The Formation of Amorphous Si Induced by Eutectic Reaction and Ag-Nodule Mediated Bonding

Metastable phases such as supersaturated solid solutions, supercooling, and amorphous phases are well-known in metallurgy. They are often composed in non-equilibrium states and can be transformed into a stable phase by overcoming an energy barrier with driving forces. Particularly, it has been widely used for material strengthening and heterogeneous nucleation of precipitates in solids is mainly induced by heat treatments for supersaturated solid solutions. However, little is known about the metastable phases of the Ag-Si alloy, although it is a well-known simple binary eutectic alloy. We show that the metastable phases composed of amorphous Si and supersaturated Ag solid solution are induced by the eutectic reaction under rapid cooling of Ag-Si. Conventionally, the underlying concept of amorphous alloys has been considered as the mixing of multi-atomic compositions, which leads to the stabilization of the liquids due to the negative values of heat mixing. Here, we present a new concept of amorphization that induced by the significant supercooling due to the deep eutectic depression of the liquidus temperature near the eutectic composition of Ag-Si. The similar amorphization was recently found in the Al-Si eutectic system, but the amorphous phases were expected to appear in the hypereutectic region of 25-45 at % Si in Al-Si. Furthermore, the solute Si in the Ag matrix reacts with oxygen to precipitate Ag by-products, which grow as nodules. The Ag nodules have high crystallinity and robust interfacial structures, and the nodule growth leads to the formation of cross-links between the Ag-Si particles. We also demonstrate the Ag nodule-mediated bonding where the rapidly cooled Ag-Si ribbon is directly used as a bonding medium, indicating the possibility of using it as a high-temperature bonding material with low-temperature processes.