Grain Size In Nanocrystalline Materials Research Articles

Anisotropic grain growth kinetics in nanocrystalline nickel

ABSTRACTIntriguing properties exhibited by nanocrystalline metals, including a high level of mechanical strength, arise from their nanometer-scale grain sizes. It is critical to determine the evolution of grain size of nanocrystalline materials at elevated temperature, as this process can drastically change the mechanical properties. In this work, a nanocrystalline Ni foil with grain size ∼ 25 nm was annealed in situ in an X-ray diffractometer. X-ray diffraction peaks were analysed to determine the grain growth kinetics. The grain growth exponents obtained were ∼ 2–4 depending upon the crystallographic direction, indicating the anisotropic nature of the grain growth kinetics.

Crystal instability in nanocrystalline materials

As the grain size in nanocrystalline materials is made ever smaller, the questions of what the smallest grain size could be and what factors influence it become highly relevant to material synthesis and application. Using extensive atomistic simulation and theoretical analysis, this paper shows that the crystalline phase instability sets the ultimate limit for grain size reduction below which amorphization occurs. The instability is caused by the combined effect of structural disorder present at grain boundaries and the internal inhomogeneous strain fields associated with solutes or impurities. A phase diagram describing the instability or crystal-to-glass transition is constructed from a Ginzburg–Landau theory based on the effects of the two types of disorders and their interactions. The mean critical grain size is shown to range from several nanometers to tens or hundreds of nanometers, depending on the impurity or solute concentration.

Generation of dislocation loops in deformed nanocrystalline materials

A theoretical model is suggested which describes the generation of lattice perfect, lattice partial and grain boundary dislocation loops (DLs) at pre-existent DLs in mechanically loaded nanocrystalline materials (NCMs). The energy characteristics of various modes of the DL generation are calculated and compared. With these calculations and comparison, the two basic ranges of the grain size in NCMs are revealed each is characterized by its specific set of effectively operating modes of the DL generation and plastic deformation. The role of the DL generation in plastic and superplastic deformation processes in NCMs is discussed.

On the analysis of grain size in bulk nanocrystalline materials via x-ray diffraction

The Warren-Averbach (WA) analysis and other simplified methods that are commonly used to determine the grain size of nanocrystalline materials are discussed in terms of accuracy and applicabilities. The nanocrystalline materials used in the present study are prepared by cryomilling of A1 powders and subsequent consolidation (hot isostatic pressing and extrusion). Transmission electron microscopy observations of the as-extruded nanocrystalline A1 reveal a bimodal distribution of grain sizes centered around 50 to 100 nm and 250 to 300 nm. It is shown that the grain size determined by the WA analysis agrees with the lower bound grain size (e.g., 50 to 100 nm) observed experimentally. In the case of the integral method, it is useful to use a parabolic (Cauchy-Gaussian (CG)) relationship to approximate instrumental broadening and separate the intrinsic broadening. Compared to the Cauchy-Cauchy (CC) and Gaussian-Gaussian (GG) approximations, this is shown to give the best results. In addition, the reliability of the Scherrer equation is also discussed.

623 分子動力学シミュレーションによるナノ多結晶体の内部構造と力学特性の粒径依存性に関する研究

623 分子動力学シミュレーションによるナノ多結晶体の内部構造と力学特性の粒径依存性に関する研究

Percolative composite model for prediction of the properties of nanocrystalline materials

A physical percolating composite model is presented for description of the changes in the transport-type properties with grain size in nanocrystalline materials. The model is based on hierarchial percolation through the different microstructural components such as grain boundaries, triple lines, and quadruple nodes at grain sizes when their respective percolation thresholds are reached. The model yields critical grain sizes at which the properties may change significantly. These grain sizes depend on the grain boundary thickness. Master curves were calculated for the elastic modulus and compared to the experimental data from the literature. Better fit was found with the experimental data in comparison to Hill's approximation model. The critical grain size at grain boundary percolation threshold is suggested as a criterion for definition of materials to exhibit nanocrystalline properties.

Variation of the interfacial energy with grain size in nanocrystalline materials

The transformation of an amorphous NiP alloy was investigated calorimetrically. In particular, the formation of the interfaces in nanocrystalline NiP samples with grain sizes ranging from a few nanometers to 60 nm was studied. A linear increase in the interfacial excess energy was found in increasing grain size on a nanometer scale. However, a rather small interfacial excess energy was obtained when the grain size was a few nanometers. The variation of interfacial excess energy with grain size was found to correlate with that of the interfacial excess volume derived from experimental results.