Microstructural studies of oxide dispersion strengthened austenitic steels

Oxide dispersion strengthened (ODS) austenitic steel (Fe-16.8Cr-22Ni-2W-2.4Mo-1.5Mn-0.62Si-0.15La-0.2Ti+0.35Y2O3, wt%) was prepared by mechanical alloying and consolidated by hot extrusion followed by solutionization at 1150°C for 1h. The composition of this steel is designed to suppress the formation of deleterious delta (δ-) ferrite and sigma (σ-) FeCr phases, which reduces the life the materials. Various microscopic, spectroscopic and diffraction techniques were employed to study the hierarchical microstructural features of this alloy. X-ray diffraction studies confirm the austenitic phase and no traces of δ-ferrite phase were observed in as-extruded and solutionized conditions. The solutionized austenitic ODS steel exhibited ultra-fine grain structure having mean sizes of 280±5 and 440±9nm in transverse and longitudinal sections of the bar, respectively. Evenly dispersed high density of Y-Ti-O nano-oxides with a mean size of 9nm was achieved along with low fractions of TiC and La rich Y2Si2O7. The crystal structure of Y-Ti-O nano-oxides was determined by combining HR-TEM and FFT techniques and was found to be pyrochlore Y2Ti2O7. These dispersoids and ultra-fine grain structure of austenitic steel contribute to an enhanced hardness of 346±25HV.