A microstructure study of nanostructured Fe–Mo + 1.5 wt.%SiO2 and +1.5 wt.%TiO2 powders compacted by spark plasma sintering

Two Fe–1.5 wt.%Mo powdered alloys modified with 1.5 wt.%SiO2 and 1.5 wt.%TiO2 compacted through spark plasma sintering have been studied. Transmission electron microscopy was used to characterize the nano-scale microstructure. The results showed that sintered FeMo + SiO2 powder is able to keep a nanometric scale grained structure up to a sintering temperature of 815 °C. Mean grain size of the FeMo + TiO2 compacted powder maintained its nanometric range up to 900 °C. Residual porosity decorates the grain boundary and the triple grain junctions in all the studied sintering conditions. Moreover, nano-porosity accounted for a maximum of 0.26% volume fraction. Direct comparison between X-ray diffraction grain size evaluation (from previously published data) and transmission electron microscopy investigations, showed a significant underestimation when using the first approach. Nano-indentation tests confirmed that the micro-hardness values tend to increase with sintering temperature. Berkovich nano-indentation tests clearly showed that micro-hardness values in nanostructured compacts lead to overestimation of the real hardness values.