  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.
