Porosity defects of bulk metallic glass casting

Casting porosity defects constitute a critical bottleneck hindering the engineering application of bulk metallic glass (BMG). Their formation mechanisms remain insufficiently elucidated due to the unique rapid vitrification process inherent to BMG. This study investigated a Zr‑based BMG, employing multi‑scale characterization and thermodynamic analysis to reveal the formation mechanisms of porosity defects. The results indicate that gas pores are the predominant type of porosity defect. The fundamental cause lies in the layer‑by‑layer-like solidification mode, in which the rapid advance of the supercooled melt from the casting surface toward the core kinetically traps bubbles. Melt-turbulence-driven gas entrainment is the primary gas source for pore formation, while oxide inclusions interact with pre-existing bubbles, reduce the bubble nucleation energy barrier, and promote bubble retention in the melt. This work demonstrates that achieving smooth melt filling coupled with melt purification provides a viable pathway for controlling porosity defects, thereby furnishing a theoretical basis for producing high‑quality BMG castings.