Cyclic fatigue-crack growth and fracture properties in Ti3SiC2 ceramics at elevated temperatures

The cyclic fatigue and fracture toughness behavior of reactive hot-pressed Ti3SiC2 ceramics was examined at temperatures from ambient to 1200 degreesC with the objective of characterizing the high-temperature mechanisms controlling crack growth. Comparisons were made of two monolithic Ti3SiC2 materials with fine- (3-10 mum) and coarse-grained (70-300 mum) microstructures. Results indicate that fracture toughness values, derived from rising resistance-curve behavior, were significantly higher in the coarser-grained microstructure at both low and high temperatures; comparative behavior was seen under cyclic fatigue loading. In each microstructure, DeltaK(th) fatigue thresholds were found to be essentially unchanged between 25 degrees and 1100 degreesC; however, there was a sharp decrease in DeltaK(th) at 1200 degreesC (above the plastic-to-brittle transition temperature), where significant high-temperature deformation and damage are first apparent. The substantially higher cyclic-crack growth resistance of the coarse-grained Ti3SiC2 microstructure was associated with extensive crack bridging behind the crack tip and a consequent tortuous crack path. The crack-tip shielding was found to result from both the bridging of entire grains and from deformation kinking and bridging of microlamellae within grains, the latter forming by delamination along the basal planes.