Abstract
This paper reports the first atom probe field-ion microscopy (APFIM) study of the ageing reactions in a Co-bearing (C-300) maraging steel (Fe-l8.5%Ni-9.0%Co-4.8%Mo-0.6%Ti-O. l%Al-O.I%Si-0.03%C (wt%)), using both the conventional APFIM and the position-sensitive atom probe (POSAP). Two families of intermetallic phases (Fe7M06 and Ni3(Mo,Ti) types) have been found to contribute to age-hardening. The com~osition and morphology of these precipitates were studied for different ageing times (0.5- 128 hours at 510 C) to investigate the ageing sequence. The observation of the (Fe,Ni,Co)7Mog p-phase is of interest since this is contrary to most published electron diffraction work, but is supported by thermodynamic calculations. Austenite reversion has been found to start after ageing for 4 hours and the reverted austenite approaches the predicted equilibrium composition after ageing for 8 hours.
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