Effect of Mn on precipitation behaviour of NiAl phase in Fe–NiAl steels

The microstructural evolution of two model alloys, Fe–NiAl and Fe–NiAl–Mn, during aging at 500°C was investigated using atom probe tomography (APT). Vickers micro hardness measurements showed that the addition of Mn increased the hardness and decreased the time to reach the peak hardness. Both alloys exhibited a softening process at the initial stage of aging. The APT results showed that adding Mn increased the number density of NiAl nanoparticles significantly, and Mn atoms partitioned into the NiAl nanoparticles, occupying the Al sites preferentially. The precipitation of NiAl nanoparticles was accelerated by the addition of Mn because Mn increased the nucleation driving force for NiAl precipitates and reduced the strain energy for the nucleation.