Structure of oxide bifilms in nickel-aluminium bronze alloys

Abstract

Bifilm defects are proposed to be critical among the metallurgical defects generated in castings. However, direct investigations of individual bifilm defects separated from the casting matrix were absent to date. In this study, separated bifilm defects in nickel-aluminium bronze were investigated systematically. The results showed that an electrolysis method can successfully separate the bifilms from the casting matrix. Al2O3 with an amorphous state, a γ-phase structure, and an α-phase structure were observed in the separated bifilms, respectively, violating the conventional prediction of only α-Al2O3 existing in the final casting. The thickness of the bifilm defects was not uniform and varied in a wide range from several nanometres to several hundred nanometres. Upon analysis, the thinner bifilm presented a much smaller Young’s modulus in comparison to the crystallised Al2O3. The thicker bifilm was intergrown with several single crystals and covered by smooth and faceted faces. These results suggest the complex bifilms form in cast nickel-aluminium bronze alloys and more investigations are therefore required in future.