Wettability Of Metastable BNi-2 And Fe₇₈B₁₃Si₉ Alloys By Molten Sn (Bi) And Their Interfacial Microstructures

Amorphous alloys possess a number of desirable physical and chemical properties, which are of significant importance for scientific research and potential engineering applications. However, the amorphous structure is thermodynamically unstable, being sensitive to heat treatment. On the other hand, the critical thickness of the cast bulk metallic glass alloys is limited within centimeters at present time, limiting their applications as engineering materials. In order to broad their applications, it is important to develop appropriate joining techniques. Soldering, as a low-temperature joining process, is a potentially feasible method, and the wettability of the amorphous alloys by molten solder is very important to this process. It is well known that the amorphous substrates will undergo complicatedtransformations including structural relaxation, primary nucleation (nanocrystallization), wide-range polycrystallization and grain growth during the annealing treatment, which will significantly change the substrare structure and energy, influencing the atomic diffusion and interfacial reaction with the molten metal and thus affecting the wettability. However, these aspects and their correlations remain largely ambiguous. In this work, the main goal is to investigate the wettability in the Bi/Fe₇₈B₁₃Si₉ (non-reactive), Sn/Fe₇₈B₁₃Si₉ (weak reactive) and Sn/BNi-2 (strong reactive) systems. The wetting behaviors of the amorphous or their annealing-induced nanocrystaline alloys by molten Sn or Bi as well as their interfacial microstructures wre analyzed focusing on the aspects of substrate structure, surface/interface free energy, interfacial reaction, IMCs formation, diffusion layer, etc. Besides, the influences of the annealing treatment on the substrate structure, wetting behavior and interfacial reaction were addressed in detail. The results indicate that the amorphous substrates favor the wetting and spreading by molten Sn, and the structural transformation induced by the annealing treatment has a little influence on the initial contact angles for all the systems. However, different influences of the structural transformation on the wetting behavior were embodied in different systems, which are summaried as follows:1. For the non-reactive system (Bi/Fe₇₈B₁₃Si₉), the substrate annealing can alter both the solid surface energy (σsv) and solid-liquid interface energy (σsl). However, there is little change in the difference (σsv-σsl) between them. So, the variation in the contact angles is quite small, i.e., the wettability in the non-reactive wetting system is not sensitive to the substrate structure changes. There is only physical interaction between molten Bi and Fe₇₈B₁₃Si₉. The solid-liquid adhesion is smaller than the liquid cohesion, and therefore Fe₇₈B₁₃Si₉ cannot be wetted by liquid Bi.2. For the reactive systems (Sn/Fe₇₈B₁₃Si₉ and Sn/BNi-2), the primary crystallization causes a deterioration in the wettability, i.e., the structure changes (from amorphous to nanocrystalline) have a great influence on the wetting. There are chemical reactions between the solid and liquid. The wetting behaviors are not only closely related toσsv andσsl, but also determined by the atomic diffusion, interfacial reaction, IMCs formation and growth. The interfacial reaction enhances the solid-liquid affinity and reducesσsl, and thus drives the wetting. The crystallization (grain growth of the nanocrystallites) can improve the atomic cohesion in the substrate and decrease the solid-liquid affinity, thus reduceing the spreading rate (in the Sn/BNi-2 system) or inhibiting the wetting (in the Sn/Fe₇₈B₁₃Si₉ system).3. The chemical reactions between Sn and the annealed Fe₇₈B₁₃Si₉ substrate become weaker with the increase in the annealing temperature, and there is no chemical interaction between them after the substrate crystallization, i.e., Sn/Fe₇₈B₁₃Si₉ changes from a reactive system to a non-reactive one after the substrate crystallization.4. The substrate structure change shows a great influence on the wetting in the strong reactive system. The structural relaxation and primary crystallization of the BNi-2 amorphous substrates during annealing deteriorate the wettability. The substrates annealed at temperatures between the glass transition point and the onset point of crystallization possess the poorest wettability. On the other hand, the wide-range crystallization and the subsequent grain growth of the nanocrystallites in the BNi-2 substrates improve the wettability to a certain extent. Enhanced wettability corresponds to the reduced diffusivity of Sn in the BNi-2 substrate, and the overgrowth of the IMCs can impede the spreading of liquid Sn.