Curvature-driven Grain-boundary Migration Research Articles

Shape functions and kinetics of migrating grain boundaries in nanocrystalline materials

Grain boundary (GB) shapes determine driving force of curvature-driven GB migration and therefore is critical to grain growth kinetics. GB shape functions in nanocrystalline materials are hardly known due to experimental difficulties. This study represents the first one with regard to investigating shape functions of steadily migrating GBs at nanometer scale by using molecular dynamics. It is found that two types of GB migration kinetics occur depending on grain size and temperature, both differing from the one in coarse grain counterparts. The two kinetics correspond to different GB shapes. For the grains in a range of about 10–25 nm (the upper limit is expected to extend to sub-micron) at relatively low temperature, new shape functions have been successfully derived, while for the finest nanograins of around several nm at relatively high temperature their shape functions coincide with the ones developed for coarse grains. In addition, GB faceting occur for some GBs, which causes deviation of boundary shape from the functions proposed. The findings in the present study imply that the boundary shapes and migration kinetics may contribute to the specific grain growth kinetics in nanocrystalline materials.

Evolution of residual stress and its impact on Ni-based superalloy

Severe residual stresses generated by various preparative techniques can greatly affect the mechanical properties in the Ni-based superalloy parts. Limited data is currently available for deeply understanding the origin of residual stress at nanoscale and in particular, for establishing the relationship between the residual stress and mechanical property to accelerate the development of the Ni-based superalloys. Here, the effect of the heat treatment on the grain growth, residual stress and flow stress in Ni-based superalloy is investigated by using an atomic simulation combined with a physically-based theoretical model. The dissociation of grain boundary causes the coalescence of adjacent grains during annealing, which is observed by atomic simulation. However, this process is different from the previous work that the curvature-driven grain-boundary migration leads to the grain growth. The residual stress consists of the grain and grain boundary, where the average value from grain boundary is about 50% higher that from the grain based on the statistical result. The grain growth and local movement of grain boundary cause the stress relaxation, and in turn they reduce the residual stress. The evolution of the residual stress is in good agreement with the previous experiment. Further, an analytical theoretical model is proposed to qualitatively evaluate the variation trend of the residual stress with the annealing temperature. It is also proved that the residual stress decreases with the increase of the annealing temperature. The present work can be applied to determine the residual stress in the annealed structure, which greatly reduces the process flow, therefore provides a scientific basis to turn the technological process for the accelerated development of the new Ni-based alloys.

Molecular dynamics studies on the grain growth of nanocrystalline Ni and Ni3Al

With molecular dynamics simulations, the growth of face-centered-cubic nanocrystalline materials Ni and Ni3Al has been studied. It is found that grain-rotation induced grain coalescence and curvature-driven grain-boundary migration are dominant mechanisms in the nanograin growth. A detailed comparison of the nanograin growth between the two systems is discussed in terms of grain rotation and grain sliding. We also study the temperature effect and the size effect in the nanograin growth. The tendency of twinning in the nanograin growth is discussed. It is found that in Ni3Al, it seems more possible for nanograins to grow into twin-like structures than single crystal unless at very high temperatures.

Grain-Boundary Fluctuations in Two-Dimensional Colloidal Crystals

We study grain-boundary fluctuations in two-dimensional colloidal crystals in real space and time using video microscopy. The experimentally obtained static and dynamic correlation functions are very well described by expressions obtained using capillary wave theory. This directly leads to values for the interfacial stiffness and the interface mobility, the key parameters in curvature-driven grain-boundary migration. Furthermore, we show that the average grain-boundary position exhibits a one-dimensional random walk as recently suggested by computer simulations [Z. T. Trautt, M. Upmanyu, and A. Karma, Science 314, 632 (2006)]. The interface mobility determined from the mean-square displacement of the average grain-boundary position is in good agreement with values inferred from grain-boundary fluctuations.

Relation between grain growth and grain-boundary diffusion in a pure material by molecular dynamics simulations

Traditionally, due to the impurities inevitably present in real materials that limit the grain-boundary (GB) mobility, the processes of grain growth and GB diffusion are thought to involve similar activation barriers. However, recent molecular dynamics (MD) simulations of GB migration in bicrystals have suggested that, in pure materials, GB diffusion and GB migration involve distinct mechanisms and, hence, different activation barriers. Here we report MD simulations of grain growth in an impurity-free model nanocrystalline palladium microstructure containing only high-energy GBs. By contrast with the bicrystal simulations, we observe virtually identical activation energies for grain growth and GB diffusion. We discuss several mechanisms that might be responsible for this difference between the bicrystal and polycrystal results, including accommodation of the curvature-driven GB migration by the elimination of GB area and GB excess free volume and a possible finite mobility of the triple junctions. The qualitative agreement of our observations with the general experimental findings is remarkable given the absence of any impurities in our model system. This suggests that a fundamental, intrinsic process may be responsible for the similar activation energies for GB migration and GB diffusion in polycrystals. In our model system this process seems to involve the accommodation of the curvature-driven GB migration by GB diffusion to eliminate the GB area and related GB free volume at the triple junctions.

Growth study of nanocrystalline Ni and Ni3Al using molecular dynamics

AbstractThe growth of nanocrystalline materials has received much attention recently due to its importance in nanocrystalline thin film properties and fabrications. Research studies of this subject have so far focused on the face-centered-cubic metals. Two mechanisms, grain-rotation induced grain coalescence and curvature-driven grain-boundary migration are considered as the dominant mechanisms in the nanograin growth. In this work, we use molecular dynamics method to simulate the growth of Ni and Ni3Al alloy. We find the above mechanisms can describe the growth behavior well. A detailed comparison of the nanograin growth between the two systems is discussed in terms of grain rotation and grain sliding. We also study the temperature effect and the size effect in the nanograin growth. The tendency of twinning in the nanograin growth is discussed.

Role of grain rotation during grain growth in a columnar microstructure by mesoscale simulation

We use a mesoscopic simulation approach to study the coupling and competition between grain-boundary-curvature driven and grain-rotation-coalescence induced grain growth in a 〈001〉 textured columnar microstructure. The well-known variational formulation for the total dissipated power, due to Needleman and Rice, provides the formal basis for our two-dimensional simulations. A stochastic velocity Monte-Carlo algorithm is used to minimize the functional in each time step. The competition between grain-boundary migration and grain rotation introduces a physical length scale, R c, into the system, enabling the growth process to be characterized by two regimes. If the average grain size is smaller than R c, as is the case in nanocrystalline materials, grain growth is dominated by the grain-rotation-coalescence mechanism. By contrast, if the average grain size is greater than R c, then growth is dominated by curvature-driven grain-boundary migration. The growth exponents characterizing the power-law time dependence of the average grain size are different for the two growth regimes. An extended von Neumann–Mullins relation, based on averaged grain-boundary properties, is further extended to include the effect of grain rotations.

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.)