# Pattern Formation on Alloys by Cluster Ion Beam Irradiation
## 4 Conclusions  
In this study, we use MD simulations to investigate the mechanisms of nanoscale pattern formation on Au-Cu alloy surfaces irradiated by Ar cluster ion beams, focusing on the effects of alloy composition and incident angles. The fluence deposited on the material is the same in all cases. Two more details characterize the simulations: they have been performed at low temperatures so that ballistic effects predominate over diffusive effects, and to accelerate the ripple formation effect on the surface, a very simple model has been used, characterized by bombardment on a delimited surface in the form of a strip. Key findings show that intermediate-composition alloys exhibit significant suppression of ripple height and depth due to irradiation-induced amorphization. When bombarded with alloys of mixtures of similar concentration, an amorphization process occurs in the crystalline lattice. This effect causes the wave pulse to expand horizontally and drastically decrease its height. Sputtering and redistribution are the physical processes that can explain this phenomenon. The observation of maximum sputtering and minimal horizontal and vertical displacements at intermediate incidences suggests that sputtering is the predominant effect within this angular interval. In contrast, the explanation for grazing incidence proves more difficult and does not lean toward either of the two processes since the sputtering becomes minimal, the horizontal displacement depends on the mobility of the material, and the vertical displacement becomes quite small. However, the redistribution factor becomes negative for the entire angular range in alloys, indicating sputtering outweighs atomic  
Figure 8: Sputtered atoms, outward and inward relocated atoms as a function of the \(z\) position for two surfaces of different composition (at.% Cu): (**a**,**c**) 50 and (**b**,**d**) 100 and two angles of incidence: (**a**,**b**) 70\({}^{\circ}\) and (**c**,**d**) 50\({}^{\circ}\). The dashed line shows the difference between the atoms that arrive and the atoms that leave that layer. redistribution, a behavior that contrasts with pure metals. Therefore, sputtering is once again the predominant effect at grazing angles. In short, it appears that alloys are more susceptible to amorphization in processes far from equilibrium, such as bombardment. An amorphous solid hampers surface redistribution processes to the point that sputtering predominates throughout the entire angular range of incidence. Finally, we analyze the differences in repositioning between a pure material and an alloy with a similar concentration. In the latter material, the horizontal cross-sections are asymmetric on a short scale, which means that an alteration of the structure occurs at a local level. Furthermore, the formation of a large ridge by direct outward transfer of atoms is visualized only in the case of a pure material. The accounting of atoms per layer sputtered, ejected outward or returned inward along the \(z\)-axis allows us to visualize the importance of sputtering in the process. If along the \(z\)-axis the net balance of atoms expelled by redistribution is greater than that expelled by sputtering, it means that the first process will predominate. The simulation time limitations of MD are undeniable. However, future lines of work related to this article would include experimental verification of the results presented herein. From a numerical perspective, other bimetallic combinations should be used, and it should be demonstrated that the same effects occur. Methodology, J.C.J.-S.; Software, J.C.J.-S.; Investigation, S.M. and P.P. ; Writing--original draft, J.C.J.-S.; Writing - review & editing, S.M. and P.P. ; Funding acquisition, S.M. All authors have read and agreed to the published version of the manuscript. This work was supported by the Universidad Complutense of Madrid under the Project for Research Groups (Bioelectromagnetism Research Group 910305). Not applicable.