The oxidation behaviour of amorphous alloy Fe40Ni40P14B6

A combination of thermogravimetry, optical microscopy, scanning and transmission electron microscopy, electron probe microanalysis and differential scanning calorimetry has been used to investigate the oxidation behaviour of amorphous Fe₄₀Ni₄₀P₁₄B₆. The oxide layer formed on amorphous Fe₄₀Ni₄₀uB₆ has a whisker-like α-Fe₂O3 structure, which grows very rapidly to build up a thick layer of oxide. Kinetic data indicate that the oxidation of amorphous Fe₄₀Ni₄₀P₁₄B₆ obeys a parabolic rate law as long as the alloy remains amorphous and the rate controlling process is diffusion of iron in the amorphous alloy matrix. However, the oxidation rate drops sharply if crystallization of amorphous Fe₄₀Ni₄₀P₁₄B₆ takes place during oxidation annealing. Crystalline Fe₄₀Ni₄₀P₁₄B₆ also obeys a parabolic rate law but with as much smaller rate constant than the amorphous alloy. The rate controlling process for oxidation of crystalline Fe₄₀Ni₄₀P₁₄B₆ is diffusion of iron and nickel in the multiphase oxide layer, which consists of a fine scale mixture of NiO, Fe₃O₄, Fe₂O₃ and NiFe₂O₄, crystals. The difference in oxidation behaviour between amorphous and crystallized Fe₄₀Ni₄₀P₁₄B₆ is caused by the different alloy microstructures.