Mo¨ssbauer study of ball-milled Fe-Ge alloys

A. F. CABRERA AND F. H. SANCHEZ

PHYSICAL REVIEW B - SOLID STATE

APS

Año: 2002 vol. 65 p. 942021 - 942029

0556-2805

In this work the mechanical alloying of Fe1002xGex 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 A2!D03 transition was investigated for 9<x<27.5. The detailed annealing temperature dependence of short- and longrange ordering into the D03 structure was studied for the case of Fe75Ge25 by means of Mo¨ssbauer-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 Mo¨ssbauer 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 disorderedGe-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 tc , 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.