Crystallization of amorphous Ni₆₀Nb₄₀

Abstract

The crystallization of Ni₆₀Nb₄₀ metallic glass during continuous heating and isothermal annealing at temperatures from 845 to 904 K, was studied by differential scanning calorimetry (DSC), electrical resistance measurements, X-ray diffraction and transmission electron microscopy. Crystallization occurred in four clearly defined stages. In the initial stage a metastable phase, with structure similar to the M-phase in the Ni-Nb-Al ternary system, forms in the amorphous matrix. In the two subsequent stages the remaining glass crystallizes to the Ni₃Nb- andΜ-phases found in the equilibrium Ni-Nb phase diagram. The M-phase transforms into the equilibrium Ni₃Nb- andΜ-phases only at high temperature. The crystallization of the M-phase could be described by Johnson-Mehl-Avrami kinetics with the time exponentn=1.3 and activation energyE ₁=628 kJ mol⁻¹. M-phase crystal growth was apparently diffusion controlled and the diffusion coefficient was estimated to be 4.2×10⁻²⁰ m² sec⁻¹. Activation energies for the second and third stages of crystallization were found to beE ₂=446 kJ mol⁻¹ andE ₃ = 430 kJ mol⁻¹.