Strengthening mechanisms of different oxide particles in 9Cr ODS steel at high temperatures

The influence of different oxide particles on deformation mechanisms of 9Cr ODS steels at temperatures 400-600 °C is tested. Two ODS variants with Al2O3 and Y2O3 dispersed particles respectively and one variant without particles have been prepared by powder metallurgy. The deformation mechanisms are investigated by the tensile and stress-relaxation test at temperatures 400 °C and 600 °C. The analysis is based on a comparison of athermal and thermal stress components of the flow stress, apparent activation volume and a strain rate hardening. Microstructural parameters are evaluated upon SEM and TEM observations. The contributions of different strengthening mechanisms are assessed using the Taylor, Hall-Petch, dispersed barrier and Orowan models. Alumina particles provide the highest strengthening which can be explained by the stress decrease around the particles induced by a high difference between Young's moduli of alumina and ferritic matrix. This stress field can trap the dislocation in the vicinity of particles. Due to this stress decrease, alumina particles provide strengthening even at 600 °C in contrast to yttria particles that lose their effect due to their overcoming by a non-conservative motion of dislocations.