Deformation Behavior in Medium Mn Steel of Nanometer-Sized α' plus γ Lamellar Structure

Yielding and work-hardening phenomena in an Fe-10.62Mn-2.84Al-0.17C-0.5Mo steel, which is composed of nanometer-sized lamellae of alpha' and gamma, are described on the basis of the Hall-Petch relations. Unlike the general expectation, yielding in the steel, which consists of lamellae of alpha' and mechanically stable gamma, occurs through the propagation of pileup dislocations from alpha' to gamma. However, when gamma is mechanically unstable, yielding occurs through the stress-assisted martensitic transformation (SAMT) within the unstable gamma region, resulting in a low YS of about 500 MPa. The overall prominent work-hardening behavior of this steel after yielding is due to the active SAMT, which does not accompany the increase in mobile dislocation density and so causes the high elastic strain rate. The carbon partitioning treatment increases the SAMT starting strength to about 980 MPa, which is caused by the mechanical stabilization of gamma. The overall low work-hardening behavior of this case is mainly attributed to the active propagation of pile-up dislocation from alpha' to gamma which causes the high plastic strain rate through the abrupt increase of mobile dislocation density.