The Research On The Periodic Layered Structure In The Ni₃Si/Zn Diffusion Couples

The alternating layer microstructure was found in the Fe₃Si/Zn diffusion couples by Osinski in 1982, which was called the solid-state periodic layered structure. This discovery has attracted many scholars'attention. They have many exploratory researches on it. However, the formation mechanism of periodic layer structure remains controversial, many phenomena have to be clarified. Since the phase diagram is the foundation of this science research, the Zn-Ni-Si ternary phase diagram was determined. The periodic layered structure was investigated based on the theory of diffusion path and interface reaction.With a series of experiments, this work has completed the measurement of the zinc-rich corner of the Zn-Ni-Si system at 450℃and determined the zinc-rich corner of the Zn-Ni-Si system at 500℃using the equilibrated alloys. The specimens were investigated by means of scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) and X-ray powder diffraction (XRD). The experimental results indicate that the 450℃isothermal section includes five three-phase equilibrium phase regions: Liq.+NiSi₂ +Si, Liq.+NiSi₂ +T, Liq.+δ+ T ,γ+Τ+β₁ andγ+Τ+δ; and a ternary phase T(Ni₂Zn₃Si). Comparing with the 450℃isothermal section, there was noδphase in the 500℃isothermal section. The 500℃isothermal section includes four three-phase equilibrium phase regions: Liq.+ NiSi₂+Si, Liq.+NiSi₂+T, Liq.+γ+Τandγ+Τ+β₁; six two-phase equilibrium phase regions: Liq.+ Si, Liq.+NiSi₂, NiSi₂+Si, Liq.+ T, NiSi₂+T andγ+ T. The diffusion couple experiments were carried out. The periodic layered structure was observed both in Zn/Ni Si liquid-solid diffusion couples and Zn/Ni Si solid-solid diffusion couples.The periodic layered structure was determined by using scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). The periodic layered structure consists of parallel alternating layers: theγphase layer and the T phase layer. Based on the phase relationship, the formation mechanism of periodic layer structure was analyzed using the theory of diffusion path. Theγphase nucleates and precipitates in the T phase near the interface of Ni₃Si/T when the concentration of Ni reaches a certain critical value required for nucleation of theγphase. As the reaction continues, the particles form a band that is calledγ₂ phase layer, which divides the T phase into two parts: the T₁ phase layer and the T₂ phase layer, so a new layer pair forms. The process is periodic, which results in the formation of the periodic layer structure.