Atom Probe-Field Ion Microscopy and Mössbauer Spectroscopy of Low Temperature Phase Decomposition in a Ferritic Stainless Steel

Abstract

A ferritic stainless steel, nominal composition Fe-28%Cr-5%Ni was aged at 673–748 K for up to 5000 h and the phases produced by ageing were characterized by means of atom probe-field ion microscopy (AP-FIM) and Mössbauer effect.By ageing, the Fe-28%Cr-5%Ni steel decomposes by the spinodal mechanism. The result was compared with that of a Fe-28%Cr steel to evaluate the effect of nickel addition. In the spinodal decomposition, the Cr-rich phase and the Fe-rich phase form an isotropic network so that the random area atom probe analysis readily picks up the decomposition.The Fe-rich phase produced contains 10-12 at%Cr determined by both the atom probe and the Mössbauer effect. On the other hand, the Cr-rich phase contains about 85 at%Cr determined by the Mössbauer effect, while it contains about 56 at%Cr determined by the atom probe. This indicates that the size of the Cr-rich precipitates is smaller than the probe aperture of the atom probe and therefore some averaging of the composition occurs. The addition of nickel to the Fe-28%Cr steel appears to promote the decomposition by decreasing the activation energy for diffusion.