PHASE-SEPARATION IN THE FE-CR-NI SYSTEM

The phase relations and microstructures of a series of isothermally ag ed Fe-45%Cr-5%Ni alloys have been determined with a combination of ato m probe field ion microscopy, transmission electron microscopy, electr on probe microanalysis and optical microscopy. The position of the mis cibility gap has been determined. The phases composing the alloy varie d with the isothermal ageing temperature. At the highest temperatures, 650 and 600 degrees C, only the sigma phase was observed, At the lowe st temperature, 400 degrees C, the alloy was composed entirely of the alpha and alpha' phases. At intermediate temperatures, the near-surfac e region of the sheet was composed of the a phase and the interior of the sheet of the alpha and alpha' mixture. The morphology of the alpha ' regions was a complex interconnected network structure at 400 and 50 0 degrees C and isolated regions at 550 degrees C. The addition of nic kel to the binary Fe-45%Cr alloy was observed to raise the critical te mperature of the miscibility gap and skew it to higher chromium concen trations, In the alpha end alpha' two-phase mixture, the nickel was fo und to partition preferentially to the cu phase, The experimental resu lts are compared with the predictions of the Thermocalc(TM) program.