RESUMO
Casting of metallic glasses (MG) sometimes induces surface crystallisation despite the fact that the surface is expected to be the region where the cooling rate is the highest. This phenomenon has been observed on various MG, even for those with large critical diameters. Such surface crystallisation can be detrimental when the target applications are focused on surface properties, such as corrosion resistance for biomedical applications. In this paper, a Zr56Co28Al16 bulk metallic glass (BMG) with a large critical diameter was used. We reveal that samples processed using common copper-mould suction casting present surface crystallisation up to 20 µm in thickness, greatly deteriorating corrosion resistance. Using in-house highly reproducible suction casting and injection micro-casting processes, the influence of the processing parameters (mould material and temperature, working atmosphere, applied pressure) were investigated. The origin of surface crystallisation was found to arise from the complex thermal history of the alloy depending on the alloy/mould contact quality. By ensuring a tight contact between the solidifying alloy and the mould, BMG samples without crystalline surface defects were obtained.
RESUMO
The phenomenon of supercooling in metals-that is, the preservation of a disordered, fluid phase in a metastable state well below the melting point-has led to speculation that local atomic structure configurations of dense, symmetric, but non-periodic packing act as the main barrier for crystal nucleation. For liquids in contact with solids, crystalline surfaces induce layering of the adjacent atoms in the liquid and may prevent or lower supercooling. This seed effect is supposed to depend on the local lateral order adopted in the last atomic layers of the liquid in contact with the crystal. Although it has been suggested that there might be a direct coupling between surface-induced lateral order and supercooling, no experimental observation of such lateral ordering at interfaces is available. Here we report supercooling in gold-silicon (AuSi) eutectic droplets, enhanced by a Au-induced (6 x 6) reconstruction of the Si(111) substrate. In situ X-ray scattering and ab initio molecular dynamics reveal that pentagonal atomic arrangements of Au atoms at this interface favour a lateral-ordering stabilization process of the liquid phase. This interface-enhanced stabilization of the liquid state shows the importance of the solid-liquid interaction for the structure of the adjacent liquid layers. Such processes are important for present and future technologies, as fluidity and crystallization play a key part in soldering and casting, as well as in processing and controlling chemical reactions for microfluidic devices or during the vapour-liquid-solid growth of semiconductor nanowires.
