Intermetallic compounds are usually so brittle that they simply cannot be efabricated into useful shape components. Recent developments of physical and metallurgical principles for the structural materials have led to the conclusion that the deformability of several intermetallic systems can be substantially improved. Among them, in this lecture, the nature of grain boundary structure in L12-type intermetallics is first discussed.
Then, the possibilities and instances to overcome the brittleness are shown. Polycrystalline L12 -type Ni3Al, even prepared from high purity metals, exhibits brittle intergranular fracture. On the other hand, single crystals of Ni3Al are ductile even at room temperature. Thus, it is said that the grain boundaries of this alloy are intrinsically weak. Our recent analyses showed that there exist several configurations of atomic bonds, resulting in the heterogeneous electronic environments which can be regarded as “cavities”. The drastic ductility improvement of Ni3Al alloy by a small boron addition, which was found in 1979, can be interpreted as segregated boron atoms at grain boundaries to modify the electronic environments of boundaries and to reinforce boundary cohesion, thus suppressing intergranular fracture. From a quite similar point of view, modification of boundary structures can be achieved by substituting proper elements for the constituents of compounds or by selecting atom combinations of compounds. Thus, the ductility of Ni3Al could be improved by substituting a few percent of Mn or Fe for Al. Also, by selecting atom combination, several L12-type alloys such as Co3Ti, Cu3Pd, Ni3Mn and Ni3Fe have been found as ductile intermetallics.