Precipitation Phase Transformation in Nanocrystalline Fe-Mo Alloys
Precipitation phase transformation was studied in nanocrystalline Fe-rich Fe-Mo alloys with the use of X-ray diffraction and Mössbauer spectroscopy. Alloys up to 5 at% Mo in Fe were synthesized by mechanical alloying and formed in α phase bcc solid solutions with average grain sizes in the range of 10-13 nm. The precipitation transformation (α → α + λ) was found to proceed via a Mo clustering that was correlated with the size of the nanograins. This was understood in terms of the Gibbs Thomson effect with a concept of negative surface energy contribution to the Gibbs free energy of mixing in a nanocrystalline alloy with positive internal energy of mixing. This contribution increased the stability of the solid solution for nanosized grains, and the Mo precipitation started once the grains grew beyond a critical size. We argue that the Mo precipitation takes place in the grain boundary regions, and the Mo-rich λ phase also precipitates directly in the grain boundary regions, in contrast to the microcrystalline alloys, where the Mo clusters formed within the grains and were first dissolved in the Fe matrix before the λ phase was formed.
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Document Type: Research Article
Publication date: 2004-01-01
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