  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.
