Fracture of Ti-Al3Ti metal-intermetallic laminate composites:: Effects of lamination on resistance-curve behavior

The fracture toughness and resistance-curve (R-curve) behavior of Ti-Al3Ti metal-intermetallic laminate (MIL) composites have been studied in the crack-divider orientation, by examining the effect of ductile-laminate-layer thickness and volume fraction. The MIL composites were fabricated in open air by a novel one-step process, and the final structure consists of alternating layers of ductile Ti and brittle Al3Ti. Such a laminate architecture in conjunction with a relatively low volume fraction of tougher Ti (18 to 40 pct) was seen to augment the fracture toughness of the inherently brittle intermetallic by over an order of magnitude. Additionally, as a result of their low density, MIL composites exhibit a specific fracture toughness (K/rho) on par with tougher but relatively denser ductile metals such as high-strength steel. Such vast improvements may be rationalized through the toughening provided by the ductile Ti layers. Specifically, toughening was obtained through plastically stretching the intact ductile Ti layers that formed bridging zones in the crack wake, thus reducing the crack driving force. Such toughening resulted in R-curve behavior, and the toughness values increased with an increase in the volume percentage of Ti. Weight-function methods were used to model the bridging behavior, and they indicated that large bridging zones (similar to 2 to 3 min) were responsible for the observed increase in toughness. The role of large-scale bridging (LSB) conditions on the resistance curves is explored, and steady-state toughness (K-SS) values are estimated using small-scale bridging (SSB) approximations. A new approach to gage the potential of laminate composites in terms of their true fracture-toughness values as determined from a cyclic crack-growth fatigue test is proposed, wherein small-scale specimens can be utilized to obtain fracture-toughness values.