Formation of a face-centered-cubic metastable phase in the Fe-Nb system by solid-state reaction

In the bcc-structured Fe-Nb system, two types of metastable crystalline phases, i.e., an fee (namely fcc-I) and an enlarged fee (namely fee-II) were formed by solid-state reaction at medium temperatures in Fe-Nb multilayered films with 15-80 at% Nh. The fcc-I was only found in the Fe20Nb80 films. The fee-II phase was found in all the multilayered films, i.e. Fe20Nb80, Fe35Nb65, Fe55Nb45 and Fe85Nb15. However, the lattice constant of the formed fee-II decreased with increasing overall Fe atomic concentration in the as-deposited films. Thermodynamically, the formation of the fee phases are favored in the range of 15-83 at% Nh, based on a constructed Gibbs free energy diagram of Fe-Nb system. Kinetically, the formation of the fee phases can take place through a bcc-fce transition by shearing and then sliding. (C) 1999 Elsevier Science S.A. All rights reserved.