Acoustic microscopy of room-temperature fatigue damage in aluminium single crystals

Different forms of acoustic microscopes such as the time-resolved acoustic microscope and the line-focus-beam acoustic microscope were applied to the study of fatigue damage in aluminium single crystals which were cyclically deformed under constant resolved shear stress amplitude (4 MPa) control, R = –1, in air, at room temperature. Acoustic imaging revealed the non-uniformity of persistent slip bands, and the secondary slip in a macroband on the surface (side surface) containing the primary slip vector. The nature of extrusions, intrusions and macrobands on the side surface was characterized using the time-resolved acoustic microscope. The net irreversible slip accumulation or macroband phenomena found in the aluminium sample probably arose because some of the irreversible slip processes proceeded more easily in the tension phase than in the compression phase of the cycle. We believe that the net irreversible slip accumulation effect is significant to crack initiation. Using the line-focus-beam acoustic microscope a significant decrease in Rayleigh wave velocity was found on the side surface after 5 × 10⁶ cycles. The Rayleigh velocity change was associated with fatigue damage in the surface and might therefore be used to evaluate the surface damage as a function of the number of cycles in single-crystal aluminium. The short crack geometry was also measured by time-resolved acoustic microscopy. The result indicated that an early-stage short crack usually had a shape much shallower than a semicircle, which is probably a characteristic of the short fatigue crack in aluminium single crystals.