The influence of boron segregation on the structure and mechanical properties of boundaries in bicrystals of Ni 3Al

Abstract

This paper reports that in contrast to the high ductilities reported for single crystals of ordered ({gamma}) Ni{sub 3}Al, the ductility in polycrystalline form is extremely limited. The discovery by Aoki and Izumi that microalloying with boron greatly increases the ductility of polycrystalline Ni{sub 3}Al has initiated spirited scientific investigation into the structure and properties of grain boundaries in this material. One consequence of the addition of boron could be an increase in the cohesive properties of the grain boundaries. However, at present the most widely accepted explanation for the improved ductility of the grain boundaries is the boron cause compositional disordering of the grain boundary region. As first suggested by King and Yoo, the total number of possible dislocation reactions is generally much larger at a grain boundary in disordered ({gamma}) material (the FCC structure) than in the ordered Ll{sub 2} phase. consequently, for the disordered case, the absorption or transmission of dislocations across the boundary should be easier, large stress concentrations should not develop as readily at the boundary, and the propensity for intergranular fracture should be reduced.