Morphology and fracture of oxide bifilm defects in nickel-aluminium bronze

Most of the metallurgical defects in castings can be initiated by oxide bifilms reported in various alloys. However, investigations of bifilm defects in nickel-aluminium bronze castings and their fracture behaviors were absent to date, which was explored in the present work. The results showed that there are two types of defects, i.e. smooth oxide film separating from the alloy matrix (old bifilm) and crinkled oxide film tightly adhering to the matrix (young bifilm). Thermodynamics and energy spectrum analysis proved that the bifilm was Al2O3. The inferred bifilm thickening speed curve also suggests that they come from different stages of the casting process. A fracture model was proposed to understand the fracture behaviors associated with the Young's modulus of the two types of bifilm defects. These results will shed new light on the understanding of bifilm defects failure and improve the defect control of castings.