Evolution of interphase precipitation during phase tranformation from austenite to ferrite in Ti-Mo microalloyed steels

• Small-angle neutron scattering and atom probe tomography were employed. • Number density of interphase precipitates peaks at the end of phase transformation. • Mo increases the number density and volume fraction of interphase precipitates. • The effect of Mo on phase transformation kinetics depends on temperature. • Balancing precipitate size and volume fraction is crucial to maximise strength. In this work, small-angle neutron scattering (SANS) was combined with dilatometry, transmission electron microscopy (TEM), and atom probe tomography (APT) to comprehensively investigate the interphase precipitation behaviour in Ti-Mo and Ti microalloyed steels. It is shown that both the precipitate volume fraction and number density reach peak values at the end of the phase transformation. Subsequent coarsening and partial dissolution of interphase precipitates then leads to the formation of random precipitates. In comparision to the Ti steel, the larger precipitate volume fraction in the Ti-Mo steel can be attributed to the increased number density, whereas the volume fraction increase in Ti steel is mainly attributed to precipitate growth, as number density changes only slightly during transformation. The elevated number density in the Ti-Mo steel is attributed to a reduced α/γ interface velocity due to the Mo addition, which thereby enhances precipitate nucleation. Site-specific APT lift-out analysis of the transformation interface for the Ti-Mo steel revealed numerous Ti-rich clusters specifically located at the α/γ interface.