ON THE NANOCRACK NUCLEATION AND PROPAGATION MECHANISMS OF FE3AL INTERMETALLICS

The initiation and propagation of nanometre scale cracks have been inv estigated in detail in in situ TEM observations for the intermetallic compound Fe3Al under Mode I loading. When cracks propagate directly fr om the thin edge of a double-jet hole, no dislocation is deteced and n o dislocation emission is found. In thicker regions of the foils, thin ning takes place because a great number of dislocations are emitted fr om the crack tip, and then an electron semi-transparent region is form ed in front of the crack tip. A superdislocation model is used and it indicates that the maximum normal stress occurs in the crack tip regio n but not at the crack tip. Nanometre scale cracks are initiated disco ntinuously ahead of the main crack tip in the highly stressed zone. Th e size of the smallest observed was about 3 nm. The radius of the main crack tip was about 2.5 nm. Distances between discontinuous nanocrack s and the main crack tip were 5-60 nm which depends on the applied ten sile loading. The distance increases with the tensile loading, which i s consistent with an ''elastic-plastic'' theoretical model that maximu m normal stress location has the relation d alpha K-I(2), where K-I is the applied stress intensity factor. Hydrogen charging effects on the cracking behaviour are studied. In contrast to the non-charged case, the crack propagates preferably from the main crack tip. The propagati on rate is higher and the crack radius is smaller.