Structure of oxide bifilms in nickel-aluminium bronze alloys.

• The oxide bifilm defects were separated from the casting matrix through electrolysis method for the first time. • The composition, phase structure and morphology of individual bifilm defects were measured directly. • The mechanical properties of amorphous alumina bifilm were tested. • The formation reasons of bifilm with different crystal types were analyzed. 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. Al 2 O 3 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-Al 2 O 3 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 Al 2 O 3. 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. [ABSTRACT FROM AUTHOR]