A nanotwinned surface layer generated by high strain-rate deformation in a TRIP steel

• Surface mechanical grinding treatment (SMGT) can produce gradient nanotwinned layers with high hardness in a TRIP steel.
• Deformation mechanism can change from TRIP effect to TWIP effect in a SMGT process.
• De-twinning plays a key role in the recovery and recrystallization of deformation twins.
• Thermal stabilities of deformation layers produced by SMGT can be greatly enhanced.

Gradient nanotwinned layers (GNTLs) with high hardness of approximately 500 HV and good thermal stability were produced in a Fe–20Mn–3Al–3Si TRIP steel by means of a high strain-rate surface mechanical grinding treatment (SMGT). The effect of the strain-rate on the plasticity-enhancing mechanisms in steels was investigated. It is found that although it primarily exhibits transformation-induced plasticity (TRIP) when cold-rolled, this steel shows twinning-induced plasticity (TWIP) during the SMGT process, illustrating that the steel may undergo a transition from TRIP to TWIP under high-strain-rate deformation. The martensite induced by deformation is thermally unstable and can easily transform back to austenite during annealing. In contrast, the deformation twins (DTs) are thermally much stable, since de-twinning, which plays a key role in the recovery of DTs, is more difficult to occur during thermal annealing. As such, undergoing the same annealing at 600 °C for 1 h, the GNTLs containing a great many of DTs maintain high hardness, whereas the cold-rolled counterpart samples containing deformation-induced martensite softens drastically.

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