Migration Of Low-angle Grain Boundaries Research Articles

Dynamic Recrystallization Modeling and Mechanisms in Inconel 690 Alloy during Hot Compressive Deformation

The dynamic recrystallization (DRX) behavior of INCONEL 690 (IN690) alloy was studied using electron backscatter diffraction (EBSD) and transmission electron microscopy. Isothermal compression tests were performed on a Gleeble-3500 simulator under temperatures 950-1100 °C, strain rates 0.01-1 s−1, and the maximum true strain 0.8. An Avrami-type model was developed to investigate the DRX behavior of IN690; it was found to be extremely sensitive to the processing parameters of temperature, strain rate, and true strain. As temperature increased and strain rate decreased, the activation energy increased, thus accelerating movement of dislocation and migration of grain boundaries, causing DRX behavior acceleration. The DRX behavior is accompanied by dislocation rearrangement and annihilation. The DRX mechanism of IN690 is dominated by discontinuous dynamic recrystallization (DDRX) and supplemented by continuous dynamic recrystallization (CDRX). A key characteristic of DDRX is grain-boundary bulges, while CDRX features progressive sub-grain rotation, leading to migration of low-angle grain boundaries to high-angle grain boundaries. A series of constitutive models were embedded into finite-element method software to study the DRX behavior of IN690. The results show that the microstructure evolution regularity obtained by the simulation method is consistent with the experimental values, which provides a basis for computer simulations of the hot machining process.

On the lower limit of misorientation of grain boundaries in hot forging of AA7050

The goal of this work is to find a reasonable norm to measure the area of low angle grain boundaries (LABs) using EBSD data, which is useful for the metal forming community involved in developing appropriate constitutive models for dynamic recovery. Typically, LABs are defined as the grain boundaries with misorientation between 2° and 10°. However, it is found in this study that reasonable variations of area per volume of LABs (Slab) with strain level (ε), strain rate (έ) and temperature is not be observed for AA7050 with this definition. There is a competition among different processes, including sub-grain formation and coarsening, rotation of sub-grain/cell boundary and migration of LABs and high angle grain boundaries (HABs) during recovery and continuous dynamic recrystallisation. By analysing the evolution of Kernal Angle Misorientation (KAM), it is shown that recovery has more effect on the lower angle part (< 1°) of local misorientation. If the lower limit of LABs is shift to 0.5°, the evolution of Slab with ε and έ shows clear pattern and increasing trend of Slab with temperature is consistent with temperature effect on sub-grain formation process. With such definition, we can have a better understanding of the microstructure evolution in the dynamic restoration of AA7050.

Low cycle fatigue properties and microstructure of P92 ferritic-martensitic steel at room temperature and 873 K

Low cycle fatigue tests for P92 heat-resistant steels (P92 HRS) were conducted at room temperature (RT) and elevated temperature (873 K). The fatigue test results showed that P92 steel showed softening characteristics during fatigue at RT and 873 K, and the increase of temperature and strain amplitude was the main reason for the decrease of uniaxial fatigue life. The specimens before and after fatigue test at two temperatures were observed and analyzed by transmission electron microscope (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS) and selected area electron diffraction (SAED). Microstructure observations revealed that the cyclic softening was caused by the annihilation of dislocations, as well as the fragmentation and polygonization of lath structure. High temperature promoted the formation of subgrains by accelerating the motion of dislocations and migration of low-angle grain boundaries (LABs). With the continuously decreasing of dislocation density, the growth of equiaxed subgrains and the weakening of the precipitation strengthening, the softening rate of the material was further increased until the final fracture occurs. Therefore, the evolution of microstructure of 9Cr martensitic steel during the low cycle fatigue process was deduced and described.

Stress-Driven Migration of Low-Angle Grain Boundaries Near Crack Tips in Nanocomposites Containing Incoherent Nanoinclusions

Abstract Theoretical model describing stress-driven migration of low-angle grain boundaries (GBs) in the vicinity of growing crack in metal matrix nanocomposites with reinforcing (metallic or ceramic) incoherent nanoinclusions is proposed. Using two-dimensional discrete dislocation dynamics approach profiles of migrating GBs are analytically calculated and critical stress for transition into unstable migration mode is found. It is shown that the presence of crack always promotes stress-driven migration and thus grain growth.

Influence of coherent nanoinclusions on stress-driven migration of low-angle grain boundaries in nanocomposites

A theoretical model that effectively describes stress-driven migration of low-angle tilt grain boundaries in nanocomposites with nanocrystalline or ultrafine-grained metallic matrices containing ensembles of coherent nanoinclusions has been developed. Within this model, low-angle tilt boundaries have been considered as walls of edge dislocations that, under the influence of stress, slip in the metallic matrix and can penetrate into nanoinclusions. The dislocation dynamics simulation has revealed three main regimes of the stress-driven migration of low-angle grain boundaries. In the first regime, migrating grain boundaries are completely retarded by nanoinclusions and their migration is quickly terminated, while dislocations forming grain boundaries reach equilibrium positions. In the second regime, some segments of the migrating grain boundaries are pinned by nanoinclusions, whereas the other segments continue to migrate over long distances. In the third regime, all segments of grain boundaries (except for the segments located at the boundaries of inclusions) migrate over long distances. The characteristics of these regimes have been investigated, and the critical shear stresses for transitions between the regimes have been calculated.

Effects of incoherent nanoinclusions on stress-driven migration of low-angle grain boundaries in nanocomposites

Stress-driven migration of grain boundaries (GBs) is theoretically described as a plastic deformation mode in metal matrix nanocomposites containing incoherent reinforcing (ceramic or metallic) nanoinclusions. We considered the exemplary case of low-angle tilt boundaries migrating in nanocrystalline or ultrafine-grained metallic matrixes and analytically calculated the effects of reinforcing nanoinclusions on the GB migration process. In doing so, migrating low-angle tilt boundaries are represented as walls of edge lattice dislocations that cooperatively glide in a metal matrix but cannot penetrate wire nanoinclusions. It is theoretically revealed that the nanoinclusions typically hamper the stress-driven GB migration. At the same time, in the situation with small (ultrafine) nanoinclusions, they cause an anomalous effect enhancing (or, in other terms, decreasing the critical stress for unlimited migration) the stress-driven GB migration in metal–metal and metal–ceramic nanocomposites. The results of our theoretical examination are consistent with the corresponding experimental data reported in the literature.

Stress-driven migration of deformation-distorted grain boundaries in nanomaterials

Stress-driven migration of low-angle grain boundaries (LAGBs) having deformation-distorted structures is theoretically described as a special mode of plastic deformation in nanocrystalline and ultrafine-grained materials. Equilibrium migration distances, energy and critical stress characteristics for stress-driven migration of deformation-distorted LAGBs are calculated as those depending on their structural and geometric parameters. It is theoretically revealed that deformation-distorted LAGBs tend to split under shear stresses, and these splitting processes lead to formation of new nanoscale (sub)grains. The role of migrating, deformation-distorted LAGBs in diminishing grain size in ultrafine-grained metallic materials during severe plastic deformation is discussed. Also, widening of deformation-distorted LAGBs in the absence of external stresses is theoretically described. Generalization of the suggested model to the case of high-angle grain boundaries is discussed. The results of our theoretical examination are compared with the corresponding experimental data and computer simulations reported in the literature.