AP-FIM study of ultra-fine precipitates at the peak hardening stage in a tempered Fe-Mo-C alloy

Morphology and chemical compositions of the precipitates formed at the peak stage of secondary hardening in an Fe-Mo-C alloy were examined by atom probe field ion microscopy (AP-FIM). AP-FIM analysis has confirmed the existence of not only needle-like Mo₂C precipitates along the 〈100〉_α-Fe directions but also plate-like precipitates lying parallel to the {100}_α-Fe planes. The latter were deduced to be clusters of Mo and a small fraction of Fe atoms that contain neither C nor N. These Mo-rich clusters are finer and more densely distributed than Mo₂C. Thus, its presence may contribute to the secondary hardening of Mo steels.