Effect Of Substrate Annealing On The Wetting Of Three Kinds Of Metastable Alloys By Molten Sn/Sn-Bi

The wettability of solid by liquid, which is closely related to the surface structure of thesubstrate, plays a key role in determining the processability of the materials and theirperformance. The amorphous alloys are characterized by short-range order and long-rangedisorder, without the presence of any grain boundary in their crystallographic structure.However, the amorphous structure is thermodynamically metastable, and it will undergocomplicated transformations including structural relaxation, primary crystallization,wide-range polycrystallization and grain growth during the annealing treatment, which willsignificantly change the substrare structure and energy, and thus influence the wettability.In this thesis, we introduced a dispensed sessile drop method to investigate thewettability and spreading dynamics of molten Sn or Sn-Bi on the metastableCo₆₅Fe₄Ni₂Si₁₅B₁₄, Ni₈₀P₂₀ and Fe₄₀Ni₃₈Mo₄B₁₈ substrates annealed at different temperatures,and employed XRD, micro-XRD, TEM, SEM and EDS means to study the interfacialmicrostructures. We further examined the differences in wettability among them andexplored their origins based on the substrate structure, surface/interface free energy, atomicdiffusion, interfacial reaction, the nucleation and growth of the intermetallic compounds（IMCs） as well as precursor film. The major results of the present study are as follows:1. The annealing at different temperatures changes the substrare structure and energy,which are closely related to the interfacial reaction, and thus changes the solid-liquidatomic interaction and the bond strength. The amorphous substrates with high activesurface atoms favor the wetting and spreading.2. The interfacial reaction enhances the atomic bonds between the solid and liquid, rapidlyreducing σsl, and thus drives the wetting. The strong chemical interaction promotes theovergrowth of the IMCs and thus roughens the interface, which may retard or eveninhibit the wetting and spreading （in the Sn/Co65Fe4Ni2Si15B14）. The crystallization canimprove the atomic bonding in the substrate and decrease the solid-liquid affinity, thusdeteriorating the wetting. 3. The formation of precursor film leads to excellent wettability. The wide range ofnanocrystallization increases the amount of grain and phase boundaries at the substratesurface. The liquid could then diffuse or enter into the narrow grooves produced bycrystallization ahead of the triple line, so that the movement of the liquid can be assistedby capillary force inside the grooves, which also improves the wetting to a certain extent.A wider precursor film also means better wetting. The width of the precursor films isrelated to the amount of the grain and phase boundaries.4. The inward diffusion of Sn to the bulk of the substrates weakens its spreadability.Enhanced wettability corresponds to the reduced diffusivity of Sn in the substrate. Thesubstrate structure and solid-liquid atomic affinity are the major factors in determiningthe wettability and atomic diffusion in the substrate. When the substrates are in theamorphous state, the interfacial reaction promotes the crystallization of the substrateclose to the reaction layer, yielding the free volume, and thus accelerate the atomicdiffusion; on the other hand, the wide-range crystallization and transition of precipitatedphases reduce the solid-liquid affinity and thus inhibit the wetting. The diffusion layerthen gradually disappears.5. The structural transformation of the amorphous substrate induced by the annealingtreatment does not significantly affect the initial contact angles; when the annealingtemperature exceeds a certain value, the initial contact angles gradually decrease withincreasing annealing temperature.6. For the Sn/Co₍65_Fe₄Ni₂Si₁₅B₁₄ system, the structural relaxation and primarycrystallization in the amorphous substrates remarkably deteriorate the wettability andspreadability of the molten Sn as a result of changing a weak chemical interaction to aphysical one for the atoms at the interface; the wide-range polycrystallization（nanocrystallization） improves the wettability because of transformation into thechemical interaction again. For the Sn/Fe₄₀Ni₃₈Mo₄B₁₈ system, the structural relaxationand primary crystallization do not substantially change the wettability because ofinvariable interfacial reaction.The above results are expected to be helpful to the joining of amorphous andnanocrystalline materials using a soldering technique.