FUNDAMENTAL-ASPECTS OF FATIGUE AND FRACTURE IN A TIAL SHEET ALLOY

The fatigue mechanisms in a TiAl sheet alloy, heat treated to the lame llar and equiaxed microstructures, were studied to determine the effec ts of microstructure on the initiation of microcracks and their subseq uent growth into large cracks. The nucleation and growth history of in dividual microcracks were followed. For comparison, fatigue crack grow th and fracture toughness were also characterized using specimens cont aining a machined notch with a fatigue precrack. The results indicated that microcracks initiated at grain/colony boundaries and at slip ban ds. Most microcracks were arrested after nucleation, but a few grew at stress intensity ranges below the large crack threshold. The populati ons of nonpropagating and propagating cracks varied with life fi actio ns. Ligaments in the wake of a fatigue crack were more severely strain ed than the crack-tip region of the main crack, and, as a result, they were more prone to fatigue failure. The destruction of the crack-wake ligaments is expected to result in lower fracture resistance in mater ials under cyclic loading than those under monotonic loading.