THERMAL-STABILITY OF DISPERSOIDS IN FERRITIC OXIDE DISPERSION-STRENGTHENED ALLOYS

To study the thermal stability of dispersoids in ferritic oxide-dispersion-strengthened (ODS) alloys, specimens of two alloys, F10 (PM2000) and F11 (PM2010), with different yttria contents were isothermally annealed at 1623 K or isochronally heat treated for 110 h in the temperature range 1273-1623 K. The investigations were performed by small angle neutron scattering and transmission electron microscopy in conjunction with quantitative image analysis. Two overlapping size distributions of dispersoids in the approximate size ranges from 5 to 40 nm and from 40 to 100 nm were found. In addition, a third size distribution of larger particles with a mean value above 250 nm was present. The size distributions determined with each technique are in good agreement for the dispersoids in the size range 5-100 nm, which influence the creep resistance of ODS alloys at high temperatures. Since the aluminium content of the dispersoids increases with increasing heat treatment time and phase transformations take place, the coarsening behaviour of the dispersoids cannot be described by Ostwald ripening. A variation in yttria content in the alloys of between 0.5% and 1% had no significant effect on the number density of dispersoids smaller than 100 nm.