Study of size effects in structural transformations of bcc Zr films by molecular-dynamics simulation

The effect of the film thickness and temperature on structural phase transformations in infinite films of bcc zirconium with (0 0 1) crystallographic orientation is studied by the molecular dynamics method with a many-body potential of interatomic interaction obtained within the embedded atom model. It is shown that for bcc (0 0 1) films there exists a critical thickness (∼6.1 nm) below which they are unstable at any temperatures, while at a thickness above 6.1 nm the films are stable in a certain temperature range. The thin bcc (0 0 1) films (<6.1 nm) experience an orientational transition into the bcc (1 1 0) phase through an intermediate metastable fcc phase, and then, on cooling, a diffuse bcc (1 1 0) → hcp transition is observed. In films 6.1 to 8.2 nm thick there forms, on cooling, a twin fcc phase as a result of shear deformation, so that the film surface acquires stepped relief. With further increase of the bcc (0 0 1) film thickness there occurs martensitic transformation with the formation of a twin hcp structure, and the film has a wavy surface. lk and elastic moduli are calculated for the bcc and fcc lattices. It is shown that for the fcc phase the lattice stability conditions are satisfied in both bulk and film systems.