Size- and Interface-Constrained Tensile Behavior of Ti/Ni Polycrystalline Nanolaminates: Insight from Molecular Dynamics

Metallic nanolaminates (MNLs) exhibit excellent mechanical properties due to unique modulation and interface structures. However, the correlation between the deformation of nanostructures and the mechanical behavior of the materials remains inadequately elucidated. Molecular dynamics method is performed to investigate coupled effect of grain size (d = 7.5~25.0 nm) and layer thickness (λ = 1.31~15.15 nm) on the tensile behavior of Ti/Ni polycrystalline nanolaminates (PNLs). A plastic co-deformation mechanism involving crystalline phases, interface, and grain boundary under strong size and interface constraints is discovered. The dominant plastic deformation in Ti layer is size-independent HCP-BCC-HCP phase transformation. Dislocations propagation in Ni layer shifts with increasing layer thickness, which manifests as extended dislocations sliding, interaction between moving dislocations and interface dislocations, respectively. When grain sizes or layer thicknesses are small, interface migration, grain boundary diffusion, and grain boundary migration become prominent plastic deformation carriers. The coordinating effect of grain boundary and interface on deformations of different nanostructures endows materials with relatively favorable plastic properties. Moreover, a dimensionless parameter d/λ accounting for grain morphology and interface structure is found to predict the variations in flow stresses and characterize the dominating plastic deformation mechanisms of the stretched Ti/Ni PNLs.