Mössbauer-Effect Study of Solid-Solution and Precipitated Fe-Rich Fe-Mo Alloys

Abstract

A detailed study of the precipitation reaction in an Fe-6 at.% Mo alloy at 550° and 650°C was made using the Mössbauer effect and Young's modulus. Mössbauer spectra of Fe2Mo, Fe3Mo2, and alloys of 2.5 and 20 at.% Mo were also obtained. The Mössbauer spectra of the Fe-base Mo solid solution were analyzed on the basis of a 14 near-neighbor model. The six peaks of the Fe spectrum in the alloy were broadened into six envelopes. These were analyzed as a superposition of three sets of peaks, one set each for Fe atoms with 0, 1, and 2 Mo atoms among the 14 near-neighbors giving in the as-quenched Fe-6 at.% Mo alloy hyperfine fields of −335±1, −296±1, and −255±2 kOe and the isomer shifts relative to pure Fe of +0.01±0.005, −0.02±0.01 and −0.05± 0.02 mm/sec, respectively. The precipitation in the Fe-6 at.% Mo alloy at 650°C occurred by a one-step process while the 550°C aging sequence occurred by a two-stage reaction, first clustering of Mo and then precipitation. An activation of 59 kcal/mole and a rate exponent of 1.2 were determined from modulus and Mössbauer data suggesting diffusion-controlled growth of thin long platelets. The solvus composition Cs in the temperature range 650°–870°C was found by quantitative analysis of the Mössbauer spectra to follow the empirical relation Cs = exp − (7400/RT) at.% Mo assuming the precipitate to be Fe2Mo. A mass balance ruled out Fe3Mo2 (or Fe7Mo6). The Fe2Mo precipitate was confirmed in the Fe-20 at.% Mo alloy by x-ray diffraction.