Kinetics of transformation, border of metastable miscibility gap in Fe-Cr alloy and limit of Cr solubility in iron at 858 K

TL;DR

This study investigates the transformation kinetics and solubility limits of Cr in Fe-Cr alloys at 858 K, revealing a two-stage process and precise measurements of the metastable miscibility gap and Cr solubility.

Contribution

It provides new kinetic data and accurate determination of the metastable miscibility gap and Cr solubility limit in Fe-Cr alloys at 858 K using Mossbauer spectroscopy.

Findings

Metastable miscibility gap at 24.5 at.% Cr

Cr solubility limit at 20.3 at.% Cr

Transformation occurs in two stages with different kinetics

Abstract

The study was aimed at determination of the position of the Fe-rich border of the metastable miscibility gap (MMG) and of the solubility limit of Cr in iron at 858 K. Towards this end a Fe73.7Cr26.3 alloy was isothermally annealed at 858 K in vacuum up to 8144 hours and M\"ossbauer spectra were recorded at room temperature after every step of the annealing. Three spectral parameters viz. the average hyperfine field, <B>, the average isomer shift, <IS>, and the probability of the atomic configuration with no Cr atoms in the two-shell vicinity of the probe Fe atoms, P(0,0), gave evidence that the transformation process takes place in two stages. All three parameters could have been well described in terms of the Johnson-Mehl-Avrami-Kolmogorov equation, yielding kinetics parameters. The first stage, associated with the phase decomposition, proceeded much faster than the second stage, associated with the alpha-to-sigma phase transformation. The most reliable estimation of the position of the MMG and that of the value of the Cr solubility limit was obtained from the annealing time dependence of <B>, namely 24.5 at.% Cr for the former and 20.3 at.% Cr for the latter. A comparison of these figures with the recent phase diagrams pertinent to Fe-Cr system was done.