Enhanced superplasticity in an Al-alloyed multicomponent Mn–Si–Cr–C steel

Excellent superplasticity (elongation ∼720%) is observed in a novel multi-component (Mn–S–Cr–Al alloyed) ultrahigh carbon steel during tensile testing at a strain rate of 2 × 10−3 s−1 and a temperature of 1053 K (just above the equilibrium austenite–pearlite transformation temperature). In order to understand superplasticity in this material and its strong Al dependence, the deformation-induced microstructure evolution is characterized at various length scales down to atomic resolution, using X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, energy-dispersive X-ray spectroscopy and atom probe tomography. The results reveal that 1 wt.% Al addition influences various microprocesses during deformation, e.g. it impedes Ostwald ripening of carbides, carbide dissolution, austenite nucleation and growth and void growth. As a result, the size of the austenite grains and voids remains relatively fine (<10 μm) during superplastic deformation, and fine-grained superplasticity is enabled without premature failure.