| The interface diffusion plays an important part in the diffusion welding. The Al/Fe/Ni ternary diffusion couples made by riveting method were heat-treated at different temperatures of 500℃, 550℃and 600℃. The microstructure characteristics of the diffusion zones were studied by means of optical metallography, backscattering technique and energy dispersive spectrometer. The experiment results show that, there are diffusion layers at the interfaces. Based on the experiment data, the thickness and configuration of the diffusion layers, together with their influence factors are studied. Intermetallic compound Fe2Al5 appears at the Al/Fe interface after heat-treatment, and its shape likes shells or branchs due to the Al atoms diffusing along Fe grain boundary. The intermetallic compound Ni2Al3 appears at the Al/Ni interface when diffusion couples are annealed at 500℃and 550℃, and the Ni2Al3 and NiAl3 appear at 600℃. Only solid solution can be observed at Fe/Ni interface in the specimen annealed at 700℃for 200h. Qua the third kind of atoms, Ni atoms and Fe atoms can dissolve in the Al/Fe and Al/Ni binary intermetallic compound layers, but no Al-Fe-Ni ternary compound can be seen in entire ternary diffusion couples. The Fe atoms diffuse into Al/Ni diffusion layers, which form mixed phase through replacement. The Al-Ni and Al-Fe system products in the mixed phase have different growth rate, so the interface move unsteadily and generate bunchy diffusion layer. The growth of a new intermetallic compound at the interface is controlled by both the kinetics and thermodynamics. The composition ratio of the phases is in agreement with the diffusion flux ratio according to the kinetics principle, and the diffusion flux lies on the ability of diffusion of atoms. Premising kinetics qualification is adequate, the new phase which has the strongest driving force of thermodynamics is prone to form at first. Calculating the enthalpy of formation according to Miedema model can investigate the new phases'formation.
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