In-Situ Nanoscopic Visualization of Stress Corrosion Cracking of High-Strength Aluminum Alloy by Scanning Atomic Force Microscopy

Abstract
An atomic force microscope (AFM) equipped with a small three-point bending testing machine was applied to perform in situ visualization of intergranular stress corrosion (SC) crack growth under a constant displacement. The tests were conducted on a high-strength 7075-T6 aluminum alloy in laboratory air. The AFM was capable of imaging surface topography of growing SC crack in the order of nanometer. The AFM has extremely high spatial resolution, and it was capable of monitoring very slowly growing SC crack. Even when it grew at the order of 0.1nm/s, it grew continuously at the order of microns. When the crack grew along the grain boundary inclined to tensile stress direction, not only Modes I and II crack tip displacement, but also Mode III displacement was observed. However, Mode I stress intensity derived from crack tip displacement was responsible for its crack growth. The tip of a growing SC crack in laboratory air was very sharp. However, when the environment was changed to vacuum, the crack tip became blunt, and the crack retarded. When the environment was rechanged to laboratory air, the crack restarted after some crack retardation time, and the tip became sharp again. We discuss the SC crack growth mechanisms based upon nanoscopic in situ visualization by using AFM.