Effect of the alloy/mould contact on surface crystallisation of a biocompatible ZrCoAl bulk metallic glass.

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 ZrCoAl 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.