Damage tolerance in intermetallic and ceramic materials at ambient and elevated temperatures: role of extrinsic vs. intrinsic mechanisms

Recent experimental results on the fracture toughness and fatigue-crack growth behavior of intermetallics, ceramics and their composites are reviewed. In particular, the role of extrinsic crack-tip shielding on crack propagation under monotonic and cyclic loading is contrasted with intrinsic damage mechanisms, both at ambient and elevated temperatures. For example, the fracture and fatigue properties in a toughened SiC ceramic are almost exclusively controlled by extrinsic shielding; marked resistance-curve toughening is achieved under monotonic loading by grain bridging, which is severely diminished under cyclic loading through progressive wear at the sliding grain interfaces. Cyclic crack growth rates in ceramics, as a result, are extremely sensitive to K_max and less dependent on ΔK at room temperature. At elevated temperatures, behavior in ceramics is influenced by extrinsic and intrinsic effects associated with deformation and damage of grain boundary amorphous films. Conversely, damage tolerance characteristics of intermetallics like TiAl, TiNb/TiAl and Nb/MoSi₂ are found to be a function of both intrinsic and extrinsic mechanisms, intermediate to behavior in metals and ceramics at all temperatures. Extrinsically, ductile-particle and shear-ligament bridging are dominant under monotonic loading and become less effective under cyclic loading; crack-propagation rates are moderately sensitive to both ΔK and K_max. From an intrinsic stand point, fatigue properties of γ-TiAl intermetallic alloys are slightly degraded in the 600-650°C range, just below the ductile-to-brittle transition temperature for TiAl. The strong dependence of cyclic crack growth rates on the applied stress intensity (K_max or ΔK) in advanced ceramics and intermetallics necessitates the use of design and life-prediction procedures based on crack initiation, small crack or fatigue-threshold philosophies.