Papers by Keyword: Abnormal Grain Growth

Abstract: The phase field models have been built to study the influence of the nonuniform grain boundary energy for abnormal growth of grains in the AZ31 magnesium alloy in the real time and space. The simulated results show that if the grains of a certain orientation with low grain boundary energy in the AZ31 Mg alloy, abnormal grain growth will occur after annealing treatment, and only if the local low grain boundary energy is less than 0.98σ_0, can the certain grains grow abnormally in the microstructure.

Abstract: In this work, the effect of pre-strain cold rolling on thermal stability of friction-stir welded AA6061-T6 alloy was studied. The pre-strain rolling was found to be very effective in suppression of abnormal grain growth during standard post-weld T6 heat treatment. It was also shown that the efficiency of this approach essentially depends on rolling path and the rolling along welding direction was the most effective rolling schedule.

Abstract: This study aims to highlight the grains growth of tungsten carbide in the nickel binder phase, in function of sintering temperature of a hot isostatic pressed - HIP (at 60 bar) material, targeting the morphological behavior of the grain size and densification of the composite WC-8Ni. Cylindrical samples 25 mm x 15 mm were sintered in compression at temperatures ranging from 1380 °C to 1460 °C. The results showed that increasing the sintering temperature, there is an increase in WC grain size, the hardness decreases and the porosity is reduced increasing the specific gravity.

Abstract: AA1235 aluminum alloys were twin roll cast into a 7.0 mm-thick billet and further rolled into a plate with 70% reduction, followed by annealing at 500 C for 8 h. Abnormal grains were found to be formed on the plate bottom surface. SEM, EBSD, and micro X-ray diffraction were used to analyze the mechanism underlying the abnormal growth of these grains. Results showed that the {100} <001> texture was formed in the surrounding normal grains adjacent to the abnormal grain. The orientation angle between the abnormal grain and its adjacent normal grains was 45°. The {100} <001> texture formed during rolling and annealing accelerated abnormal grain growth. The partially dissolved Fe precipitates were heterogeneously distributed in the plate bottom heterogeneously, which also mainly explained abnormal grain growth.

Abstract: The grain coarsening temperature in Nb microalloyed steels is investigated by multiphase-field model. In this study, the pinning force is treated as time-dependent using mean-field kinetics of precipitates including volume fraction and their size. The grain size is calculated with time under various temperature range. The grain coarsening temperature is determined by the ratio of the largest radius of grain vs. the average grain radius criteria (R_max / R_avg > 2.94) in two-dimensional growth. Through this model, it is possible to simulate grain growth in microalloyed steels more precisely.

Abstract: The samples with a length of 50mm and cross section area of 20mm2 were machined from extruded P/M Nickel-base superalloy billet and the isothermal deformation tests were carried out at constant strain rate in a vacuum environment using the Thermecmastor. All the samples were deformed to a true strain of 1.04 at deformation temperature 1070°C and 1100°C and strain rate 0.001s-1~1s-1 respectively. The result is the deformation strain rate determine the position and the deformation temperature determine the severity of abnormal grain growth (AGG). The initial grain size and dislocation density is not the only reason to AGG. The AGG is related the recrystallization nucleation process is inhibited in the heat treatment. The AGG region usually has more low energy and low migration rate boundary than other regions.

Abstract: The Nickel-base superalloy samples were prepared by the isothermal forging in different strain rates at 1070°C. The isothermal deformation tests were carried out at constant strain rate in a vacuum environment using the Thermecmastor (Fuji Electronic Industrial Co., Japan) with a capacity of 30 ton compressive force. All the samples were deformed to a true strain of 1.04 at deformation temperature 1070°C and strain rate 0.001s-1~0.1s-1 respectively. The result is The deformation strain rate determine the position and domain of AGG and the temperature determine the severity of AGG by deformation temperature and temperature rise. The most serious region of AGG is related to the specific CSL boundaries. The fraction of Ʃ3 boundary reaches the peak point value and fraction of Ʃ7 boundary reaches the valley point value in all the samples with different strain rates. The most serious region of AGG is also related to the residual strain. In the most serious region of AGG, more dislocation is used to prefer grain growth to recrystallization, which can refine grain.

Abstract: β-Tricalcium phosphate (β-TCP) ceramics are of interest for bone requirements implants due to resoption behavior. The mechanical properties of β-TCP, however, are not yet sufficient to allow load bearing application of implants. The aim of this work was to investigate the effect of Mg²⁺ substitution on the strength sintered TCP. Due to promotion of a liquid phase at 1200°C, Calcium pyrophosphate (CPP-C₂P₂O₇) was used to improve the sintering of the samples. The introduction of CPP was promoted by use of a Ca/P molar ratio of 1.45. The powders were synthesized using a mixture of Ca (OH)₂ suspension and diluted H₃PO₄ with addition of MgO and calcined at 750 °C, 900 °C and 1050 °C. The cold isostatic pressing compacts were sintered at 1200 °C and 1300 °C, respectively.It was shown that a small Mg content (1.5 mol%) increased both compressive strength and fractional density of the TCP material sintered at 1200 °C from 132 ± 39 MPa at 92.1 % of fractional density to 193 ± 29 MPa at 94.5 % of theoretical density. Higher amounts of Mg inhibited the grain growth provoking a increase of the boundary mobility activation energy. Abnormal grain growth (AGG) was observed after sintering at 1300°C, as result CPP - liquid phase formation. Increase of Mg content promoted AGG, due to inhibition of grain growth during normal grain growth resulting in a increase of the residual elastic energy of the system.

Abstract: During abnormal grain growth, a few Goss grains grow exclusively fast and consume the matrix grains. The Goss abnormally-growing grain (AGG) has peculiar features which are irregular grain boundaries and very high frequency of peninsular grains nearby the growth front of AGG and island grains trapped inside AGG. These features might provide a clue for clarifying the mechanism of Goss AGG. The experimentally-observed microstructural feature and grain boundary characterization of Goss were approached by the solid-state wetting mechanism. In this study, observing the three-dimensional wetting morphology in serial section images of Goss AGG by EBSD, we report some direct microstrucrual evidence supporting solid-state wetting mechanism for Goss AGG. The solid-state wetting mechanism for the evolution of the Goss AGG in Fe-3%Si steel explains the microstructural features evolved during secondary recrystallization, which cannot be approached by the conventional theories based on the grain boundary mobility.

Abstract: Under certain circumstances abnormal grain growth occurs in Nickel base superalloys during thermomechanical forming. Second phase particles are involved in the phenomenon, since they obviously do not hinder the motion of some boundaries, but the key parameter is here the stored energy difference between adjacent grains. It induces an additional driving force for grain boundary migration that may be large enough to overcome the Zener pinning pressure. In addition, the abnormal grains have a high density of twins, which is likely due to the increased growth rate.