On the Ti-Mo-Fe-C atomic clustering during interphase precipitation in the Ti-Mo steel studied by advanced microscopic techniques

Highlights: • The Ti-(Mo)-(Fe)-C clusters co-existed with Ti-Mo interphase nano-precipitates. • Cluster hardening mechanism of ferrite was proposed. • The clusters had the same crystal structure as the solvent phase. • Mo participates in cluster composition after 1700 s of isothermal hold. • Nano-precipitates were formed via transition of clusters and metastable precipitates. Site-specific transmission electron microscopy (TEM) and atom probe tomography (APT) have been used to study the Ti-Mo-Fe-C atomic clustering during interphase precipitation at the early stages of austenite to ferrite transformation in a Fe–0.03C–1.5Mn–0.2Si–0.29Cr–0.22Mo–0.05Ti–0.025Al (in wt.%) steel. We observed: (i) the formation of only clusters at the beginning of austenite-to-ferrite transformation and co-existence of numerous clusters along with interphase nano-scale precipitates during development of the transformation, (ii) the nano-precipitates were formed via transient metastable precipitates, and (iii) a cluster-hardening mechanism was proposed based on hardness test and APT and TEM characterization. It was suggested that the rapid hardening reactions, due to the formation of solute clusters, can accompany the early stages of solid solution decomposition, which precede the formation of interphase precipitates.