Microstructural influences on the decomposition of an Al-containing ferritic stainless steel

The behaviour of as-received and recrystallised (homogenised) MA 956, a Cr-rich ferritic stainless steel, aged at 475°C for up to 588 h has been investigated. The convolution of recovery and phase separation results in aged as-received material contrasting with hardening due to phase separation only in homogenised material. Atom probe microanalysis of the decomposition products reveals that Al does not partition significantly to the Fe-rich phase, as calculated by phase diagram modelling. The wavelength of the decomposition is found to be significantly larger in aged as-received material compared to aged homogenised material. The spinodal decomposition of the MA 956 supersaturated solid solution is possibly a paraequilibrium transformation with respect to Al. The behaviour of Al is discussed in terms of competing chemical and stress gradients in the nanophase material and the influence of a such a structure on its thermodynamic description.