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 very slow growth of an SC crack:even when it grew on the order of 0.1 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, but also Mode III crack tip displacement was observed. However, the Mode I stress intensity derived from the crack tip displacement was responsible for the crack growth. The tip of an SC crack growing in laboratory air was very sharp. However, when the environment was changed to a vacuum, the crack tip became blunt, and the growth of the crack was 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 discuss the SC crack growth mechanisms based on nanoscopic in situ visualization using AFM.