Aus Wissenschaft und Forschung / TAE-Plenarvorträge 38 Tribologie + Schmierungstechnik · 67. Jahrgang · 4/ 2020 1 Introduction CuNi alloys are used in shipbuilding, offshore oil production, power plants, for pipes and fittings, coinage, as well as in the aviation industry [1]. It is generally accepted that sliding contact may cause irreversible changes to the involved surfaces, but many of the mechanisms that govern microstructure evolution are not well understood [2], which limits the development of tailormade microstructures. While experiments are indispensable for studying the occurring phenomena, it is not easy to identify a mechanism from a set of “before/ after” pictures. Therefore, in this work we study the microstructural response of five FCC CuNi alloys with a broad range of stacking fault energies subjected to sliding using large-scale molecular dynamics (MD) simulations. 2 Model All simulations were carried out using the open-source code LAMMPS [3]. The setup of the polycrystalline MD model of 85 x 85 x 40 nm 3 , or approximately 25 million atoms, is explained in detail in [4,5]. For an exploded view of the tribosystem annotated with some simulation parameters, see Figure 1. Interactions within the FCC CuNix samples are controlled by an embedded atom alloy potential from [6]. The initial grains measure approximately 40 nm in diameter to ensure that plasticity is not dominated by grain boundary sliding, so our polycrystalline aggregate exhibits dislocation pile-up, twinning, and grain refinement analogous to polycrystals with much larger grains. Tailored Lennard-Jones potentials were adopted for modelling the interactions between the mated surfaces to imply a third body. This reflects a sliding process that explicitly considers friction, rather than a sheared interface between two pure metal surfaces in ultra-high vacuum [7]. A Langevin thermostat provided the basis an electron-phonon coupling scheme where the electrons act as an implicit heat bath to mimic the electronic contribution to the thermal conductivity in a metal [8]. 3 Results We analyzed the depth-resolved time development of the grain size, shear, twinning, and the stresses in the aggregate. Over a series of data distillation processes, the computational micrographs visualizing the material response can be used to produce a deformation mechanism map for CuNi alloys [9]. This is done by first eliminating the lateral resolution, leading to microstructure evolution maps, which can be simplified to time-dependent curves of the key quantities mentioned above. Next, by eliminating time, we obtain load-dependent trends of derived parameters that can be attributed to changes in the way the system deforms plastically. Finally, by fitting these load-dependent trends, the resulting characteristic normal pressures σ z can be entered into a composition de- MD Simulations of FCC Alloys under Dry Sliding Yield a Mechanism Map for Near-Surface Microstructural Development Stefan J. Eder, Manel Rodríguez Ripoll, Ulrike Cihak-Bayr, Daniele Dini, Carsten Gachot* * Stefan J. Eder 1,2)* Manel Rodríguez Ripoll 1) Ulrike Cihak-Bayr 1) Daniele Dini 3) Carsten Gachot 1,2) *Corresponding author: stefan.eder@ac2t.at 1) AC2T research GmbH, Wiener Neustadt, Austria 2) Institute of Engineering Design and Product Development, TU Wien, Vienna, Austria 3) Department of Mechanical Engineering, Imperial College London, London, UK Figure 1: Annotated overview of the model CuNi tribosystem Aus Wissenschaft und Forschung / TAE-Plenarvorträge 39 Tribologie + Schmierungstechnik · 67. Jahrgang · 4/ 2020 pendent map featuring well-defined curves that divide between regions where different plasticity mechanisms dominate, see Figure 2. This map captures the predominant microstructural phenomena occurring for a given composition and normal pressure. We compared tomographic visualizations of our atomistic model with focused ion beam images of the near-surface regions of real CuNi alloys that were subjected to similar loading conditions. For an example of the near-surface microstructures of experimental and simulated Cu samples that have undergone grain refinement due to sliding, see Figure 3. 4 Conclusion We have demonstrated how large-scale MD simulations are suitable for producing a deformation mechanism map that is able predict the evolving microstructure in sliding CuNi contacts. Our model system clearly showed how alloying, by which the stacking fault energy is varied, leads to a range of microstructural responses under sliding. We assume that our simulations capture the relevant acting mechanisms so that the deformation mechanism map, while derived for the example of CuNi, is suitable to be applied to other FCC alloy systems as well. This will aid engineers in optimizing materials/ surfaces to work within a required operating range. Acknowledgements This work was supported by the Austrian COMET program (project K2 XTribology, no. 849109) and carried out at the “Excellence Centre of Tribology” (AC2T research GmbH). The government of Lower Austria supported the endowed professorship tribology at the TU Vienna (grant no. WST3-F-5031370/ 001-2017). D.D. acknowledges the support of the Engineering and Physical Sciences Research Council (EPSRC) via his Established Career Fellowship EP/ N025954/ 1. References [1] “Copper-Nickel Alloys: Properties, Processing, Application”, Booklet, German Copper Institute (DKI), English translation. [2] Greiner, C., Gagel, J., & Gumbsch, P. (2019). Solids Under Extreme Shear: Friction: Mediated Subsurface Structural Transformations. Advanced Materials, 31(26), 1806705. [3] Plimpton, S. (1995). Fast parallel algorithms for short-range molecular dynamics. Journal of computational physics, 117(1), 1-19. [4] Eder, S. J., Cihak-Bayr, U., Gachot, C., & Rodriguez Ripoll, M. (2018). Interfacial Microstructure Evolution Due to Strain Path Changes in Sliding Contacts. ACS applied materials & interfaces, 10(28), 24288-24301. [5] Eder, S. J., Bianchi, D., Cihak-Bayr, U., & Gkagkas, K. (2017). Methods for atomistic abrasion simulations of laterally periodic polycrystalline substrates with fractal surfaces. Computer Physics Communications, 212, 100-112. [6] Bonny, G., Pasianot, R. C., Castin, N., & Malerba, L. (2009). Ternary Fe-Cu-Ni many-body potential to model reactor pressure vessel steels: first validation by simulated thermal annealing. Philosophical magazine, 89(34-36), 3531-3546. [7] Eder, S. J., Cihak-Bayr, U., Vernes, A., & Betz, G. (2015). Evolution of topography and material removal during nanoscale grinding. Journal of Physics D: Applied Physics, 48(46), 465308. [8] Eder, S. J., Cihak-Bayr, U., Bianchi, D., Feldbauer, G., & Betz, G. (2017). Thermostat influence on the structural development and material removal during abrasion of nanocrystalline ferrite. ACS applied materials & interfaces, 9(15), 13713-13725. [9] Eder, S. J., Rodríguez Ripoll, M., Cihak-Bayr, U., Dini, D., and Gachot, C. (2019) Unravelling and Mapping the Mechanisms for Near-surface Microstructure Evolution in CuNi Alloys under Sliding, under review. Figure 2: Deformation mechanism map for CuNi alloys Figure 3: Comparison of the microstructures after sliding obtained from an SRV experiment (top) and MD simulation (bottom)