Structural Stability of Nano-Sized Clusters
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Structural Stability of Nano-Sized Clusters
J.Th.M. De Hosson, G.Palasantzas, T.Vystavel, S. Koch Dept. of Applied Physics, Materials Science Centre and the Netherlands Institute for Metals Research, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands. ABSTRACT This contribution presents challenges to control the microstructure in nano-structured materials via a relatively new approach, i.e. using a so-called nanocluster source. An important aspect is that the cluster size distribution is monodisperse and that the kinetic energy of the clusters during deposition can be varied. Interestingly the clusters are grown in extreme nonequilibrium conditions, which allow obtaining metastable structures of metals and alloys. Because one avoids the effects of nucleation and growth on a specific substrate one may tailor the properties of the films by choosing the appropriate preparation conditions. INTRODUCTION Basically two key factors of the microstructure determine the material strength of a nanostructured material: the concentration of lattice defects and the details of the numerous interfaces, including the topology of the triple points between the interfaces. As a rule of thumb all the phases in a nano-structured materials must be made of strong material and the grain boundaries must be sharp by optimizing the hardness/stiffness ratio. For hard materials the key challenge is to avoid grain boundary sliding but as far as the toughness is concerned more compliant (amorphous) boundary layers might be more beneficial. Actually, boundary sliding may have a positive effect on wear resistant coatings by optimizing the ratio of hardness over the stiffness [1,2,3]. Knowledge of the deformation mechanisms in the nano-structured materials is simply very rudimentary at the moment. For instance grain boundary ledges for the generation of dislocations may become increasingly important in the production of dislocations at smaller grain sizes. When the grain size drops below 20 nm or so in a homogeneous nano-structured material the number of triple junctions per unit volume becomes appreciably large. Since the triple points possess disclination character they may contribute substantially to the ductility of nanocomposites and softening is expected at the smallest grain sizes [1,2]. The misorientation across short grain-boundaries in nano-structured materials may only be partly accomplished by grain boundary dislocations and at small sizes the number of disclination dipoles will be increased. Further, crack deflection, crack branching, intergranular fracture and transgranular fracture are probably very much different in these nano-structured materials compared to their micrometersized counter parts. In evaluating the performance of a nano-structured materials it is essential to examine the defect content as well as the microstructural features [4]. In particular: grain size dispersion, distribution of interface misorientation angles, and internal strains. Clearly, defects, such as microcracks, can completely
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