Ordering of nanocrystalline Fe-Pt alloys studied by in situ neutron powder diffraction

The ordering in nanocrystalline Fe100-xPtx (x=40, 45, 50, and 55) alloys prepared by mechanical ball milling was studied by neutron powder diffraction during in situ heat treatment and differential scanning calorimetry. Independent of milling time and stoichiometry the onset of ordering is at around 300 degrees C. For alloys with a multilayer-type microstructure an enhanced kinetics of ordering is observed, characterized by an activation energy of 1.03 +/- 0.08 eV. The transformation to the L1(0) phase proceeds via a metastable state, which is a mixture of highly ordered L1(0) and L1(2) phases. For samples with homogeneous A1 phase, the A1 -> L1(0) transformation is characterized by a lower driving force and higher activation energy. For nonequiatomic compositions, the A1 -> L1(0) transformation is considerably decelerated. (c) 2006 American Institute of Physics.