Correlative Electron Microscopy Analysis of Precipitates in a V–Nb–Mo Micro-alloyed Steel

Correlative microscopy is an advanced technique that has been performed to elucidate precipitation behavior of a V–Nb–Mo micro-alloyed steel. The same sample region was analyzed using transmission electron microscope (TEM) and transmission Kikuchi diffraction (TKD) to comprehend the impact of microscale grain boundary character on the intragrain nanoscale precipitates. Thermo-mechanical simulation yielded ferrite microstructure with predominantly nanoscale (V, Mo)C precipitates. The microstructural analysis revealed finer grain size of 4 ± 1.9 µm with nanoprecipitates of 2 to 5 nm size in 650 °C isothermally held sample and 6.6 ± 3.4 µm size grains comprised 5 to 10 nm size precipitates in 700 °C isothermally held sample. Preferential nucleation and growth of (V, Mo)C precipitates were observed on the dislocations and high-angle grain boundaries due to the rapid diffusion of solute atoms through disordered lattice. The consumption of solute atoms near the dislocations and high-angle grain boundaries resulted coarser precipitates and precipitates free zone (PFZ). In contrast, low-angle grain boundaries and coincidence site lattice (CSL) boundaries were discovered to be less favorable sites for precipitates because of their low energy. The pre-existing dislocations in the austenite phase locally alter the lattice orientation, resulting in the creation of domains of interphase precipitates with varying row alignment. Correlative microscopy analysis provided broad understanding about the formation of precipitates in the V–Nb–Mo micro-alloyed steel by correlating the multiscale microstructural features.