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

The microstructural evolution in a set of Mn-based maraging steels (7–12 wt% Mn) when aged at 460–500 ºC for various durations up to 10,080 min and the influences on mechanical properties are systematically investigated. The improved yield strength of peak-aged samples is attributed to the formation of Ni₂TiAl precipitates and the precipitation strengthening is governed by Orowan mechanism. Segregation of Mn at grain boundaries in the initial aging stage resulted in severe intergranular brittleness. During further aging, accumulated Mn segregation leading to the formation of ductile lath-like reverted austenite removed the embrittlement and significantly improved the ductility. In the overaged condition, the steady work hardening after yielding compensates the loss of yield strength resulting from the coarsening of precipitates and softening of α^′-martensite matrix. There was only limited evidence of the TRIP effect in the reverted austenite, indicating that work hardening was associated with other deformation mechanisms. Increasing the aging temperature or the Mn content of alloy that promotes austenite reversion was demonstrated to accelerate the improvement of ductility.