Abstract
Microstructure-sensitive fatigue crack growth in 304 metastable austenitic stainless steel containing nanotwin bundles was studied using a miniature compact tension specimen combined with post-fatigue metallographic examinations. The single-variant nanotwinned specimens exhibited high crack growth resistance compared to the coarse-grained specimen without nanotwin bundles, regardless of lamellar orientation. In the nanotwin bundles, martensite variants are formed with the habit plane parallel to the twin plane during crack propagation. Even when the crack propagates along the twin boundary, it tends to proceed by activation of multiple slip systems into the formed martensite phase, which inhibits brittle twin boundary separation. Post-fatigue transmission electron microscopy revealed that detwinning occurred ahead of the crack tip. Therefore, introducing nanotwin bundles into the metastable austenitic steel changes the course of damage accumulation through detwinning and martensite formation, enhancing the fatigue crack growth resistance.
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