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

TL;DR

This study investigates the phase separation kinetics and solubility limits of chromium in iron at 832 K using Mössbauer spectroscopy, revealing a two-stage decomposition process and the border of the metastable miscibility gap.

Contribution

It provides new insights into the kinetics of phase decomposition and determines the Cr solubility limit in iron at 832 K through detailed spectroscopic analysis.

Findings

Border of the metastable miscibility gap at 832 K is 20.9 at.% Cr.

Cr solubility limit in iron at 832 K is 19.8 at.% Cr.

Second-stage activation energy is approximately 12 kJ/mol higher.

Abstract

Kinetics of phase decomposition accompanied by precipitation of sigma-phase in a Fe73.7Cr26.3 alloy isothermally annealed at 832 K was studied by means of M\"ossbauer spectroscopy. Two stage decomposition process has been revealed by three different quantities viz. the average hyperfine field, <H>, the short-range parameter, alpha1, 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 ca.300 h of annealing, has been associated with the decomposition into Fe-rich phase in which the concentration of Cr, determined as 20.9 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 sigma. 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 19.8 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 ca.12 kJ/mol higher.