Hyperfine Interactions 126 (2000) 187–191 187 A new approach to the study of the α–σ phase transformation J. Cie´ slak a , S.M. Dubiel a and B. Sepiol b a Faculty of Physics and Nuclear Techniques, The University of Mining and Metallurgy, al. Mickiewicza 30, PL-30-059 Krak´ ow, Poland b Institut f¨ ur Materialphysik, Universit¨ at Wien, Strudlhofgasse 4, A-1090 Wien, Austria It is demonstrated that the kinetics of the α–σ phase transformation in the Fe–Cr based alloys can be studied from the average isomer shift. Using this new approach it is shown that the size of grains significantly affects the formation of the σ-phase, while that of the α-phase does not depend on the microstructure. The σ-phase was found in over fifty binary transition-metal alloys [1]. It is of great interest for both scientific and technological sake. Scientifically it is related to gathering knowledge of physical properties of the phase and the mechanisms of its formation. Technological interest originates from the fact that the σ-phase often occurs in materials that are technologically important and its presence drastically deteriorates their properties. One class of such materials are high-chromium steels which have superior creep- and heat-resistant properties and, consequently, they have found a wide application in various branches of industry, e.g., in oil refinery and nuclear power plants. The steels are composed to over 95% of Fe–Cr alloy, hence it has been regarded as a model system for the investigation of the σ-phase. M¨ ossbauer Spectroscopy (MS) is one of the techniques which can be successfully applied to the measurements of the kinetics of the σ-phase formation [2]. A traditional way of the phase analysis by means of the MS is based on a spectral area. In the case of a two-phase sample, which is discussed here, a M¨ ossbauer spectrum is analyzed in terms of two subspectra corresponding to the two phases present. A typical spectrum of this kind recorded at room temperature can be seen in figure 1(a). It consists of a pseudo single-line with a spectral area S σ which originates from the σ-phase, and a broadened sextet having a spectral area S α which originates from the α-phase. The two subspectra have not only different shape but also different spectral parameters, therefore the decomposition of the overall spectrum is an easy and unique task. Based on the above-described approach, the relative amount of the σ-phase present A σ can be determined from the following equation: A σ = S σ f α S α f σ + S σ f α , (1)  J.C. Baltzer AG, Science Publishers