Interface sharpening and broadening during annealing of Cu Ni multilayers: A kinetic Monte Carlo study

Kinetic Monte Carlo simulations are performed to study the evolution of interfaces in a model alloy system whose parameters are chosen so as to reproduce the main features of the Cu-Ni system, in particular the asymmetry of diffusion coefficients. Recently, Erdélyi [Phys. Rev. Lett. 89, 165901 (2002)] have shown that, in alloy systems forming ideal solid solutions, a large asymmetry can lead to the unexpected transient sharpening of interfaces that were initially flat and diffuse. In the present work we first remove two simplifications made by these authors and we study the consequences on the sharpening and broadening of interfaces during the annealing of a multilayered sample. First, in our diffusion model, atoms migrate by vacancy-assisted jumps instead of direct atom exchanges. Second, we investigate the effect of the initial roughness of interfaces on their evolution during annealing. In addition, we also study the influence of the short-range order that arises from a nonideal solid solution, since the Cu-Ni phase diagram displays a miscibility gap at low temperatures. While we observe that the transient sharpening phenomenon can still take place with a vacancy mechanism in alloys with large diffusional asymmetries, we find that the sharpening amplitude is reduced when short-range order is present and even suppressed when interfaces are initially rough. In the latter case, the in-plane wavelength of the roughness plays a determinant role and we identify a crossover from sharpening to broadening when this wavelength exceeds a threshold value. The practical implications of these effects are discussed.