Phase separation in the FeCrNi system

The phase relations and microstructures of a series of isothermally aged Fe45%Cr5%Ni alloys have been determined with a combination of atom probe field ion microscopy, transmission electron microscopy, electron probe microanalysis and optical microscopy. The position of the miscibility gap has been determined. The phases composing the alloy varied with the isothermal ageing temperature. At the highest temperatures, 650 and 600°C, only the σ phase was observed. At the lowest temperature, 400°C, the alloy was composed entirely of the α and α′ phases. At intermediate temperatures, the near-surface region of the sheet was composed of the σ phase and the interior of the sheet of the α and α′ mixture. The morphology of the α′ regions was a complex interconnected network structure at 400 and 500°C and isolated regions at 550°C. The addition of nickel to the binary Fe45%Cr alloy was observed to raise the critical temperature of the miscibility gap and skew it to higher chromium concentrations. In the α and α′ two-phase mixture, the nickel was found to partition preferentially to the α phase. The experimental results are compared with the predictions of the Thermocalc™ program.