Precipitation behaviour of an Fe-Co-Mo-alloy during non-isothermal ageing

Fe-Co-Mo alloys show hardening behaviour comparable to that of precipitation hardened Al-alloys, which is characterised by a low hardness after solution annealing and a significant increase in the hardness during subsequent ageing at higher temperatures. In this study, the precipitation behaviour of Fe-25 % Co-15 % Mo (wt.%) has been studied applying non-isothermal ageing. High sensitivity differential scanning calorimetry and atom probe tomography have been used to characterise the precipitation sequence. Three overlapping exothermic reactions, which are related to precipitation reactions, are observed in the temperature range of 400 to 700 degrees C. The investigations have shown that precipitation starts with diffusion of Mo, followed by the formation of the intermetallic mu-phase which reaches its equilibrium composition at a temperature of about 615 degrees C.