Structural relaxation of residual amorphous matrix and modulus oscillation in nanocrystalline FeSiNbCuB ribbons

Abstract

Amorphous Fe_73.5Cu₁Nb₃Si_13.5B₉ ribbons prepared by melt-spinning were annealed at 500°C for different periods. The dependence on annealing time (t) of the average grain size (d) and volume fraction (V_c) of α-Fe(Si) phase, as well as the Young’s modulus (E) of the ribbons were investigated by X-ray diffraction (XRD) and tensile test, respectively. The grain size of the α-Fe(Si) phase grew in stages with annealing time, exhibiting plateaus between rapid growth periods. The Young’s modulus oscillated with annealing time and the peaks in the E-t curve appeared at the times when grain growth slow down, i.e. the plateau region. The chemical compositions of the amorphous layer surrounding the growing α-Fe(Si) phase and that of the amorphous matrix were estimated. The time taken for the composition equilibration between the amorphous shell ahead of the crystallization front and the amorphous matrix was calculated with Fick’s diffusion equation. The calculated time had the same order of magnitude as that of the period of the staged-growth of the α-Fe(Si) phase. This staged-growth process, controlled by the diffusion of niobium, and the periodical structural relaxation of the amorphous matrix give rise to the oscillating behavior of the E vs. t relationship.