Mössbauer study of ball-milled Fe-Ge alloys

Abstract

In this work the mechanical alloying of ${\mathrm{Fe}}_{100\ensuremath{-}x}{\mathrm{Ge}}_{x}$ 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 ${B8}_{1}$ (NiAs type) ${\mathrm{Fe}}_{3}{\mathrm{Ge}}_{2}$ was found. Thermally induced $A\stackrel{\ensuremath{\rightarrow}}{2}{D0}_{3}$ transition was investigated for $9<~x<~27.5.$ The detailed annealing temperature dependence of short- and long-range ordering into the ${D0}_{3}$ structure was studied for the case of ${\mathrm{Fe}}_{75}{\mathrm{Ge}}_{25}$ by means of M\"ossbauer-effect spectroscopy and x-ray diffraction. The solid solution orders gradually and homogenously into the ${D0}_{3}$ structure. To follow and quantify the evolution of short-range order a special fitting routine for M\"ossbauer spectra was used that takes into account local and nonlocal contributions to the hyperfine interactions experienced by a ${}^{57}\mathrm{Fe}$ probe, and that assumes nonlinearity dependences on the number of neighbors of a given class. By this means the short-range ${D0}_{3}$ order parameter S was determined for as milled steady states as well as annealed states for $9<~x<~27.5.$ In the case of ${\mathrm{Fe}}_{75}{\mathrm{Ge}}_{25}$ 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 $S\ensuremath{\approx}0.3$ 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.