Structural characterization of nano-scale precipitates in Cu bearing extra-low-carbon steels

Abstract

The morphologies, size distributions, and crystal structures of sulfide and nitride precipitates in the copper (Cu) bearing extra-low-carbon steel matrix were investigated by transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectrometry (EDS). An atomic-scale structural analysis by high-resolution electron microscopy (HREM) also was carried out. The (Cu, Mn)S precipitates manifested a core–shell structure, the shell part containing Cu and S elements, and the core part Mn and S elements. Most of the nano-scale nitrides of wurtzite structure were less than 30 nm in diameter; indeed, the average diameter as estimated was about 20 nm. The TEM analysis also revealed two nitride precipitation modes: hexagonal AlN precipitates of relatively large size (several tens of nm) and elongated/cuboidal morphology, and cubic TiN precipitates of square and rectangular shape.