Stability of alumina- and mullite-based fibers by thermal exposure: experimental study and phase-field modeling
Final Report Abstract
In the project, the microstructural changes of oxide fibers embedded in alumina-based matrices were studied by combining experiments and phase-field modeling. For that, minicomposites with Nextel fibers and a porous oxide matrix were designed, prepared, and tested, varying the matrix composition, processing conditions, and heat treatment temperature. In addition to conventional sintering (CS), fast firing (FF) and two-step sintering (TSS) were investigated as possible methods for sintering the porous ceramic matrix of the minicomposites. Among the tested temperatures and holding times, TSS at 1200°C-900°C-10h led to the highest tensile strength. The higher strength observed for TSS in comparison to CS was credited to the lower amount of sintering-induced cracks, lower fiber degradation, and higher porosity achieved. During CS, the densification of the matrix caused the formation of cracks and the strength decreased with longer exposure times. For TSS, most of the densification takes place during the first step, while the matrix is slowly strengthened during the second step at a lower temperature without impairing the minicomposite strength. Since temperature is a key factor for the processing and application of these materials, appropriate selection of temperature and holding time in TSS could overcome some drawbacks of commonly used CS. For improving the thermal stability, doping of the ceramic matrix with SiO2 and MgO was studied. The thermal degradation of the minicomposites was evaluated in terms of microstructure changes and mechanical properties after exposure to 1300°C and 1400°C for 2 h. The relatively low sintering temperature and short sintering time (1200°C for 2 h) could be the reasons why minicomposites with and without doping have very similar microstructure and mechanical properties in as-processed state. However, their microstructure is not stable when subjected to temperatures similar or above the processing conditions. After the thermal exposures, reduction of the open porosity, grain growth and strength loss were observed. Because the inward diffusion of Mg decreases fiber grain growth, MgO-doped minicomposites show higher strength retention in comparison to non-doped composites. Therefore, the doping of alumina matrices with MgO can be a promising alternative to produce Ox-CMCs with enhanced thermal stability without reducing their room-temperature properties. An efficient phase-field approach was developed to simulate the grain growth in polycrystalline ceramic materials in the presence of pores with various mobilities and diffusion coefficients. It was shown that the model can be applied to the simulation of the interaction of the grain boundaries with pores and also with coherent and non-coherent particles. The parameters of the model allow the reproduction of the theoretical equilibrium dihedral angle in the triple junction of a pore or a particle and a grain boundary. The effects of the pore dynamics on the grain size evolution in ceramic materials was investigated and compared with reported theoretical predictions and experimental data. An anisotropic phase-field model of grain coarsening in polycrystalline ceramic fibers with the abnormal grains is developed based on the multi-phase model of the grain growth which takes into account the crystal anisotropy of alumina. Anisotropic interface mobility and a recrystallization-type driving force are considered here as the main reasons for the abnormal grain growth. The numerical study shows the effects of the interface energy and interface mobility anisotropy on the shape of the abnormal grains and the grain size distribution during the heat treatment. In the case of the anisotropic interface mobility due to the different misorientation between abnormal and normal grains, the best fit to the experimental data is obtained. An extended model for describing grain boundary diffusion and segregation behavior in ceramic materials was developed to simulate the effect of dopants on abnormal grain growth. The model takes into account the dependency of the grain boundary mobility on the concentration, in particular, to the cases of silica diffusion from the fiber to the matrix and magnesia diffusion from the matrix to the fiber. The inverse method is developed and applied to adjust the unknown grain boundary diffusion coefficients of dopants by comparison to the experimental grain size distribution. The simulation results are in good agreement with the theoretical prediction of diffusion profiles. It was found by numerical tests (1200°C and 1300°C with different times) with different diffusion coefficients that the diffusion of Si from the fiber to the matrix in the samples without MgO dopant causes the increase of the average grain size on the rim of the fiber, the diffusion of Mg from the matrix to the fiber causes the decrease of the average grain size on the rim of the fiber due to the increasing concentration of MgO on the rim, the combined diffusion of Mg and Si causes the decrease of the average grain size in a full sample due to the faster diffusion of Mg. The grain boundary diffusion coefficient of Mg is estimated to be larger than that of Si by 6–7 times which is in agreement with the data in the literature.
Publications
- Numerical investigation of the recrystallization kinetics by means of the KWC phase-field model with special order parameters. Model Simul Mater Sc. 2017; 25: 045008
Kundin J
(See online at https://doi.org/10.1088/1361-651X/aa6a2a) - Phase-field modeling of pores and precipitates in polycrystalline systems. Model Simul Mater Sc. 2018; 26: 065003
Kundin J, Sohaib H, Schiedung R and Steinbach I
(See online at https://doi.org/10.1088/1361-651X/aacb94) - Obtaining complex-shaped oxide ceramic composites via ionotropic gelation. Journal of the American Ceramic Society. 2019; 102: 53-57
Almeida RSM, Pereira TFS, Tushtev K and Rezwan K
(See online at https://doi.org/10.1111/jace.15990) - Joining oxide ceramic matrix composites by ionotropic gelation. International Journal of Applied Ceramic Technology. 2020; 17: 1574-1581
Almeida RSM, Farhandi H, Tushtev K and Rezwan K
(See online at https://doi.org/10.1111/ijac.13507) - Phase-field simulation of abnormal anisotropic grain growth in polycrystalline ceramic fibers. Computational Materials Science. 2020; 185: 109926
Kundin J, Almeida RSM, Salama H, Farhandi H, Tushtev K and Rezwan K
(See online at https://doi.org/10.1016/j.commatsci.2020.109926) - Role of inclination dependence of grain boundary energy on the microstructure evolution during grain growth. Acta Materialia. 2020; 188: 641-651
Salama H, Kundin J, Shchyglo O, Mohles V, Marquardt K and Steinbach I
(See online at https://doi.org/10.1016/j.actamat.2020.02.043) - Increasing the tensile strength of oxide ceramic matrix mini-composites by two-step sintering. Journal of the American Ceramic Society. 2021; 00 1– 11
Farhandi H, Karim MN, Almeida RSM, Tushtev K and Rezwan K
(See online at https://doi.org/10.1111/jace.18212) - Phase-field modeling of grain growth in presence of grain boundary diffusion and segregation in ceramic matrix mini-composites. Computational Materials Science. 2021; 190: 110295
Kundin J, Farhandi H, Ganesan KP, Almeida RSM, Tushtev K and Rezwan K
(See online at https://doi.org/10.1016/j.commatsci.2021.110295)
