The influence of critical points and structure and microstructural evolution in iron rich Fe-Si alloys

Abstract

The evolution of microstructure in Fe-8.7 to 14.2 at.% Si alloys in the neighbourhood of higher order transition lines has been investigated. Systematics of various features of the Fe-Si phase diagram, highlighting the multicritical points, have been presented. The possible influence of these critical points on the kinetic pathways for the microstructural evolution has been discussed and compared with experimental results. It is shown that below the metastable tricritical point the alloy undergoes continuous ordering and a conditional spinodal followed by the renucleation of B2 phase in the DO₃ matrix. The latter process leads to the anomalous contrast in the B2 superlattice darkfield images. It is also shown that above the metastable (extrapolated) higher order B2 → DO₃ transition line a small domain exists in the two phase field where decomposition to two phase B2 + DO₃ takes place by classical nucleation of fine DO₃ domains in B2 matrix. No evidence for DO₃ ordering could be obtained in alloy containing less than 11 at.% Si.