Sarkar Subhajit, Bansal Chandrahaas
School of Physics, University of Hyderabad, Hyderabad-500 046, India.
J Nanosci Nanotechnol. 2004 Jan-Feb;4(1-2):203-8.
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 alpha phase bcc solid solutions with average grain sizes in the range of 10-13 nm. The precipitation transformation (alpha-->alpha + lambda) 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 lambda 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 lambda phase was formed.
利用X射线衍射和穆斯堡尔谱研究了富铁的纳米晶Fe-Mo合金中的沉淀相变。通过机械合金化合成了铁中钼含量高达5原子%的合金，其形成了α相体心立方固溶体，平均晶粒尺寸在10-13纳米范围内。发现沉淀转变（α→α+λ）通过与纳米晶粒尺寸相关的钼聚集过程进行。这可以根据吉布斯-汤姆逊效应来理解，即对于具有正混合内能的纳米晶合金，负表面能对混合吉布斯自由能有贡献。这种贡献增加了纳米尺寸晶粒固溶体的稳定性，一旦晶粒生长超过临界尺寸，钼沉淀就开始了。我们认为钼沉淀发生在晶界区域，富钼的λ相也直接在晶界区域沉淀，这与微晶合金不同，在微晶合金中，钼团簇在晶粒内形成，在λ相形成之前先溶解在铁基体中。
