IN-SITU OBSERVATION OF STRESS-CORROSION CRACKING OF HIGH-STRENGTH ALUMINUM-ALLOY BY SCANNING ATOMIC-FORCE MICROSCOPY AND INFLUENCE OF VACUUM

An atomic force microscope (AFM) equipped with a small three-point ben ding testing machine was used for in situ visualization of intergranul ar stress corrosion (SC) crack growth under a constant displacement. T he tests were conducted on a high-strength 7075-T6 aluminum alloy in l aboratory air. The AFM was capable of imaging the surface topography o f a growing SC crack on the nanometer order. The AFM has extremely hig h spatial resolution, and it was capable of monitoring very slow growt h 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 gre w along the grain boundary inclined to the tensile stress direction, n ot only Mode I and II, but also Mode III crack tip displacement was ob served. However, the Mode I stress intensity derived from the crack ti p displacement was responsible for the crack growth. The tip of an SC crack growing in laboratory air was very sharp. However, when the envi ronment was changed to a vacuum, the crack tip became blunt, and the g rowth of the crack was retarded. When the environment was changed agai n to laboratory air, the crack growth restarted after a crack retardat ion period, and the tip became sharp again. We discuss the SC crack gr owth mechanisms based on nanoscopic in situ visualization using AFM.