| Alloys based on gamma titanium aluminide, (TiAl), are of considerable interest as potential structural materials for a number of aerospace and automotive applications. The reasons for this interest are that these materials exhibit attractive elevated temperature mechanical properties, good oxidation and corrosion resistance and low density. The major barriers to their use, however, is their low room temperature fracture toughness and ductility. In addition, there is a need for a comprehensive understanding of the fatigue and fracture behavior of TiAl-based alloys, particularly the micro-mechanisms of deformation and fracture.; The purpose of this program of work therefore, is to conduct a detailed investigation of the micro-mechanism; of crack-tip deformation and fracture under monotonic and cyclic (fatigue) loading conditions in a wide range of TiAl-based alloys. Crack-tip deformation mechanisms are elucidated by crack-tip transmission electron microscopy examination. Twin process zone dimensions and the degree of deformation-induced twinning are also determined via optical interference and transmission electron microscopy techniques, respectively. A micro-mechanics-based model is proposed for assessment of the effects of twin toughening via crack-tip shielding under monotonic or cyclic loading. The model, which is based on non-linear fracture mechanics concepts, assumes an average 'smeared' plastic stress distribution across the twin process zone. Differences in the resistance to fatigue crack growth in gamma-based alloys are related to intrinsic microstructural features and crack-tip shielding phenomena. |
