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

Abstract

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