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To explore the mechanical behavior and microstructure evolution of nanostructured bainite under deformation at high strain rates (> 10 3 s − 1 ), split Hopkinson pressure bar experiments were conducted. It was found that nanometer-sized twinning occurred in both blocky austenite and film austenite. In the former case, the intersections of different twin variants brought about strain-induced α’-martensite, while in the latter case, the lamellar twinned structures were occasionally accompanied by a small amount of ε martensite. It is concluded that twinning occurs first in blocky austenite, enhancing the initial work-hardening; subsequent twinning takes place in film austenite, promoting further straining. [ABSTRACT FROM AUTHOR]

In this work, a possible cause of the extraordinary low impact toughness of nanostructured bainite has been investigated. The microstructure of nanostructured bainite consisted chiefly of carbide-free bainitic ferrite with retained austenite films. X-ray diffractometry (XRD) measurement indicated that no retained austenite existed in the fractured surface of the Charpy-impact-tested specimens. Fractographs showed that cracks propagated mainly along bainitic ferrite platelet boundaries. The change in microstructure after impact loading was verified by transmission electron microscopy (TEM) observations, confirming that retained austenite was completely transformed to strain-induced martensite during the Charpy impact test. However, the zone affected by strained-induced martensite was found to be extremely shallow, only to a depth of several micrometers from the fracture surface. It is appropriately concluded that upon impact, as the crack forms and propagates, strain-induced martensitic transformation immediately occurs ahead of the advancing crack tip. The successive martensitic transformation profoundly facilitates the crack propagation, resulting in the extremely low impact toughness of nanostructured bainite. Retained austenite, in contrast to its well-known beneficial role, has a deteriorating effect on toughness during the course of Charpy impact. [ABSTRACT FROM AUTHOR]