Transmission Electron Microscopy Study on the Grain Boundary Precipitation of an Fe-Ni-Mn Maraging Steel

Abstract

Transmission electron microscopy was used to study grain boundary precipitation of an Fe-10Ni-7Mn (wt pct) maraging steel during isothermal aging at 753 K. Precipitation of a face-centered tetragonal (fct) θ-NiMn phase in association with solute-depleted precipitate free zone (PFZ) was found at prior austenite grain boundaries. Precipitation-induced puckering of grain boundaries in the early stages of aging initiates grain boundary displacement, creating PFZs behind the displaced grain boundary. Further grain boundary migration is driven by the manganese and nickel concentration gradients across the migrating boundary and the elastic strain energy and interphase boundary energy of fine fct θ-NiMn particles in the grain that is to be swept by the migrating boundary. Consequently, coarsening of the fct θ-NiMn particles occurs via grain boundary diffusion as they are passed by the migrating boundary. As rationale, the reaction is defined as discontinuous coarsening of grain boundary precipitates, featuring a nodular morphology of discontinuous coarsening aggregate. Continuous precipitation within the grain facing reaction front decreases overall reaction driving force at later stages, which leads to a decelerated reaction rate.