Atomic Structure and Chemical Composition of Grain Boundaries in L12 Intermetallic Compounds: Relation to Intergranular Brittleness

Intergranular fracture in L1₂ compounds such as Ni₃Al poses two basic questions. First, whether the grain boundary brittleness is intrinsic and if yes what is its origin and why it is not found L1₂ alloys such as Cu₃Au. Second, how and why is the brittleness affected by alloying and deviations from stoichiometry. These problems are discussed here in the light of the results of the Monte Carlo atomistic studies of the structure and composition of grain boundaries. For stoichiometric Ni₃Al virtually no compositional disorder occurs even at very high temperatures, while in Cu₃Au a significant disordering takes place already at room temperature. As suggested earlier [27, 30, 31], preservation of the compositional order in grain boundaries may be the principal reason for their intrinsic brittleness and, therefore, these results may explain why grain boundaries are brittle in Ni₃Al but not in Cu₃Au. In non-stoichiometric Ni₃Al the compositions of grain boundaries are very different in nickel rich and aluminum rich alloys, respectively. Whereas both components segregate to the boundaries when in surplus, nickel invokes disordering while aluminum does not. This may be the reason why the intergranular brittleness can be alleviated by additional alloying, for example by boron, in nickel rich alloys only.