Computational materials science 

In 2015 the computational materials research activities were marked by several PhD graduations -- a total of 5 group members graduated with a PhD. All of them received postdoc positions somewhere in the world.

Scientifically, one of the highlights was the discovery of the mechanism by which nanoscaled energetic multilayer materials release energy during self-propagating reactions on a micrometer-microsecondscale [1].  Working together with experimental collaborators at the University of Saarbrucken, we demonstrated a way by which robust single-phase synthesis guaranteed the ductility of Ru/Al multilayer materials. Measuring maximal propagation velocities of 11 m/s and reaction temperatures of at least 1950 °C, we verified the maximal energy density. Our molecular dynamics calculations were in notably good agreement with the experiments, and - even further – enable us to reveal the RuAl formation mechanism under the non-equilibrium conditions. Overall, the work providse a solid fundament to define a new family of Ru/Al-based ternary systems showing compromise of high energy density, small dimensions and ductility, however, with varying property accentuation. [1].

Another major highlight was the first ever direct comparison of cluster sizes produced during irradiation between simulations and experiments [2]. Working together with collaborators from Oxford University, we directly observed nano-scale defects formed in ultra-high purity tungsten by low-dose high energy self-ion irradiation at 30K. At cryogenic temperature lattice defects have reduced mobility, so these electron microscopy observations gave directly the initial, primary damage caused by individual collision cascade events. Electron microscope images provide direct evidence for a power-law size distribution of nano-scale defects formed in high-energy cascades, confirming the theoretical prediction we made a couple of years earlier [3]. Furthermore, the analysis of pair distribution functions of defects observed in the micrographs showed significant intra-cascade spatial correlations, consistent with strong elastic interaction between the defects.

In a rather different line of work, we examined the surface segregation that describes the phenomenon of variations in chemical composition between the surface and the bulk of an alloy, This can have a vital effect on its physical and chemical properties, and is even more pronounced in nanoalloys, i.e. alloy systems comprising of nanoparticles with significant surface-to-volume ratios. In 2015, we studied this together with collaborators at OIST in Japan for the case study of two different ways of gas-phase synthesis of Ni-rich NiCr alloy nanoparticles with numerous potential applications, including magnetic hyperthermia for cancer treatment. We found strong correlation between surface segregation upon thermal treatment and magnetic behavior, such as non-saturated M-H loops with reduced coercivity values at low temperatures and uncompensated high Curie temperature values compared to the stoichiometric NiCr bulk. We confirmed our findings by direct observation using transmission electron microscopy (TEM) and compositional analysis. Simultaneously, we utilized atomistic computer simulations (molecular dynamics (MD) and Metropolis Monte Carlo (MMC)) in order to investigate the resultant structures, and discovered energetics arguments that explain the driving force behind this segregation [4].

[1] K. Woll, A. Bergamaschi, K. Avchachiov, F. Djurabekova, S. Gier, C. Pauly, P. Leibenguth, C. Wagner, K. Nordlund, and F. Mucklich, Ru/Al multilayers integrate maximum energy density and ductility for reactive materials, Sci. Rep. 6, 19535 (2015).

[2] X. Yi, A. E. Sand, D. R. Mason, M. A. Kirk, S. G. Roberts, K. Nordlund, and S. L. Dudarev, Direct observation of size scaling and elastic interaction between nano-scale defects in collision cascades, EPL 110, 36001 (2015).

[3] A. E. Sand, S. L. Dudarev, and K. Nordlund, High energy collision cascades in tungsten: dislocation loops structure and clustering scaling laws, EPL 103, 46003 (2013).

[4] M. Bohra, P. Grammatikopoulos, R. Diaz, V. Singh, J. Zhao, J. Bobo, A. Kuronen, F. Djurabekova, K. Nordlund, and M. Sowwan, Surface Segregation in Bi-metallic Alloy Nanoparticles and Its Effect on their Magnetic Behavior, Chemistry of Materials 27, 3216 (2015).