Plasticity enhancement through disordering at grain boundaries

In seeking to explain the boron effect in Ni3Al, Frost first suggested that disordering at the grain boundary might lead to enhanced plasticity by relieving some of the geometric constraints upon dislocation interactions with these normally brittle interfaces. This idea was further investigated by King and Yoo, who performed an extensive survey on the possible reactions between lattice dislocations or superdislocations and coincidence-related grain boundaries in the L12 structure. It was found that, indeed, the number of available reactions in any case was multiplied by as much as a factor of four if the requirement for maintaining chemical order in the grain boundary was removed. This suggests that it may be considerably easier to relax plastic strain at grain boundaries in the case where order is not imposed than where it is, and thus that the plastic energy contributing to the fracture process may be partly relieved. This would contribute to ductilizing the grain boundaries. There ar, in fact, several mechanisms by which grain boundary disordering may contribute to improved interfacial ductility. These mechanisms primarily relate to the fat that smaller Burgers vectors are available for the grain boundary dislocations in the case where order is not preserved, in much the same way that superdislocations are not required in the matrix if the material is disordered. This paper discusses the contributions to ductility