Strengthening and strain hardening mechanisms in a precipitation-hardened high-Mn lightweight steel

Abstract

We report on the strengthening and strain hardening mechanisms in an aged high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C, wt.%) studied by electron channeling contrast imaging (ECCI), transmission electron microscopy (TEM), atom probe tomography (APT) and correlative TEM/APT. Upon isothermal annealing at 600 degrees C, nano-sized kappa-carbides form,, as characterized by TEM arid APT. The resultant alloy exhibits high strength and excellent ductility accompanied by a high constant strain hardening rate.& para;& para;In comparison to the as-quenched kappa-free state, the precipitation of kappa-carbides leads to a significant increase in yield strength (similar to 480 MPa) without sacrificing much tensile elongation. To study the strengthening and strain hardening behavior of the precipitation-hardened material, deformation microstructures were analyzed at different strain levels. TEM and correlative TEM/APT results show that the kappa-carbides are primarily sheared by lattice dislocations, gliding on the typical face-centered-cubic (fcc) slip system {111 }<110>, leading to particle dissolution and solute segregation. Ordering strengthening is the predominant strengthening mechanism. As the deformation substructure is characterized by planar slip bands, we quantitatively studied the evolution of the slip band spacing during straining to under stand the strain hardening behavior. A good agreement between the calculated flow stresses and the experimental data suggests that dynamic slip band refinement is the main strain hardening mechanism. The influence of kappa-carbides on mechanical properties is discussed by comparing the results with that of the same alloy in the as-quenched, kappa-free state.