Mössbauer study of ball-milled Fe-Ge alloys

In this work the mechanical alloying of Fe_100-xGe_x was studied as a function of alloying composition (9<~x<~40) processing time and annealing treatment. The alloys were prepared using a high-energy vibratory ball mill. For 9<~x<~27.5 a single phase A2 solid solution was formed while for x>~30 besides the solid solution an increasing amount of disordered B8₁ (NiAs type) Fe₃Ge₂ was found. Thermally induced A2⃗D0₃ transition was investigated for 9<~x<~27.5. The detailed annealing temperature dependence of short- and long-range ordering into the D0₃ structure was studied for the case of Fe₇₅Ge₂₅ by means of Mössbauer-effect spectroscopy and x-ray diffraction. The solid solution orders gradually and homogenously into the D0₃ structure. To follow and quantify the evolution of short-range order a special fitting routine for Mössbauer spectra was used that takes into account local and nonlocal contributions to the hyperfine interactions experienced by a ⁵⁷Fe probe, and that assumes nonlinearity dependences on the number of neighbors of a given class. By this means the short-range D0₃ order parameter S was determined for as milled steady states as well as annealed states for 9<~x<~27.5. In the case of Fe₇₅Ge₂₅ the evolution of mechanical alloying with time was investigated. It was found that alloying proceeds in two stages. In the first one, iron rich (with A2 structure) and disordered Ge-rich regions coexist along with an almost equiatomic A2 Fe-Ge solid solution, which was interpreted as an interphase physically located between the former two. The three phases collapse into just one bcc solid solution with S≈0.3 for a critical time t_c, which was identified as the chemical mixing time reported by other authors. The second stage of mechanical alloying produces the complete homogenization of the solid solution. Plausible basic mechanisms of mechanical alloying of Fe-Ge mixtures are discussed on the basis of the results presented here.