Phase-Separation, Partitioning and Precipitation in MA956, an ODS Ferritic Stainless Steel

The behaviours of as-received and recrystallised (homogenised) MA 956, an Al-containing Cr-rich ferritic stainless steel, aged at 475 °C for up to 2900 hours have been investigated. Atom probe microanalysis of the decomposition products revealed that Al did not partition significantly to the Fe-rich phase after ≈600 hours ageing, contrary to thermodynamic predictions. Ageing to 2900 hours, however, resulted in partitioning. Further thermodynamic analysis showed that the chemical potential of Al in the Cr-rich α' phase increased more rapidly at later stages of phase separation. The wavelength and amplitude of decomposition were found to be significantly larger in aged as-received material compared to aged homogenised material, consistent with coarsening accelerated by the enhanced solute mobilities associated with the highly-dislocated as-received material. Ti- and Si-rich precipitates were found at the α/α' interfaces at later stages of ageing