H. Gleiter

Effective thermal conductivity of particulate composites with interfacial thermal resistance

A methodology is introduced for predicting the effective thermal conductivity of arbitrary particulate composites with interfacial thermal resistance in terms of an effective medium approach combined with the essential concept of Kapitza thermal contact resistance. Results of the present model are compared to existing models and available experimental results. The proposed approach rediscovers the existing theoretical results for simple limiting cases. The comparisons between the predicted and experimental results of particulate diamond reinforced ZnS matrix and cordierite matrix composites and the particulate SiC reinforced Al matrix composite show good agreement. Numerical calculations of these different sets of composites show very interesting predictions concerning the effects of the particle shape and size and the interfacial thermal resistance.

Nanocrystalline materials an approach to a novel solid structure with gas-like disorder?

Randomly oriented “nanocrystal” with sizes d below 10 nm, were compacted into a nanocrystalline solid. Studies by X-ray diffraction, Mossbauer spectroscopy, and magnetic measurements suggest a novel type of solid structure which consists of crystalline domains and a connective matrix without any short or long range order, corresponding structurally to a “gas-like solid”.

Deformation twinning in nanocrystalline Al by molecular-dynamics simulation

Abstract We use a recently developed, massively parallel molecular-dynamics code for the simulation of polycrystal plasticity to elucidate the intricate interplay between dislocation and GB processes during room-temperature plastic deformation of model nanocrystalline-Al microstructures. Our simulations reveal that under relatively high stresses (of 2.5 GPa) and large plastic strains (of ~12%), extensive deformation twinning takes place, in addition to deformation by the conventional dislocation-slip mechanism. Both heterogeneous and homogeneous nucleation of deformation twins is observed. The heterogeneous mechanism involves the successive emission of Shockley partials from the grain boundaries onto neighboring slip planes. By contrast, the homogeneous process takes place in the grain interiors, by a nucleation mechanism involving the dynamical overlap of the stacking faults of intrinsically and/or extrinsically dissociated dislocations. Our simulations also reveal the mechanism for the formation of a new grain, via an intricate interplay between deformation twinning and dislocation nucleation from the grain boundaries during the deformation. The propensity for deformation twinning observed in our simulations is surprising, given that the process has never been observed in coarse-grained Al and that the well-known pole mechanism cannot operated for such a small grain size. It therefore appears that the basic models for deformation twinning should be extended with particular emphasis on the role of grain-boundary sources in nanocrystalline materials.

Length-scale effects in the nucleation of extended dislocations in nanocrystalline Al by molecular-dynamics simulation

The nucleation of extended dislocations from the grain boundaries in nanocrystalline aluminum is studied by molecular-dynamics simulation. The length of the stacking fault connecting the two Shockley partials that form the extended dislocation, i.e., the dislocation splitting distance, rsplit, depends not only on the stacking-fault energy but also on the resolved nucleation stress. Our simulations for columnar grain microstructures with a grain diameter, d, of up to 70 nm reveal that the magnitude of rsplit relative to d represents a critical length scale controlling the low-temperature mechanical behavior of nanocrystalline materials. For rsplit>d, the first partials nucleated from the boundaries glide across the grains and become incorporated into the boundaries on the opposite side, leaving behind a grain transected by a stacking fault. By contrast, for rsplit<d two Shockley partials connected by a stacking fault are emitted consecutively from the boundary, leading to a deformation microstructure similar to that of coarse-grained aluminum. The mechanical properties of nanocrystalline materials, such as the yield stress, therefore depend critically on the grain size.

Grain-boundary diffusion creep in nanocrystalline palladium by molecular-dynamics simulation

Molecular-dynamics (MD) simulations of fully three-dimensional (3D), model nanocrystalline face-centered cubic metal microstructures are used to study grain-boundary (GB) diffusion creep, one mechanism considered to contribute to the deformation of nanocrystalline materials. To overcome the well-known limitations associated with the relatively short time interval used in our MD simulation (typically <10−8 s), our simulations are performed at elevated temperatures where the distinct effects of GB diffusion are clearly identifiable. In order to prevent grain growth and thus to enable steady-state diffusion creep to be observed, our input microstructures were tailored to (1) have a uniform grain shape and a uniform grain size of nm dimensions and (2) contain only high-energy GBs which are known to exhibit rather fast, liquid-like self-diffusion. Our simulations reveal that under relatively high tensile stresses these microstructures, indeed, exhibit steady-state diffusion creep that is homogeneous, with a strain rate that agrees quantitatively with that given by the Coble-creep formula. The grain-size scaling of the Coble creep is found to decrease from d−3 to d−2 when the grain diameter becomes of the order of the GB width. For the first time a direct observation of the grain-boundary sliding as an accommodation mechanism for the Coble creep, known as Lifshitz sliding, is reported.

Diffusion in nanocrystalline material

The paper reports on first investigations of the diffusion in nanocrystalline materials. The self-diffusion of the radioisotope 67Cu in nanocrystalline copper has been measured by serial sectioning with the aid of ion-beam sputtering. The values of the diffusion coefficients which were found at 353 K and 393 K are 2×10−18 m2/s and 1.7×10−17 m2/s, respectively, i.e., they are about 16 or 14 orders of magnitude larger than the bulk self-diffusion and about 3 orders of magnitude larger than the grain-boundary self-diffusion in copper. In comparison to the bulk, small values for the activation enthalpy, 0.64 eV, and the pre-exponential factor of the self-diffusion coefficient, 3×10−9 m2/s, have been observed.

Investigation of low energy grain boundaries in metals by a sintering technique

Abstract The validity of proposed grain boundary models is tested by comparing the predicted and the experimentally observed boundaries of low energy. The method used to identify the boundaries of low energy was the coherent rotation during annealing of single crystal balls of copper sintered onto a copper single crystal plate. The experimentally observed boundaries of low energy are at variance with the predictions of grain boundary models based on geometry. The observations support a grain boundary model based on a periodic arrangement of structural units, the energy of which is controlled by the interaction energy between the atoms. Furthermore, the experimental results suggest that two types of boundaries may be distinguished in metals: “electron insensitive” and “electron sensitive” boundaries. To a first approximation the energy of electron insensitive boundaries is controlled by the geometry of the atomic arrangement in the boundary, whereas the energy of the second group depends on the electron structure. In all metals with the same lattice structure, the electron insensitive boundaries are obtained for identical orientation relationships. The orientation relationships corresponding to low energy boundaries of the “electron sensitive” type vary as a function of the electron structure of the material.

Investigation of nanocrystalline iron materials by Mössbauer spectroscopy

Nanocrystalline materials, which have been proposed to represent a new solid state structure, are investigated by Mossbauer spectroscopy. Nanocrystalline materials are polycrystals with a crystal size of typically 1–10 nm. These materials consist of two components of comparable volume fractions: a crystalline component and an interfacial component, formed by the atoms located either in the crystals or in the interfacial regions between them. As the atomic configurations of both components are different, two kinds of Mossbauer spectra are expected. Iron nanocrystalline material is found to exhibit a two‐component Mossbauer spectrum, consisting of a crystalline component and a second one with different Mossbauer parameters. The Mossbauer parameters of the second subspectrum are consistent with the model of the interfacial component of a nanocrystalline material.

Mechanisms of grain growth in nanocrystalline fcc metals by molecular-dynamics simulation.

To elucidate the mechanisms of grain growth in nanocrystalline fcc metals, we have performed fully three-dimensional molecular-dynamics simulations with a columnar grain structure and an average grain diameter of 15 nm. Based on the study of coarse-grained materials, the conventional picture is that grain growth is governed by curvature-driven grain-boundary migration. However, our simulations reveal that in a nanocrystalline material grain rotations play an equally important role, at least during the early stages of grain growth. By eliminating the grain boundary between neighboring grains, such rotations lead to grain coalescence and the consequent formation of highly elongated grains. A detailed analysis exposes an intricate coupling between this mechanism and the conventional grain-boundary-migration dominated mechanism. Incorporation of these insights into mesoscopic models should enable more realistic mesoscopic simulations of grain growth in nanocrystalline materials. (A short movie showing the overall evolution of the grain microstructure can be viewed at http://www.msd.anl.gov/im/movies/graingrowth.html.)

Dislocation–dislocation and dislocation–twin reactions in nanocrystalline Al by molecular dynamics simulation

We use massively parallel molecular dynamics simulations of polycrystal plasticity to elucidate the intricate dislocation dynamics that evolves during the process of deformation of columnar nanocrystalline Al microstructures of grain size between 30 and 100 nm. We analyze in detail the mechanisms of dislocation-dislocation and dislocation-twin boundary reactions that take place under sufficiently high stress. These reactions are shown to lead to the formation of complex twin networks, i.e. structures of coherent twin boundaries connected by stair-rod dislocations. Consistent with recent experimental observations, these twin networks may cause dislocation pile-ups and thus give rise to strain hardening.