Evaluation of threshold stress of the MA957 ODS ferrtic alloy

Threshold stress of ODS (Oxide Dispersion Strengthened) alloy is well known as a value which can quantitatively shows the effect of oxide dispersion strengthening. However, the threshold stress cannot be clearly observed in the result of creep tests for the commercial ODS alloy MA957. In this work, the threshold stress of MA957 was carefully evaluated using TEM (Transmission Electron Microscope) with EDS (Energy Dispersive X-ray Spectrometer). From the result, it was revealed that MA957 has an inhomogeneous dispersion of oxide particles. The inhomogeneous dispersion caused different threshold stresses between observed areas. This is probably a reason why the threshold stress cannot be clearly seen in the result of creep tests. This inhomogeneous dispersion appeared to be partly caused by aluminum contamination, because aluminum made much larger complex oxide particles than the fine oxide particles with no aluminum. Consequently, aluminum should be strictly controlled or eliminated for the homogeneous dispersion of oxide particles of MA957.