Atomic understanding of Kelvin–Helmholtz instability at single-crystal copper interface with an initial corrugated disturbance under different tangential velocities

The evolutionary mechanisms of Kelvin–Helmholtz instability at single-crystal copper–copper interfaces with initial sinusoidal perturbation under different tangential velocity discontinuities (0.5, 1.0, 2.0, and 3.0 km/s) are investigated through molecular dynamics simulations. Distinct characteristics of stable and unstable interface morphologies are identified. The interface contact length, rather than the perturbation amplitude, extracted by an edge detection method, is selected as an appropriate physical quantity to indicate the state of the interface. The interface is stable for a tangential velocity discontinuity of 0.5 km/s, since the contact length remains essentially unchanged, but it is unstable for the other three velocities, since the contact length increases continuously. The contact lengths of unstable interfaces exhibit distinctly different nonlinear growth patterns. The microstructural evolutions for stable and unstable interfaces are also revealed. Sparse distributions of microstructures, such as dislocations and stacking faults, are observed in a stable interface, whereas extensive propagation, indicating large areas of plastic deformation, is found in unstable interfaces. Furthermore, a large number of metastable phase atoms emerge during plastic deformation, and an amorphous belt whose thickness grows as the velocity increases is formed near unstable interfaces. A thicker amorphous belt corresponds to greater plastic work, which produces a wider high-temperature belt near the interface. It also leads to the formation of a thicker shear belt with a high gradient velocity profile, creating a dynamic condition to promote the instability and thus distort the interface.