Cavity growth and filament formation of superplastically deformed Al 7475 Alloy

Abstract

Superplastic deformation induced cavitation of aluminium alloys usually results in the material performance degradation. In this study the cavitation behaviour of Al 7475 was characterised, using samples deformed at temperature ranging from 480 to 530°C and at an initial strain rate of 10 −3s −1. The results showed that the cavity growth rate parameter increased slightly as the test temperature increased. The cavitation ratio at fracture increased evidently with increasing temperature from 480 to 500°C, and reached a plateau for any further increase of temperature. Various morphological filaments were observed at cavities and fracture surface, as evidence of the presence of liquid phase along grain boundaries. The effects of test temperature and thermal history on cavitation were found to be closely related to the presence of the liquid phase. The presence of liquid phase will improve the ability of the materials to tolerate high volume fraction of cavities before fracture. On the other hand, when liquid phase is anisotropically distributed along grain boundaries, it will cause the preferential interlinkage of cavities along the weak grain boundaries, and result in corporate grain boundary sliding (CGBS). It is concluded that the critical factor is to achieve appropriate quantity, high property, and uniformly distributed liquid phase along grain boundary. This highlights a new clue in searching for an economical and practical way to alleviate cavitation.