The crystallography of cleavage fracture in Al₃Sc

Abstract

The intermetallic compound Al₃Sc is a trialuminide with the L1₂ structure, which deforms easily in compression at room temperature, with yield stresses around 100 MPa. As shown by single crystal compression experiments, slip occurs on {111} planes. In tension Al₃Sc fractures transgranularly in a brittle manner. The predominant cleavage plane is {011}. In a scanning electron microscope (SEM) numerous cleavage steps, which are aligned in three major crystallographic directions, are observed. The fracture surfaces almost always show microscopic waviness along the traces of intersecting slip planes. Regions that are flat within the experimental resolution of the SEM are only occasionally observed. Some of the cleavage steps consist of {111} or {001} planes, but others are not distinctly crystallographic. Plastic deformation involving dislocation motion or twinning may have occurred at these steps. Reactions among different types of steps are also observed. One type of cleavage pattern found is strikingly similar to the typical appearance of fracture surfaces of fcc or fcc related materials after stress corrosion cracking. However, this particular pattern is only rarely observed in Al₃Sc and can therefore not be taken as evidence for stress corrosion cracking in this material. Our observations indicate that an interpretation of cleavage fracture in Al₃Sc in terms of surface energies alone is unlikely to be successful. Similarly, any criterion that categorizes its fracture behavior into either fully brittle or fully ductile is faced with difficulties. A full understanding of the fracture morphology of Al₃Sc will therefore require detailed atomistic simulations.