Microstructural evolution and mechanical properties of nanocrystalline Fe–Mn–Al–C steel processed by high-pressure torsion

Abstract In this study, microstructural evolution and mechanical properties of Fe–Mn–Al–C steel were investigated with the variation of shear strain imposed by high-pressure torsion (HPT). Two different initial grain sized steels were used: (1) fine grained (FG: ≈10 μm) and (2) coarse grained (CG: ≈100 μm) steels, and the amount of strain was varied by increasing the number of revolution (R) of HPT. At the maximum R (10R) of HPT, FG and CG microstructures were refined to the average grain size of 20.7 ± 5.2 nm and 51.0 ± 11.0 nm, respectively. With increasing R, ultimate tensile strength (UTS) of both FG and CG steels increased, and however, FG 10R showed lower UTS (Δσ ≈ 350 MPa) than that of FG 5R, indicating a softening phenomenon of inverse Hall-Petch (IH–P) relation. TEM observation of the finest grained FG 10R revealed the absence of deformation twins and the formation of numerous tilt/twist nanocrystalline boundaries, which might explain the softening behavior in this regime.