Kinetics of phase separation, border of miscibility gap in Fe–Cr and limit of Cr solubility in iron at 832 K

Abstract

Kinetics of phase decomposition accompanied by precipitation of σ-phase in a Fe–Cr alloy with 26.3 at% Cr isothermally annealed at 832 K was studied by means of Mössbauer spectroscopy. Two stage decomposition process has been revealed by three different quantities viz. the average hyperfine field, <H>, the short-range parameter, α1, and the probability of atomic configuration with no Cr atoms within the first two coordination shells around the probe Fe atoms, P(0,0). The first stage, that has terminated after ∼300 h of annealing, has been associated with the decomposition into Fe-rich phase in which the concentration of Cr, determined as 20.7 at.%, can be interpreted as the border of the metastable miscibility gap at 832 K. The second stage can be regarded as a continuation of the phase decomposition process combined with a precipitation of σ. Its termination can be associated with the limit of Cr solubility estimated as 19.7 at% Cr. The three relevant parameters for this stage have also saturation-like behavior vs. annealing time and the saturation can be interpreted as termination of the two processes. The concentration of Cr in the Fe-rich phase has been determined as 19.7 at.% and this value can be regarded as the limit of Cr solubility in iron at 832 K. Both stages of the kinetics were found to be in line with the Johnson-Mehl-Avrami-Kolgomorov equation yielding values of the rate constant and the Avrami exponent. The activation energy of the second-stage process was determined to be by ∼12 kJ/mol higher.