A study of thermodynamic properties of dilute Fe-Ru alloys by ⁵⁷Fe Mössbauer spectroscopy

The room temperature Mössbauer spectra of ⁵⁷Fe were measured for Fe_1−xRu_x solid solutions with x in the range 0.01 ≤ x ≤ 0.08. The obtained data were analysed in terms of short-range order parameter (SRO) and the binding energy E_b between two ruthenium atoms in the studied materials using the extended Hrynkiewicz-Królas idea. The extrapolated value of E_b for x = 0 was used to compute the enthalpy of solution H_FeRu of Ru in Fe matrix. The result was compared with corresponding values given in the literature which were derived from experimental calorimetric data as well as with the value resulting from the cellular atomic model of alloys by Miedema. It was found that all the H_FeRu values are negative or Ru atoms interact repulsively. At the same time, the Mössbauer data were used to determine values of the short-range order parameter α₁. For the as-obtained samples in which atoms are frozen-in high temperature state, close to the melting point, the negative α₁ values were found. The findings indicates ordering tendencies in such specimens. On the other hand, in the case of the annealed samples where the observed distributions of atoms should be frozen-in state corresponding to the temperature 700 K, the Fe_1−xRu_x alloys with x ≥ 0.05 exhibit clustering tendencies (a predominance of Fe-Fe and Ru-Ru bonds), which manifest themselves by positive values of the calculated SRO parameter. The clustering process leads to a local increase in ruthenium concentration and nucleation of a new ruthenium-rich phase with the hcp structure.