On the strain hardening mechanisms of a high-Mn lightweight steel

The aim of this thesis, is to clarify the strain hardening mechanisms of a fully austenitic high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C wt.%) in the as-quenched state and in the precipitation-hardened state. Interrupted tensile tests at room temperature were performed in order to study the microstructural evolution. In the as-quenched state pronounced planar dislocation glide leads to the formation of slip bands, which are defined as crystallographic planes of high dislocation density. The slip band structure refines during straining by reduction of the spacing between the slip bands. On the basis of this observation a novel strain hardening mechanism is proposed, which is described as dynamic slip band refinement. Isothermal holding of the studied alloy for 24h at 600°C promotes the formation of κ-carbides. These nano-sized cuboidal precipitates exert strong influence on the mechanical properties. The yield strength of the precipitation-hardened material is increased by about 480MPa compared to the precipitation-free material, without sacrificing much of the good ductility. TEM investigations show that κ-carbides are sheared by dislocations, which is concluded to be the main contribution to precipitation-strengthening. It is shown that strain hardening in the κ-carbide strengthened state is surprisingly also caused by dynamic slip band refinement.