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 A1 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 A1 in the Cr-rich a' 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. Tiand Si-rich precipitates were found at the ala' interfaces at later stages of ageing.