Micromechanics Modeling of Creep Fracture of the Ultra-High Temperature Ceramic Composites

Ultra-high temperature ceramics has great applications as refectory materials at high temperature. ZrB2-SiC has been considered as an excellent candidate of the ultra-high temperature ceramics due to its relatively low density and excellent refractory ability. However, the creep fracture of ZrB2-SiC limits its potential utilities. To mediate the creep fracture is thus imperative. It has been concluded from several experiments that the creep resistance of ZrB2-SiC reduces when an elevated temperature increases. There also exists a transition region of creep resistance for temperature above 1500oC for ZrB2-SiC composites. Moreover, recent experiments have shown that above 1800oC, dislocations were observed within the ZrB2 grains but the SiC grains behaved like a rigid body.
The thesis is aiming to develop a micromechanics model by means of finite element analysis to simulate the creep fracture and related deformation mechanisms. Since several length scales are involved in creep fracture, there are three major micromechanisms: grain creep, grain boundary diffusion and sliding. A micromechanics model considering grain boundary separation, grain boundary sliding and the creep or dislocation slip of grain was hence developed to simulate intergraunlar creep fracture of the ZrB2-SiC at elevated temperature in the present study. By finite element method, this micromechanics model was implemented in a commercial software ABAQUS. An isotropic creep deformation behavior and a crystal plastic model considering crystalline slip were implemented via ABAUQS UMAT, a user-defined material, for grains. For grain boundaries, a rate–dependent cohesive zone model was implemented using the user-defined element (UEL) in ABAQUS.
The influence of creep resistance by the effect of grain boundary heterogeneity was studied first. The creep resistance of an isotropic, homogeneous ZrB2 polycrystalline material is affected by the applied strain rate and the grain boundary properties. Grain boundary heterogeneity would initiate the microcrack and thus lead to fracture. A polycrystalline model composed by grain interiors constituted by the isotropic creep (UMAT) and grain boundaries modeled by a rate-dependent cohesive zone model (UEL) was built to study the heterogeneity effect on grain boundary, e.g. grain boundary nucleation and diffusivity, and rate dependent effect on different applied strain rates was studied. Simulation results indicate that the grain boundary heterogeneity reduces the creep resistance for ZrB2 polycrystalline materials.
Our focus then moved to the inhomogeneous grain aggregates without slip deformation. An isotropic grain interior modeled by UMAT along with the grain boundary simulated by a rate-dependent cohesive zone modeling (UEL) was constructed to study the creep fracture of ZrB2-20%SiC composites. The micromechanism of ZrB2-20%SiC composites when temperature is raised above 1500 oC is predominated by grain boundary sliding. Numerical results indicate that cavity nucleation at ZrB2-SiC interface is a necessary accommodation for grain boundary sliding. The phenomena of grain rotation jeopardize the creep resistance of ZrB2-20%SiC at high temperature region.
An advanced micromechanics model which composed by crystal plasticity for grain interior and cohesive zone model for grain boundary was used to study the creep mechanism for ZrB2-SiC composites when temperature is above 1800oC. The slip deformation of ZrB2 grain is considered as a complementary mechanism for grain boundary sliding. The simulation results showed a good agreement with experimental EBSD data, such that the grain boundary heterogeneity provided a preferred site for cavity nucleation which is necessary to grain boundary sliding. The simulation results also revealed the dislocation was induced by an incompatible deformation nearby the ZrB2-SiC interface.