate applied.

Effect of rotation speed on microstructure evolution and mechanical properties of nugget zone in 2195-T8 Al-Li alloy friction stir welding joints

Complicated plastic deformation and welding temperature were generated in the nugget zone (NZ) of friction stir welding (FSW) joints leading to different regions being developed, such as shoulder influence zone (SIZ), pin influence zone (PIZ) and swirl zone (SZ). To find the effect of rotation speed on microstructure evolution and mechanical properties of these regions during friction stir welding, serials of AA2195-T8 plates with 7.5 mm thickness were fabricated using different rotation speeds (500, 700 and 900 rpm) and a constant welding speed (50 mm/min). Microstructural characterization was conducted out using electron back scattered diffraction (EBSD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and X-ray diffraction (XRD) technologies. Mechanical properties were measured by micro-hardness and tensile test. The results show that grains in the SIZ were mainly shown in wedge-shaped structure and had the high density of low angle boundaries (LABs) showing dominant (100) orientation. However, in the PIZ and SZ, fine and equiaxed grains with dominant (111,101) orientation were observed. With rotation speed increasing, the fraction of LABs increased in the SIZ but showed little changes in the PIZ and SZ. After welding, a large number of T1 and θ′ precipitates almost be completely dissolved in the NZ with many fine δ′/β′ being re-precipitated. It is notable that a few T1 precipitates were observed in the SIZ at a higher rotation speed (900 rpm), which were re-precipitated from the Al matrix after dissolution. In the SIZ, the dominant contribution to yield strength was dislocation strengthening which enhanced with rotation speed increasing. However, in the PIZ and SZ, grain refinement strengthening was the primary strength mechanism, which decreased with rotation speed increasing.

Effect of Yttrium Contents on the Microstructure of a Hot-Rolled Tantalum-Containing 12Cr-ODS Steel

Dual-phase oxide dispersion strengthened (ODS) steels of Fe–12Cr–xY (x=0.1, 0.2 and 0.3 wt%) have been fabricated by casting, hot-forging and subsequent hot-rolling. Microstructure of the hot-rolled samples was carefully characterized by use of electron probe microanalysis (EPMA), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The results show that all the samples consist of ferrite phase with coarse fiber grains (elongated along the rolling direction) and martensite phase in the form of fine block grains. As the Y addition increases, the ratio of ferrite to martensite and density of low-angle boundaries (LABs) within the martensite and ferrite grains do not change significantly. The Y2O3 particles introduced by self-oxidation during casting are survived without being dissolved or refined after hot-forging and hot-rolling. However, as the Y addition increases, the Y2O3 particles tend to aggregate with fine M23C6 and TaC carbides. The hardness of the sample with high Y additions is lower than that of the samples with none or low Y additions, which could be attributed to the aggregation coarsening of the yttria and TaC particles, resulting in a decrease in dispersion strengthening effect. The effect of Y additions on microtexture was also discussed.

Effect of Mesostructural Elements on Radiation-Induced Porosity in 16Cr-19Ni-2Mo-2Mn-Nb-Ti-V-P-B Austenitic Steel

Different mesostructural elements of 16Cr-19Ni-2Mo-2Mn-Nb-Ti-B austenitic steel have been examined after neutron irradiation to damage dose up to 82 dpa by scanning electron microscopy using orientation microscopy (EBSD). Radiation porosity with maximum void size up to 200 nm was observed in austenitic steel structure after neutron irradiation. Nonuniformity, related to mesostructural elements, such as general grain boundaries, special CSL boundaries Σ3 (twins), areas with high density of low-angle boundaries, is typical for radiation porosity.

Effect of pre-cold rolling on the evolution of microstructure, microtexture, and mechanical properties of the mushy state rolled in-situ Al-4.5Cu-5TiB2 composite

The mushy state rolling has been found as a promising technique for near-net forming of the cast Al-4.5 wt% Cu-5 wt% TiB2 in-situ composite to obtain microstructural refinement and moderate mechanical properties. Cold rolling with 30% reduction in thickness was carried out before mushy state rolling, for achieving a relatively more homogeneous distribution of CuAl2-TiB2 particles as well as an appropriate microstructure to enhance the mechanical properties. A comparative investigation on the microstructural and microtextural evolution, along with mechanical properties of both the mushy state rolled composite (MRC) and pre-cold rolled mushy-state rolled composite (PCMRC) was performed, and the microstructure-property correlation was established. The PCMRC exhibited the lowest homogenized lengthscale parameter (LH 29 μm) indicating the maximum uniformity of particle distribution, computed with the help of multi-scalar analysis of area fractions (MSAAF) technique. Electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM) studies revealed the presence of fine sub-grains within centrally located deformed Al grains due to dynamic recovery, as well as the particle stimulated nucleation of equiaxed fine Al grains by dynamic recrystallization or rapid solidification of liquid near edges for both the investigated composites. The PCMRC displayed a superior combination of hardness (96.82 ± 4.5 HV5kgf), 40% higher ultimate tensile strength (301.4 ± 1.8 MPa), and 68% more uniform elongation (30.8% ± 0.5%), compared to the MRC. The improvement in mechanical properties is attributed to the uniform particle distribution and presence of both higher density of low angle boundaries and recrystallized grains adjacent to the particles, obtained from prior cold rolling.

Crystallographic Features of Martensite Transformed from Ultrafine Grained Austenite Fabricated by Severe Plastic Deformation

We studied the crystallographic features, especially the orientation relationship with respect to austenite, of martensite in a steel transformed from coarse-grained equiaxed austenite (35 μm), ultrafine-grained equiaxed (2.5 μm) or lamellar (300nm) austenite fabricated by sever plastic deformation. With decreasing the grain size of equiaxed austenite, the orientation relationship changed from Kurdjumov - Sachs relationship to Greninger - Troiano relationship. We inferred that this change of orientation relationship could be attributed to the small size of martensite plate transformed from the ultrafine-grained equiaxed austenite. The martensite transformed from the ultrafine-grained lamellar austenite did not have a definite orientation relationship with austenite. We considered that a high density of dislocations or a high density of low angle boundaries within the ultrafine lamellar austenite grain resulted in the large deviation of orientation relationship.

Phonon scattering by structural defects in solid p-H2 and in p-H2–o-D2 solutions

A study is made of the influence of structural defects on the thermal conductivity in parahydrogen crystals and in parahydrogen–orthodeuterium solutions. The defects in the crystals are generated by means of a thermal shock. The temperature dependence of the thermal conductivity is analyzed in the framework of the Callaway relaxation model in the Debye approximation for the phonon spectrum with allowance for phonon–phonon scattering processes and Rayleigh scattering on isotopic defects and structural defects such as dislocations and low-angle boundaries. The effect produced in the sample by the thermal shock is found to depend on the deuterium concentration. In pure parahydrogen an increase in the dislocation density is observed, and in parahydrogen–orthodeuterium solutions an increase in the density of low-angle boundaries. The change in the density of low-angle boundaries in the solutions after the thermal shock depends linearly on the concentration of o-D2.