Abstract
In order to elucidate the fracture process in high-temperature low-cycle fatigue of a cast Ti-34wt.%Al with lamellar structure and to derive the life law of smooth specimens, the behavior of initiation and growth of microcracks is observed in detail. The results obtained are summarized as follows. (1) Microcracks, which originate from casting defects at and/or beneath the specimen surface, begin to initiate at less than 15% of the failure life. (2) The transgranular microcracks grow perpendicularly to the stress axis and transversely to the lamellar boundaries. They are neither blocked nor decelerated by the grain boundary. (3) Although the growth rate of microcracks has a large scatter, the average rate coincides with the fracture mechanics law which is characterized by the relationship between the crack growth rate, dc/dN, and the fatigue J-integral range, ΔJf. (4) The crack initiating at the earliest stage of many distributed cracks tends to grow up to be a main crack for the failure. (5) The fatigue life of smooth specimens, Nf, is successfully correlated with ΔWf-parameter which is derived by integrating fracture mechanics law of dc/dN vs. ΔJf relation.
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