Deformation and Fracture of Intermetallic Compounds Having the L12 Crystal Structure

Intermetallic compounds having the L1₂ structure show many unusual mechanical properties. For example, many show an anomalous temperature dependence of the flow stress which has been related in various models to the structure of screw dislocation cores. Others, however, show a bcc-like behavior, which also has been related to core structures. Apparently unrelated to the flow behavior, L1₂ compounds also have a tendency for brittleness at low temperatures, with failure either occurring by transgranular cleavage or by intergranular failure mechanisms. Cleavage in some L1₂ materials, e.g. Fe-modified Al₃Ti, tends to occur on a remarkably large number of planes in both poly- and single crystals. The cleavage resistance of this material can be slightly increased only by Mn additions. Ni₃Al, on the other hand, tends to fail intergranularly, unless the grain boundaries contain B, and even then, failure under triaxial stress states tends to occur intergranularly. Intergranular cracks propagate randomly through the grain boundaries of Ni₃Al with little preference for boundaries of any particular type, except that cracks tend to avoid symmetrical twin and low angle tilt boundaries. In this paper our current understanding of the flow and fracture behavior of this class of materials will be reviewed.