Mohammed Reda Chellali

Triple junction transport and the impact of grain boundary width in nanocrystalline Cu.

Triple junctions (TJ), singular topological defects of the grain boundary (GB) structure, get a dominant role for grain growth and atomic transport in nanocrystalline matter. Here, we present detailed measurements by atom probe tomography, even of the temperature dependence of TJ transport of Ni in nanocrystalline Cu in the chemical regime of interdiffusion. An unexpected variation of the effective width of merging GBs with temperature is detected. It is demonstrated that proper measurement of TJ transport requires taking into account this remarkable effect. TJ diffusion is found to be a factor of about 200 faster than GB diffusion. Its activation energy amounts to only two-thirds of that of the GB.

Interface sharpening in miscible Ni/Cu multilayers studied by atom probe tomography

Interfaces of Ni/Cu multilayers were studied by atom probe tomography. To this aim, specimens with sharp or artificially smeared interfaces were prepared and investigated before and after annealing at 773 K. Owing to three-dimensional subnanometer resolution of the atom probe, local chemical analysis of layer interfaces becomes possible without interferences of grain boundaries or geometric roughness. In contrast to the classical expectation for a miscible system, but in agreement with more recent theoretical considerations, diffusion reduces the chemical width of the interfaces by up to 50%.

Nano-analysis of grain boundary and triple junction transport in nanocrystalline Ni/Cu.

Nanocrystalline materials are distinguished by a high density of structural defects and grain boundaries. Due to the small grain size, a particular defect of the grain boundary topology, the so-called triple junction takes a dominant role for grain growth and atomic transport. We demonstrate by atom probe tomography that triple junctions in nanocrystalline Cu have 100-300 times higher diffusivity of Ni than standard high angle grain boundaries. Also, a previously unexpected systematic variation of the grain boundary width with temperature is detected. The impurity segregation layer at the grain boundaries grows from the 0.7 nm at 563 K to 2.5 nm at 643 K. This variation is clearly not controlled by simple bulk diffusion. Taking this effect into consideration, the activation energies for Ni diffusion in triple junctions and grain boundaries in Cu can be determined to be (83 ± 10) and (120 ± 15) kJ/mol, respectively. Thus, triple junctions are distinguished by considerably lower activation energy with respect to grain boundaries.

Defect analysis by statistical fitting to 3D atomicmaps

In this article we present a statistical fitting method for evaluation of atomic reconstructions which does not require a coarse-graining step. The fitting compares different models of chemical structure in their capability to explain the measured data set by a least square type merit function. Only preliminary qualitative assumptions about the possible chemical structure are required, while accurate quantitative parameters of the chosen model are delivered by fitting. The technique is particularly useful for singular defect structures with very high composition gradients, for which iso-concentration surfaces determined by coarse-graining become questionable or impossible. We demonstrate that particularly detailed information can be gained from triple junctions and grain boundaries.

Concentration Dependence of the Diffusion in the Ni/Cu System

Deviations from the Fickian-laws of diffusion in the case of concentration dependent diffusion coefficients and high composition gradients gain more and more acceptance nowadays. The cause of this phenomenon is the finite permeability of the atomic layers, or in other words “interface control”. The consequences are wide-spreading e.g. linear diffusion kinetics, deviations in the nucleation behavior of reaction products and kinetically determined interface shape in miscible alloys. Furthermore, if the original chemical interface is broader than the optimum width, even a sharpening of the interface by diffusion can be observed. Previous experiments proving these effects used more or less ideal specimens (e.g. single crystalline or amorphous samples with very flat interfaces) and some doubts can be raised whether these effects can be observed in a realistic specimen with a more complex grain structure. In this talk we will present the results of atom probe measurements on sputter deposited Ni/Cu multilayers (containing surface roughness, lattice defects, etc.). Samples with sharp and smeared Ni/Cu interfaces were produced and later annealed. We found an asymmetry on the interface width in the as-prepared specimens depending on the stacking order. After annealing this asymmetry vanished and remarkably the Cu/Ni interface sharpened by diffusion. After short diffusion time, the interface width became independent on the sample origin (sharp or smeared interface) proving the kinetic control of the interface. Atom probe tomography also allows the direct, local investigation of the grain boundary diffusion in any single grain boundaries. Surprisingly the best description of the shortcut transport can be achieved by assuming a concentration-independent grain boundary diffusion coefficient.

Physics on the Top of the Tip: Atomic Transport and Reaction in Nano-Structured Materials

Nanoscale systems show a wide variety of physical properties that cannot be observed in the bulk. Using atom probe tomography, it is possible to study nanostructured materials with almost atomic resolution in all three dimensions. In this article, we will present a short review of the latest atom-probe measurements carried out at University of Münster with particular focus on diffusion and segregation measurements in triple junctions and interface analysis.