Nanoscale alloys and core-shell materials: Model predictions of the nanostructure and mechanical properties

Atomic scale modeling methods are used to investigate the relationship between the properties of clusters of nanometer size and the materials that can be synthesized by assembling them. The examples of very different bimetallic systems are used. The first one is the ${\mathrm{Ni}}_{3}\mathrm{Al}$ ordered alloy and the second is the AgCo core-shell system. While the ${\mathrm{Ni}}_{3}\mathrm{Al}$ cluster assembled materials modeling is already reported in our previous work, here we focus on the prediction of new materials synthesized by low energy deposition and accumulation of AgCo clusters. It is found that the core-shell structure is preserved by deposition with energies typical of low energy cluster beam deposition, although deposition may induce substantial cluster deformation. In contrast with ${\mathrm{Ni}}_{3}\mathrm{Al}$ deposited cluster assemblies, no grain boundary between clusters survives deposition and the silver shells merge into a noncrystalline system with a layered structure, in which the fcc Co grains are embedded. To our knowledge, such a material has not yet been synthesized experimentally. Mechanical properties are discussed by confronting the behaviors of ${\mathrm{Ni}}_{3}\mathrm{Al}$ and AgCo under the effect of a uniaxial load. To this end, a molecular dynamics scheme is established in view of circumventing rate effects inherent to short term modeling and thereby allowing to examine large plastic deformation mechanisms. Although the mechanisms are different, large plastic deformations are found to improve the elastic properties of both the ${\mathrm{Ni}}_{3}\mathrm{Al}$ and AgCo systems by stabilizing their nanostructure. Beyond this improvement, when the load is further increased, the ${\mathrm{Ni}}_{3}\mathrm{Al}$ system displays reduced ductility while the AgCo system is superplastic. The superplasticity is explained by the fact that the layered structure of the Ag system is not modified by the deformation. Some coalescence of the Co grains is identified as a geometrical effect and is suggested to be a limiting factor to superplasticity.