Multistep Crystallization and Melting Pathways in the Free‐Energy Landscape of a Au–Si Eutectic Alloy

Abstract

Crystals do eventually melt if they are heated to their characteristic melting point. However, this is practically only the case for high‐temperature stable crystals, whereas low‐temperature metastable crystals generally transform, before melting, into a more stable solid during heating. Here, it is illustrated that low‐temperature crystals can, however, be melted via fast differential scanning calorimetry (FDSC), even in metallic systems where nucleation and growth kinetics are rapid. For a Au–Si eutectic alloy, various metastable and stable solid states, i.e., (Au–α), (Au–β), γ, and (Au–Si), which form under well‐controlled conditions and melt at high heating rates by preventing the metastable‐to‐stable solid phase transition, are isolated. It is demonstrated that Au81.4Si18.6 can fully melt at various temperatures, i.e., 294 °C, 312 °C, 352 °C, and 363 °C, with differing melting enthalpies ranging from 6.52 to 9.83 kJ mol−1. The melting and crystallization paths of the metastable solids are determined by constructing an energy−temperature diagram. This approach advances the general understanding of nucleation in metallic and other systems, and is expected to contribute to the detailed understanding of thermophysical phenomena that occur at spatially reduced dimensions and/or short time scales, for example in thin‐film deposition, nanomaterials production, or additive manufacturing.

Temporary Cohabitation: The Metastable Phase Au4Si

“…Upon heating, the DSC curve shows a signal with T onset = 495(10) K ( Figure S1 ). In agreement with ref ( 22 ), this signal corresponds to the start of decomposition of Au 4 Si and formation of Au, although in situ heating experiments with synchrotron radiation (DESY, Hamburg, Germany) revealed that very high-quality crystals of Au 4 Si withstand temperatures of up to 573 K for ∼4 min. At T onset = 635(10) K, an endothermic effect with a corresponding signal in the cooling curve is observed, in line with the T e of the eutectic mixture.…”

Erect Au Nanocones Drawn from Au Nano-Films by Nano-Size Au-Si Eutectic Clamping with High Adhesion to Substrates

“…The annealing twins and shrinkage cavities formed in Au nano-film could be attributed to the effect of non-uniform high surface tensions, which is similar with the intrinsic stress induced by high strain rolling in metals with low stacking fault energy [16]. During the annealing, the contents of Si atoms solid dissolved into Au nano-film close to the Au/Si interface dominated the number and size of Au-Si eutectics at Au/Si interfaces [17]. The ripening process of Au nano-film provides the opportunity to weld Au nano-film with Si substrate through the formation of Au-Si eutectic.…”

Five-fold Symmetry in Au-Si Metallic Glass

“…18 In recent years, although some theoretical studies reported possible alloy structures of Au-Si, [19][20][21] the local ordering of Au-Si BMG is still lacking. Interestingly, two-dimensional (2D) crystallization was observed at the surface of Au-Si metallic glass, 22,23 which has been experimentally confirmed to be an ordered rectangular structure 22 with the proposed Si-Si bond on the surface. Shpyrko and co-workers 22 claimed that the Au-Si surface ordering was induced by surface-induced freezing, 24 but the ordered prefreezing surface will only appear near the transition temperature rather than a wide range of temperatures, inconsistent with the case of Au-Si.…”