In-situ Observation of Stress Corrosion Cracking of High-Strength Aluminum Alloy by Scanning Atomic Force Microscopy and Influence of Vacuum.

Abstract

An atomic force microscope (AFM) equipped with a small three-point bending testing machine was used for 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 the surface topography of a growing SC crack on the nanometer order. The AFM has extremely high spatial resolution, and it was capable of monitoring a very slowly growing SC crack: even when it grew on the order of 0.l nm/s, it grew continuously when observed on the order of microns. When the crack grew along the grain boundary inclined to the tensile stress direction, not only Mode I and II crack tip displacement, but also Mode III crack tip displacement was observed. However, the Mode I stress intensity derived from crack tip displacement was responsible for the crack growth. The tip of a growing SC crack in laboratory air was very sharp. However, when the environment was changed to a vacuum, the crack tip became blunt, and the crack retarded. When the environment was changed again to laboratory air, the crack growth restarted after a crack retardation period, and the tip became sharp again. We discussed the SC crack growth mechanisms based upon nanoscopic in situ visualization using AFM.