INSITU OBSERVATION OF MICROFRACTURE PROCESSES IN AN 8090 AL-LI ALLOY PLATE

The main objective of the present investigation is to clarify the origin of the short transverse delamination which creates a serious decrease in the mechanical properties of an 8090 Al-Li alloy plate. Although the application of stretching generally improved the overall tensile properties of the 8090 plate, the short, transverse-oriented specimen in peak-aged condition showed an elongation of only 1 %. Such mechanical behavior was investigated in detail using an in situ scanning electron microscopy fracture technique. For the short transverse-longitudinal-oriented specimen, microcracking occurred along grain boundaries at relatively low stress levels and the crack easily propagated along the grain boundaries because of the continuous nature of the possible crack path. In contrast, it was observed from the specimen with the longitudinal-transverse orientation that the crack proceeded mainly along localized slip bands and that the cracked grain boundary did not appear to have any significant effect on the crack initiation process. Thus, direct observation of the crack initiation and propagation processes showed that the elongated shape of the grain structure strongly affected the fracture initiation processes of the short transverse-oriented 8090 plate, thereby drastically reducing the mechanical properties.