Microstructural evolution of Mn-based maraging steels and their influences on mechanical properties

A set of Mn-based maraging TRIP steels was designed by Max-Planck-Institut für Eisenforschung GmbH (MPIE) for light weight and safe automotive applications. According to their research, these Mn-based maraging TRIP steels exhibited a simultaneous increase in both strength and ductility upon aging. They attributed this surprising effect to the combination of precipitation strengthening mechanism and TRIP effect of reverted/retained austenite. This thesis carried out a further study on this type of steels with minor modification of chemical composition (7-12 wt.% Mn, with additional ~ 1 wt.% Al). The unknown precipitates were characterized as L21-ordered Ni2TiAl intermetallic phase for the first time. This type of precipitates is not only coherent but also coplanar with the martensite matrix. Their special orientation relationship together with the small lattice misfit (1.24%) led to the precipitates remaining coherent with the martensite matrix even after a long-term aging for 10080 min. Analyses on precipitate size revealed that the coarsening rate constants follows the diffusion-controlled coarsening kinetics form r ̅^3~Kt predicted by LSW theory, but the experimental precipitate size distributions (PSDs) is much broader than the theoretical PSD function. In addition, a core/shell structure was observed within the precipitates, but the exact structure of this structure is still not clear. The formation of reverted austenite nanolayers initiated at the onset of aging by a diffusionless shear mechanism since the critical Mn concentration for austenite reversion at the interface is very low. The accumulated Mn segregation at grain boundaries in the following aging led to the austenite nanolayers that grew to lath-like reverted austenite, which means the lateral growth of austenite was supported by the diffusion of Mn. Due to the low diffusion rate of Mn and the thermodynamic resistance to coalescence, the growth rate of lath-like reverted austenite is slow and thus the austenite maintained in the range 70-200 nm for a long time. The segregation of Ti and Mo on grain boundaries in the initial aging stage resulted in the Mn concentration of austenite nanolayers being far from that indicated by the equilibrium Fe-Mn phase diagram. The segregation of Ti and Mo gradually vanished with the enrichment of Mn during the succeeding aging process. The TEM-EDS analyses revealed the Mn concentration of lath-like austenite was at the level of ~24 at.% which is higher than that of retained austenite (8-12 at.%) reported in conventional Mn-based TRIP or Q&P steels. Nanoindentation testing revealed that the high stability of reverted austenite in Mn-based maraging steels was mainly attributed to the high Mn concentration of austenite. The nano-size of reverted austenite was also considered to be responsible for the high stability. Severe embrittlement occurred in samples aged at lower temperatures or for short times. Increasing aging temperatures and duration can significantly improve the embrittlement phenomena. An ultimate tensile strength (UTS) of 1120 MPa with total elongation (TE) of 18.4% was obtained in the 12% Mn alloy by aging at 500 °C for 5760 min. It was demonstrated that the dense precipitates contributed to the increase in yield strength whereas the work hardening of reverted austenite contributed to the enhanced strength and ductility after yielding. The TRIP effect of reverted austenite reported by Raabe et al. does not occur to any significant amount owing to the high stability of reverted austenite in Mn-based maraging steels.