Acicular ferrite formation by interfacial spinel phase associated with titanium oxide inclusions in welded low-alloyed steel

Acicular ferrite is a key microstructural constituent in welded low-alloyed steel due to its beneficial effect on toughness, yet the exact nucleation mechanisms at non-metallic particles often remain incompletely understood. In this study, we present examples of acicular ferrite nucleation on rhombohedral Ti 2 O 3 phase-analogues within multiphase particles and provide direct experimental evidence for the formation of a spinel-type interfacial layer between (Ti,Mn) 2 O 3 and the ferrite (α-Fe) matrix. Using four-dimensional scanning transmission electron microscopy (4D-STEM) diffraction mapping in combination with transmission electron microscopy–energy-dispersive X-ray spectroscopy (TEM-EDS) analysis, the particle–matrix interface was reconstructed, and the thickness of the interfacial layer was estimated. Notably, manganese was found in abundance at the interfacial layer and the particle, suggesting a decisive role in nucleation, particularly around Ti 2 O 3 -containing inclusions. Taken together, these findings highlight that nucleation is not necessarily a direct Ti 2 O 3 –α-Fe process but can be facilitated by transitional phases, opening new perspectives for modeling acicular ferrite nucleation, assessing suitable particle chemistries for acicular ferrite formation, and tailoring inclusion chemistries to optimize steel toughness.