Grain Boundary Distribution Research Articles

Effect of film thickness and laser energy density on the microstructure of a-GaAs films after excimer laser crystallization

A KrF excimer laser with 30 ns pulse duration is used for crystallization of a-GaAs grown on silicon substrate using molecular beam epitaxy technique. The effect of laser energy density and film thickness on grain morphology has been studied. Scanning electron microscopy and high-resolution electron backscatter diffraction have been used to study the texture and microstructure evolution during the crystallization of initially amorphous GaAs thin films. The integrated information on grain size distribution, preferred orientation, and nature of grain boundaries provides useful information to postulate the mechanism of grain growth and the likely role of different contributing parameters in the evolution of final texture under the highly transient processing conditions prevailing during the short laser irradiation. The texture ranges from weak ⟨111⟩ fiber texture to strong ⟨100⟩ texture depending on film thickness and laser influence. The grain structure and texture development are discussed based on the three melting regimes: (1) partial meting regime; (2) complete melting regime; and (3) near-complete melting regime.

Correlated grain-boundary distributions in two-dimensional networks

In polycrystals, there are spatial correlations in grain-boundary species, even in the absence of correlations in the grain orientations, due to the need for crystallographic consistency among misorientations. Although this consistency requirement substantially influences the connectivity of grain-boundary networks, the nature of the resulting correlations are generally only appreciated in an empirical sense. Here a rigorous treatment of this problem is presented for a model two-dimensional polycrystal with uncorrelated grain orientations or, equivalently, a cross section through a three-dimensional polycrystal in which each grain shares a common crystallographic direction normal to the plane of the network. The distribution of misorientations theta, boundary inclinations phi and the joint distribution of misorientations about a triple junction are derived for arbitrary crystal symmetry and orientation distribution functions of the grains. From these, general analytical solutions for the fraction of low-angle boundaries and the triple-junction distributions within the same subset of systems are found. The results agree with existing analysis of a few specific cases in the literature but present a significant generalization.

High-cycle fatigue of nickel-base superalloy René 104 (ME3): Interaction of microstructurally small cracks with grain boundaries of known character

High-cycle fatigue (HCF), involving the premature initiation and/or rapid propagation of small cracks to failure due to high-frequency cyclic loading, has been identified as one of the leading causes of turbine engine failures in aircraft. In this work, we consider the feasibility of using grain-boundary engineering to improve the HCF properties of a polycrystalline nickel-base superalloy, René 104 (also known as ME3), through systematic modification of the grain-boundary distribution. In particular, we investigate the growth of microstructurally small fatigue cracks at ambient temperature in microstructures with varying proportions of “special” vs. “random” boundaries, as defined by coincident-site lattice theory. Specifically, we examine the interaction of propagating small (∼10–900 μm) surface cracks with grain boundaries of known character, with respect both to any deflection in crack trajectory that occurs at or near the boundary, and more importantly to any local changes in crack-growth rates. In addition, finite-element calculations are performed to evaluate the effective driving force and plastic-zone profile for such small-crack propagation, incorporating information from both the local microstructure (from electron backscattering diffraction scans) and the surface crack-path profile.

A preliminary model to predict the special grain boundary fraction in commercial-purity nickel

Abstract Special grain boundaries have been shown in numerous studies to be more resistant to creep, fracture behaviour, intergranular degradation, corrosion, and minor element segregation, leading to mechanical property improvements. To manipulate the fraction of special boundaries, thermo-mechanical processing can be used whereby a series of prescribed deformation and annealing heat treatments are used. However, the ranges and combinations of manufacturing process parameters can greatly affect the type and distribution of grain boundaries and therefore their selection is crucial to the resulting material properties. This study looks at an initial experimental attempt to correlate the effects of the manufacturing processing cycle parameters with the fraction of special boundaries produced in commercial-purity nickel. Process parameters such as percent reduction in thickness by cold rolling, annealing temperature, and number of cycles were studied, with annealing time kept constant. The analysis showed complex interactions between the independent variables from processing and the special fraction of grain boundaries as observed by Orientation Imaging Microscopy.

Intergranular fracture in model polycrystals with correlated distribution of low-angle grain boundaries

Grain boundary engineering of materials is intended to increase the fraction of ``special'' boundaries that possess preferred properties. These may be low-angle boundaries with good mechanical and corrosion resistance properties. In this case, the distribution of special boundaries is spatially correlated rather than random, due to crystallographic constraints. Such a correlated distribution, in addition to the density of special boundaries, will likely affect the progression of damage and failure of a polycrystalline material under load or applied strain. This is demonstrated for a two-dimensional model polycrystal, which is an array of hexagonal grains defining a ``honeycomb'' network of grain boundary facets. The facets are assigned a high or low elasticity modulus in accordance with their ``special'' or ``nonspecial'' designation, respectively. Then damage evolution due to an increasing strain is monitored as the following sequence of two steps is repeated until material failure: (1) calculation of the stress and strain fields by use of a scalar Hooke's law and (2) rupture of the facet with greatest strain energy. For polycrystals with a high fraction of special facets, damage proceeds by a series of ``bursts'' (``avalanches'') of single-facet failures distributed over the polycrystal. These failures occur at special facets that connect two clusters of nonspecial facets, thereby creating larger clusters of ``weak'' (nonspecial plus ruptured) facets oriented roughly perpendicular to the applied strain. Eventually a weak cluster grows to a critical size such that sufficient stress is diverted through the nonspecial facets (which have served as ligaments preventing a crack from nucleating on the weak cluster) to cause those facets to fail. This crack growth leads to catastrophic failure of the model polycrystal.

Structure and Mechanical Properties of Submicrocrystalline Copper Produced by ECAP to Very High Strains

The present work is aimed at linking the microstuctutral features obtained after severe plastic deformation via ECAP to the tensile behavior and thermal stability of pure (99.98%) copper processed by routes A and Bc to different number of passes. The main conclusion one can draw unambiguously from the currently available results is that the strain path exerts relatively little effect on the resultant tensile properties when the number of pressing is sufficiently large, although there have been some marked differences in crystallographic textures and distribution of grain-boundaries. The effect of the number of pressings on the tensile ductility is considerable.

Synthesis and microstructure of bulk nanocrystalline copper

The synthesis of bulk nanocrystalline copper (NC-Cu) by powder metallurgy is presented, from compaction of nanocrystalline powders to sintering and differential extrusion. At each step of the process, the microstructure is characterized using X-ray diffraction (XRD) analysis, field-emission gun scanning electron microscopy (FEG-SEM), and transmission electron microscopy (TEM). Particular attention is given to the concept of grain size in nanostructured materials and the comparison of results from the different characterization techniques. The fully dense material has a grain size of 100 nm with a microstructure best described in terms of the distribution of high-angle grain boundaries (GBs), twin boundaries, and low-angle GBs. Dislocations occur in half the grains and at most of the twin boundaries. The GBs are shown to be crystalline, and no evidence is found for amorphous interfacial regions. It is proposed that the grain size be defined only in terms of high-angle GBs, excluding low-angle GBs, for the discussion of mechanical properties. In this respect, the microstructure is compared with the NC-Cu material produced by other synthesis techniques. Powder metallurgy (P/M) processing is revealed as an alternative for the production of large-size submicrocrystalline and NC materials.

Elongation to failure in the model of cooperative grain boundary sliding

The macroscopic effect of neck development characterizing the termination of superplasticity has been studied reasoning from certain assumptions on the deformation mechanism at the mesoscopic level. It has been established that characteristics of ultimate ductility can be described in terms of deformation-induced space-non-uniformity of a distribution of cooperative grain boundary sliding bands, and there is no need to introduce the term of macro-non-uniformity, namely, a neck nucleus initially present in the sample.

Electrical characterization of directionally solidified polycrystalline silicon

An investigation has been undertaken of polycrystalline silicon (poly-Si) thin-film transistors (TFTs) using sequential laterally solidified material. This material has a location-controlled distribution of grain boundaries (GBs), which makes it particularly useful for the investigation of their influence on the performance of poly-Si TFTs, and to address the issue of the role of spatially localized trapping states. The experimental results showed that the specific location of the GBs had a minimal effect upon TFT performance, and most aspects of TFT performance could be accurately simulated using a spatially uniform distribution of states. The conclusion to arise from this study is that, with the exception of field-effect mobility, there are no features in the device behavior, which must be specifically attributed to the spatial localization of trapping states. A limited comparison with conventional laser-crystallized poly-Si was undertaken, and, in this material, it was found that the effects of trap localization were apparent.

Finite size scaling in grain boundary wetting

Experimental studies of grain boundary invasion by a wetting fluid give clear evidence for a size dependence. In order to get a better numerical insight into grain boundary (GB) wetting as a percolation process, we have investigated size effects during gallium penetration into quasi-2D zinc polycrystalline strips of various width and during water penetration into 3D cylindrical NaCl polycrystals. Both systems are likely to be good objects for studying percolation effects because of a random distribution of wettable GB’s. Computer simulation on the square lattice, with a “wetting” probability p = 0.60 close to the number of experimental points (several dozens), shows a striking resemblance between both sets of data. Making more runs (about 105) demonstrates consistency of our model with an earlier reported work by Marrink and Knackstedt describing finite size effects in elongated lattices. Using their approach, an excellent agreement can be obtained between the experimental and simulated data, as well as between the latter and theoretical predictions.

Computational simulations of intergranular fracture of polycrystalline materials and size effect

Grain boundaries are important elements associated with micro-structural heterogeneity in polycrystalline materials. The influence of the grain boundary character distribution on intergranular fracture of polycrystals is investigated in this paper. Considering the random heterogeneity of materials, a two-dimensional stochastic finite element method (SFEM) is used to simulate the intergranular damage and failure process of polycrystals. The impact of the fraction and distribution of random grain boundaries on the fracture behavior is discussed, and the effect of model size on results is also evaluated.

Effect of strain path on structure and mechanical behavior of ultra-fine grain Cu–Cr alloy produced by equal-channel angular pressing

The effect of strain path during severe plastic deformation via equal-channel angular pressing on grain refinement and structural features of a model Cu–Cr alloy is investigated in terms of grain shape, dimensions, preferred crystallographic orientation and distribution of grain boundaries with respect to the angle of misorientation. The mechanical behavior of differently processed specimens is assessed in both monotonic and cyclic tests aimed at clarification of the role played by different structural factors in the resultant mechanical properties. It is shown, that despite the considerable microstructural differences in the grain morphology and texture, the tensile and fatigue strength is only slightly affected by the processing routes chosen in the present study. However, the strain path and, therefore, the grain shape and texture have some effect on ductility and strain localization, which is particularly pronounced during cyclic loading.

Analysis of grain boundaries in CoCrTa and CoPtCrB HDD media by analytical transmission electron microscopy

Crystallographic structures and elemental distributions of grain boundaries in cobalt (Co)-based hard disk drive (HDD) media of CoCrTa and CoPtCrB were analysed by analytical transmission electron microscopy (TEM). CoCrTa medium had crystalline grain boundaries with extremely distorted (0002) planes of hexagonal close-packed structure. The width of grain boundaries of 1.5 nm obtained by high-resolution TEM (HRTEM) imaging agreed with that of 1.5 nm obtained by energy-dispersive spectroscopy (EDS) mapping. The EDS analysis revealed that Cr was segregated to grain boundaries while Ta was not segregated to either grain boundaries or grains. Cobalt was retained within grains. CoPtCrB medium had amorphous grain boundaries and spots in grains. The EDS and electron energyloss spectroscopy analyses revealed that both Cr and B were segregated to grain boundaries and spots. Both Co and Pt were retained within grains. The width of the boundaries of 0.4 nm obtained by HRTEM differed from that of 1.5 nm obtained by high-angle angular dark-field scanning TEM; the former indicates the crystallographic boundary and the later corresponds to the magnetic boundary, which is significantly correlated to the magnetic properties of HDD media.

Measuring the five-parameter grain-boundary distribution from observations of planar sections

A stereological method is described for estimating the distribution of grain-boundary types in polycrystalline materials on the basis of observations from a single planar section. The grain-boundary distribution is expressed in terms of five macroscopically observable parameters that include: three parameters that describe the lattice misorientation across the boundary and two parameters that describe the orientation of the grain-boundary plane normal. The grain-boundary distribution is derived from measurements of grain orientations and the orientations of the lines formed where grain boundaries intersect the plane of observation. Tests of the method on simulated observations illustrate that the distribution of boundaries in a material with cubic symmetry can be reliably determined with about 10° of resolution from the analysis of 5×104 or more line segments. Furthermore, grain-boundary distributions directly observed from serial sections of a SrTiO3 polycrystal are compared to those resulting from the stereological analysis of a single plane. The comparison shows that the stereological method provides a reasonable estimate of the measured distribution. The differences between the directly observed grain-boundary distribution and that derived from the stereological analysis are consistent with the results from the simulation.

Distribution of grain boundaries in aluminum as a function of five macroscopic parameters

The grain boundary character distribution in commercially pure Al has been measured as a function of lattice misorientation and boundary plane orientation. The results demonstrate a tendency to terminate grain boundaries on low index planes with relatively low surface energies and large interplanar spacings. The most frequently observed grain boundary plane orientation is (1 1 1). However, there are also instances where boundaries terminated by higher index planes have significant populations. For example, certain twist configurations on {1 1 w} planes, which correspond to symmetric [1 1 0] tilt boundaries, also have relatively high populations. The population of symmetric [1 1 0] tilt boundaries exhibits an inverse relationship with previously measured energies.

Distribution of grain boundaries in aluminum as a function of five macroscopic parameters

The grain boundary character distribution in commercially pure Al has been measured as a function of lattice misorientation and boundary plane orientation. The results demonstrate a tendency to terminate grain boundaries on low index planes with relatively low surface energies and large interplanar spacings. The most frequently observed grain boundary plane orientation is (1 1 1). However, there are also instances where boundaries terminated by higher index planes have significant populations. For example, certain twist configurations on {1 1 w } planes, which correspond to symmetric [1 1 0] tilt boundaries, also have relatively high populations. The population of symmetric [1 1 0] tilt boundaries exhibits an inverse relationship with previously measured energies.

The distribution of internal interfaces in polycrystals

Recent advances both in experimental instrumentation and computing power have made it possible to interrogate the distribution of internal interfaces in polycrystals and the three dimensional structure of the grain boundary network with an unprecedented level of detail. The purpose of this paper is to review techniques that can be used to study the mesoscopic crystallographic structure of grain boundary networks and to summarize current findings. Recent studies have shown that grain surfaces within dense polycrystals favor the same low energy planes that are found on equilibrium crystal shapes and growth forms of crystals in contact with another phase. In the materials for which comprehensive data exists, the distribution of grain boundaries is inversely correlated to the sum of the energies of the surfaces of the grains on either side of the boundary.

Distribution of Grain Boundaries in SrTiO3 as a Function of Five Macroscopic Parameters

Measurements of the grain boundary population as a function of misorientation and boundary plane orientation show that the distribution is inversely correlated to the sum of the energies of the surfaces comprising each boundary. The observed correlation suggests that the difference between the energy of a high‐angle grain boundary and the two component surfaces is relatively constant as a function of misorientation. Two exceptions to this correlation were identified: low‐misorientation‐angle boundaries and the coherent twin boundary, where the (111) planes in the adjoining crystals are parallel to each other, but rotated by 60° around the [111] axis. In these cases, the high degree of coincidence across this interface probably lowers the boundary energy with respect to that of the component surfaces. For all other boundaries, the anisotropy of the population is accurately predicted by the surface energy anisotropy, and in general, boundaries display a preference for {100} orientations, the planes of minimum surface energy.

Statistical Modeling of Grain-Enhanced Polysilicon Thin-Film Transistor in Consideration of Grain Boundary Distribution

ABSTRACTA probabilistic model to predict the statistical distribution of grain boundaries in the thin-film transistor (TFT) with an arbitrary transistor-to-grain size ratio is proposed in this paper. Performance of the TFTs, such as carrier mobility, can be estimated and the corresponding performance variation of the fabricated devices becomes predictable when the statistical distribution of grain boundaries is known. The proposed model is still applicable even when the transistor size becomes comparable to the grain size. Reliability and accuracy of the modeling results have been extensively verified by experimental data. It is believed that the model can provide design and optimization guidelines of device variation for grain-enhanced polysilicon TFT technology.

Nonrandom percolation behavior of grain boundary networks in high-Tc superconductors

The grain boundary networks of tetragonal and orthorhombic superconductors are shown to be nonrandom through computer simulations of polycrystalline structures. For biaxially textured microstructures, the distribution of low-angle grain boundaries around triple junctions is measurably deviant from the expectations for a randomly constructed lattice. Percolation thresholds are calculated for the unhindered flow of superconducting current through a polycrystal, and are found to be significantly different from the standard random percolation threshold in two dimensions. These deviations are explained as a result of crystallographic constraints on the network topology.