Improving ductility of high strength nanotwinned steel with reverse transformation of epsilon martensite inside nanotwin lamellae

In this work, the reverse transformation of ε-martensite located inside nanotwin lamellae has been introduced to improve the ductility of a high-strength Fe–Mn–C steel with a high density of nanotwins. The steel was designed to have a low stacking-fault energy (SFE, 17.8 mJ/m2) to introduce high density of nanotwins through pre-deformation. Meanwhile, the fine ε-martensite was also obtained in nanotwin lamellae. The steel was strengthened significantly, with an increase in yield strength from 357 to 1275 MPa by the introduction of nanotwins. A subsequent intermediate annealing was applied to control the reverse phase transformation from ε-martensite to austenite to enhance the deformation ability of the nanotwinned steel (rising tensile elongation from 5 to 16%). The nanotwins with a width of 32 nm on average were stable during the heat treatment. The high strength was retained because the size of the austenite transformed from ε-martensite was restrained by the stable nanotwins. This strategy holds promise and can be widely applied to alloys with low SFE.