IN-SITU NANOSCOPIC VISUALIZATION OF STRESS-CORROSION CRACKING OF HIGH-STRENGTH ALUMINUM-ALLOY BY SCANNING ATOMIC-FORCE MICROSCOPY

An atomic force microscope (AFM) equipped with a small three-point ben ding testing machine was applied to perform in situ visualization of i ntergranular stress corrosion (SC) crack growth under a constant displ acement. The tests were conducted on a high-strength 7075-T6 aluminum alloy in laboratory air. The AFM was capable of imaging surface topogr aphy of growing SC crack in the order of nanometer. The AFM has extrem ely high spatial resolution, and it was capable of monitoring very slo wly growing SC crack. Even when it grew at the order of 0.lnm/s, it gr ew 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 obs erved. However, Mode I stress intensity derived from crack tip displac ement was responsible for its crack growth. The tip of a growing SC cr ack in laboratory air was very sharp. However, when the environment wa s changed to vacuum, the crack tip became blunt, and the crack retarde d. When the environment was recharged to laboratory air, the crack res tarted after some crack retardation time, and the tip became sharp aga in. We discuss the SC crack growth mechanisms based upon nanoscopic in situ visualization by using AFM.