| Comparing to the conventional crystalline materials, amorphous and nanocrystallinematerials possess more superior mechanical, chemical and electromagnetic properties. Thus,they are considered as a kind of potential engineering material. However, because of thesmall critical thickness of the cast bulk metallic glasses (BMGs), their applications areseverely constrained. In order to widen their application areas, it is necessary to develop anew effective connection method. As the structure of the amorphous and nanocrystallinematerials is not easy to change during soldering due to the small heat input, it is suggested asa possible method. But for the soldering process, good wettability is an important conditionfor achieving connection. On the other hand, the amorphous and nanocrystalline materialsare in the metastable state, and their structure and energy are greatly different from that ofthe conventional crystalline materials. Therefore, wetting of them by solders is likely toshow some new phenomena and variations.Based on above reasons, in this thesis we mainly investigated the wettability andinterfacial microstructures of pure Sn and Sn-base solders on the amorphous andnanocrystalline BNi-2, Fe78B13Si9substrates using a dispensed sessible method in a highvaccum. Meanwhile, the wettability and interfacial microstructures of some representativesolders and pure Sn on the Ni, Fe substrates were also studied. The main results of this studyare as follows:(1) The amorphous BNi-2substrate is well wetted by Sn and Sn-base solders and thewettability is in the general sequence of Sn–51In>Sn–37Pb>Sn–57Bi>Sn–0.7Cu>Sn–3.0Ag–0.5Cu>pure Sn; while the wettability of them on the nanocrystalline BNi-2substrate is verypoor and varies in the order of Sn–57Bi>Sn–3.0Ag–0.5Cu>pure Sn>Sn–0.7Cu>Sn–51In>Sn–37Pb. Pure Sn and Sn-base solders exhibite the best wettability on the Ni substrate, andthe wettabilty is in the order of Sn–51In>Sn–57Bi>pure Sn. The chemical interfacialreaction of Sn with Ni provides a driving force for the spreading of the molten melts, but excessive growth and coarsening of the reaction products in turn inhibits the spreading ofthem (Sn-base solders/nanocrystalline BNi-2system). The fact that the wettability of pure Snand Sn-base solders on the amorphous BNi-2substrate is worse than that of them on the Nisubstrate is presumably attributed to the crystallization of the amorphous substrate.(2) The thickness of the reaction layer and diffusion layer for the Sn-base solders onthe amorphous and nanocrystalline substrates both varies in the order of Sn–51In>pure Sn>Sn–3.0Ag–0.5Cu≈Sn–0.7Cu>Sn–57Bi>Sn–37Pb, depending mainly on the concentration ofSn in the different solders, the melting point of the solder as well as the type of the reactionproduct. For the same solder, due to the larger Sn atomic diffusion rate in the nanocrystallinesubstrate, the thickness of the reaction and diffusion layers is larger. Through the wettinginterruption experiments, Sn atomic diffusion rates in the amorphous and nanocrystallinesubstrates are measured as8.267×10–15m2/s and1.602×10–14m2/s, respectively.(3) In view of the wettability and reactivity, the Sn–51In, Sn–57Bi and Sn–37Pbsolders are suitable for the joining of the amorphous BNi-2alloy (Ni-base amorphous alloy).However, the soldering time should be properly controlled to avoid the overgrowth of theintermetallics and the extensive nanocrystallization of the amorphous substrate, especially inthe case of the selection of the Sn–51In solder.(4) The result shows that the wettability of pure Sn, Sn–0.7Cu and Sn–3.0Ag–0.5Cusolders on the amorphous Fe78B13Si9substrate are nearly the same, while that of Sn–57Bi,Sn–37Pb and Sn–51In solders are similar. The former three solders could wet the amorphousFe78B13Si9substrate, while the others could not. For the Sn/Fe system, the results that thewettability improves, the diffusion band broadens and the reaction layer becomes smoothwith the increasing of the annealing temperature are all related to the multistagedecomposition of the oxide film on the substrate surface.(5) A small amount of FeSn and FeSn2formed at the interfaces between the Sn-basesolders and the amorphous Fe78B13Si9substrate, while no products were found at theinterfaces of Sn-based solders/nanocrystalline Fe78b13Si9wetting couples.(6) In view of the wettability and reactivity, pure Sn, Sn–0.7Cu and Sn–3.0Ag–0.5Cusolders are expected to be used for soldering the amorphous Fe78B13Si9alloy (Fe-base amorphous alloy). However, the oxide film on the amorphous substrate surface should becleaned up to enhance the welding efficiency. Nannocrystalline Fe78B13Si9substrate is notsuitable for soldering.(7) The interfacial reaction triggers the crystallization of the amorphous BNi-2andFe78B13Si9substrates in and close to the reaction area. The crystallization in turn promotesthe diffusion of Sn towards the substrate, and subsequently facilitates the interfacial reaction.(8) Fundamentally speaking, the wettability in metal/metal systems should be wettedunder no influence of the oxide film, and the effects of dissolution and formation ofintermetallics at the interface on wetting are limited and even neglected. In the presence ofthe oxide film, the dissolution and interfacial reaction usually improve the wetting throghremoval of the oxide film on the substrate surface.Not only can this study provide some information for the joining of the Ni-base,Fe-base amorphous and nanocrystalline materials, but also it can enrich and develop theexisting theoris of surface and interface. |
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