Micromechanics modeling of creep fracture of high-temperature ceramics

Ultrahigh-temperature ceramics has great potential for applications as refractory materials at high temperatures. ZrB₂-SiC has been considered as an excellent candidate of the ultrahigh-temperature ceramics due to its relatively low density and excellent refractory properties. However, the creep fracture of ZrB₂-SiC limits its potential applications. Mitigation of the creep fracture is thus imperative. It has been concluded from several experiments that the creep resistance of ZrB₂-SiC decreases with the increasing temperature, and there also exists a transition of creep mechanism for temperature above 1500°C. The effects of grain boundary heterogeneity on the creep resistance were studied. The creep resistance of an isotropic and 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. An isotropic grain interior modeled by user-defined material properties (UMAT) subroutine along with the grain boundary simulated by a ratedependent cohesive zone modeling using user-defined element (UEL) subroutine was constructed to study the creep fracture of ZrB₂-20% SiC composites. The model is accounted for nucleation, growth, and coalescence of cavities along the grain boundaries in a localized and inhomogeneous manner, link up of microcracks to form macrocracks, and grain boundary sliding. For ZrB₂-20% SiC composites, a micromechanism shift form diffusional creep-control for temperatures below 1500°C to grain boundary sliding-control for temperatures above 1500°C was concluded from simulations. Also, the simulation results revealed the accommodation of grain rotation and grain boundary sliding by grain boundary cavitation for creep at temperatures above 1500°C which was in agreement with experimental observations.