Farhandi, Hedieh
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Item-typ:Veröffentlichung, Phase-field simulation of abnormal anisotropic grain growth in polycrystalline ceramic fibers(Elsevier {BV}, 2020-08-05); ; ; ; The present work proposes a phase-field approach to realistically simulate abnormal grain growth in polycrystalline ceramic fibers at temperatures above 1000 °C. Under these conditions, grain growth is characterized by the formation of large elongated rectangular grains in a matrix of normal grains. The multi-phase-field model is extended by mechanisms which are responsible for the abnormal anisotropic growth: anisotropic interface energy, anisotropic interface mobility and a recrystallization driving force. Two types of the mobility anisotropy are considered: anisotropy due to the misorientation between grains and anisotropy due to the dependency on inclination angles. The experimental data from heat treatments of the ceramic fiber Nextel 610 at 1200–1300 °C with different dwell times are examined by the proposed model. The comparison of the experiment and the simulation results at various times shows that the extended multi-phase-field model is able to simulate the microstructure realistically. In most model cases, abnormal grains have a rectangular shape, similar to the experiment. The best fit of the experimental grain size distribution and the grain shape is achieved by the simulation with the misotientation dependency of the interface mobility. Furthermore, it was shown that a strong decrease on the grain growth rate with time, observed in the experiment, can be reproduced by the simulations taking into account the segregation of impurities on grain boundaries that results in the decreasing grain boundary mobility.Wissenschaftlicher ArtikelBand:185148 177 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Enhancing thermal stability of oxide ceramic matrix composites via matrix doping(2022-02-23); ; ; To overcome the main limitation of oxide ceramic matrix composites (Ox-CMCs) regarding thermal degradation, the use of matrix doping is analyzed. Minicomposites containing Nextel 610 fibers and alumina matrices with and without MgO doping were produced. The thermal stability of the minicomposites was evaluated considering their microstructure and mechanical behavior before and after thermal exposures to 1300 C and 1400 C for 2 h. Before heat treatment, both composite types showed very similar microstructure and tensile strength. After heat treatment, densification, grain growth and strength loss are observed. Furthermore, the MgO dopant from the matrix diffuses into the fibers. As a result, abnormal fiber grain growth is partially suppressed and MgO-doped composites show smaller fiber grains than non-doped composites. This more refined microstructure leads to higher strength retention after the heat treatments. In summary, doping the matrix can increase the overall thermal stability without impairing the room-temperature properties of Ox-CMCs.Wissenschaftlicher ArtikelBand:42Heft:7181 436 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Phase-field modeling of grain growth in presence of grain boundary diffusion and segregation in ceramic matrix mini-composites(Elsevier {BV}, 2021-01-29); ; ; ; The grain boundary diffusion and segregation can influence the grain growth kinetics, the grain size distribution, and therefore the mechanical properties of the ceramic matrix composites. The present paper proposes a phasefield modeling approach to simulate the grain growth in polycrystalline alumina fibers embedded in alumina matrix at temperatures above 1000 ◦C in presence of the grain boundary diffusion of dopants from the matrix to the fiber and vice versa. The multi-phase-field model for grain growth [I. Steinbach and F. Pezzolla, Phys. D, 134 (1999) 385] is extended by the incorporation of the grain boundary diffusion, grain boundary segregation model, and the dependence of the interface mobility on the segregation concentration. The kinetic parameters of the model which allow describing the real microstructure evolution were estimated by the comparison to the experimental measurements. The simulation and experimental results of the grain growth with the diffusion of dopants in Nextel 610 fibers show the significant effect of the grain boundary diffusion on the grain size distribution. The results of numerical tests were used to adjust the values of the grain boundary diffusion coefficients by the experimental data at different temperatures by means of an inverse method. From the simulations, the diffusion coefficient of Mg was estimated to be 6–7 times higher than that of Si.Wissenschaftlicher ArtikelBand:190148 115