Microstructural characterization of Fe-9%Cr ODS steel produced via selective laser melting

Abstract

In this master thesis, the characteristics of reduced-activation ferritic Fe-9Cr oxide-dispersion-strengthened (ODS) steel, produced via mechanical alloying and laser-based additive manufacturing, are explored, with the main focus on the microstructure and the presence and size of the Y-based nano-oxides. Reference steel samples are also produced with the same processes, in order to characterize the impact of Yttria on the characteristics of the final steel. The study employs metallographic techniques, SEM, TEM, STEM and synchrotron x-ray diffraction to assess the steel properties. The results show the typical structure and microstructure of metal additive manufacturing. Melt pools and layers are clearly visible. The layer size is on average roughly consistent with the set fabrication parameter, while the melt pools’ size appear to increase with higher energy density. Two types of grains can be identified: elongated columnar grains spanning from the edge towards the centre of the melt pool, and equiaxed grains, mostly localized in the centre. Regions that are expected to have experienced lower thermal gradients during solidification seem to develop larger grains on average. Instances of abnormal grain structure are also highlighted, such as the presence of hourglass-shaped grains. Overall, the samples are clearly fine-grained, with the largest grains still below 2 µm on average. In some of the samples, a higher degree of porosity than what is expected for this consolidation technique is noticed. Possible explanations are given, such as the large spread in characteristics of the feed powder, possible material stripping during cutting and grinding and volume contraction during the melt pool formation. Nanoparticles are clearly visible throughout all of the ODS steel samples. They appear larger than expected for high grade ODS, with a large presence of dispersoids above 40 nm in diameter. Their morphology appears to be spherical in most cases and octahedral crystalline in some. There is some evidence supporting the formation of a core-shell structure and heterogeneous composition inside the particles themselves. Possible traces of V presence are also found in some ODS particles. Some dispersoids below 10 nm are also identifiable, but their chemical nature cannot be confirmed due to the limited resolution of EDXS in this study.