Phase decomposition at 402°C in an Fe-Cr alloy: Mossbauer spectroscopic study

Phase decomposition in strained and strain-free samples of Fe84.85Cr15.15 induced by an isothermal vacuum annealing at 402 degrees C up to 3888 h was studied by means of the Mossbauer spectroscopy. The measured Fe-57-site spectra were analyzed with two independent procedures viz. a hyperfine field distribution and a two-shell superposition methods. Information relevant to the kinetics of the decomposition process, concentration of Cr in the Fe-rich phase, short-range ordering and magnetic texture was obtained based on values of spectral parameters. In particular, the following quantities pertinent to the kinetics were found: the activation energy, equal to 1518 kJ/mol for the strained and to 1083 kJ/mol for the strain-free samples, the Avrami exponent = 2 for both samples, the rate constant = 0.0018 and 0.0078h(-1) for the strained and the strain-free samples, respectively. The concentration of Cr in the Fe-rich phase was found to be 11.6(4) at.% for the strained sample and 13.2(4) at.% for the strain-free one. Significant differences in the short-range ordering parameters were revealed for the two samples. An average angle between the magnetization vector and the normal to the sample's surface was equal to 52.7 for the strained sample and to 57.5 for the strain-free one.