High resolution electron microscopy in the investigation of splitting particles in Ni alloys

The mechanism of coarsening of coherent particles in solids is investigated via high-resolution electron microscopy (HREM). The origin of the so-called splitting pattern arrangement of γ' precipitate particles has been studied. This arrangement has often been interpreted as being due to splitting of a larger particle into smaller ones. HREM images along a zone axis parallel to [001] of neighboring particles are used to ascertain whether they are in-phase or out-of-phase by means of a translation order domain analysis. Ni alloys have been used for investigation including a binary Ni-Al alloy (producing different volume fractions) and two commercial multicomponent alloys with high volume fraction. About 72% of two-particle pairs forming the splitting configuration are in the out-of-phase relationship, indicating that adjacent pairs are formed at random most likely through homogeneous nucleation. Thus it is concluded that they are not formed by splitting of a large particle. In addition, a simple elasticity analysis shows that the elastic interaction energy of two γ' particles exhibits a minimum at a certain separation distance along 〈100〉. Therefore such grouping of particles in arrays can be explained on the basis of particle migration.