  Partitioning at phase boundaries of complex steels is important for their
properties. We present atom probe tomography results across martensite /
austenite interfaces in a precipitation-hardened maraging TRIP steel (12.2 Mn,
1.9 Ni, 0.6 Mo, 1.2 Ti, 0.3 Al; at.%). The system reveals compositional changes
at the phase boundaries: Mn and Ni are enriched while Ti, Al, Mo, and Fe are
depleted. More specific, we observe up to 27 at.% Mn in a 20 nm layer at the
phase boundary. This is explained by the large difference in diffusivity
between martensite and austenite. The high diffusivity in martensite leads to a
Mn-flux towards the retained austenite. The low diffusivity in the austenite
does not allow accommodation of this flux. Consequently, the austenite grows
with a Mn-composition given by local equilibrium. The interpretation is based
on DICTRA and mixed-mode diffusion calculations (using a finite interface
mobility).
