Special Grain Boundaries Research Articles

Site-specific atomic scale analysis of solute segregation to a coincidence site lattice grain boundary

A site-specific method for measuring solute segregation at grain boundaries in an Aluminum alloy is presented. A Sigma 7(Sigma 7=38 degrees 111) grain boundary (GB) in an aluminum alloy (Zr, Cu as main alloying elements) was evaluated using site-specific Local Electrode Atom Probe (LEAP). A sample containing a Sigma 7GB was prepared by combining electron backscatter diffraction (EBSD) and focused ion beam (FIB) milling to locate the GB of interest and extract a specimen. Its composition was determined by LEAP, and compared to a general high angle GB (HAGB). Zr was the only alloying element present in the Sigma 7GB, whereas the general HAGB contained both Cu and Zr. This site-specific LEAP method was found to be an accurate method for measuring GB segregation at specific GB misorientations. The method has advantages over other methods of measuring chemistry at GBs, such as spectroscopy, in that GB structure can be assessed in three dimensions.

D-Line Emission from Small Angle Grain Boundaries in Multicrystalline Si

We have characterized optical property of small-angle (SA) grain-boundaries (GBs) in high-pure multicrystalline Si by using cathodoluminescence (CL). Prior to CL measurement, the electrical activity of GBs were evaluated by using electron-beam-induced current (EBIC). The SA-GBs are categorized into two groups with room temperature (RT-) EBIC contrast. The SA-GBs with misorientation angle about 1º give weak RT-EBIC contrast and yield D3 and D4. The SA-GBs with 2.5º show strong EBIC contrast and yield D1 and D2. These correspondences reflect the dislocation density at the SA-GBs. We also found the curious distribution of D1 emission in some special GBs, which is now difficult to explain. It is noticed that large-angle GBs do not show any D-line emissions at all.

Investigation on the formation of serrated grain boundaries with grain boundary characteristics in an AISI 316 stainless steel

The formation of serrated grain boundaries (GBs) depending on the GB characteristics has been investigated by using an electron backscattered diffraction (EBSD) technique and a transmission electron microscopy (TEM) in an AISI 316 stainless steel. It was observed that at the early stage of aging treatment, the GB morphology was changed from flat to wavy at random GBs without any indication of M 23C 6 carbide formation, and no GB serration at special GBs (lower than Σ29) was found. The comparison study on the misorientation angle between two neighboring grains indicated that the occurrence of GB serration at random GBs is attributed to the reduction of the total GB energy. Random GBs with high energy tend to be serrated, resulting in the formation of two segments with lower energies. On the other hands, the special GBs may be less likely to form serrated GBs due to their lower GB energy.

Grain boundary internal friction in bicrystals with different misorientations

The grain boundary (GB) internal friction peak has been observed in pure aluminum bicrystals containing 〈1 0 0〉 and 〈1 1 2〉 tilt GBs with different misorientations. It is found that the apparent activation parameters are different among low-angle, high-angle random and special GBs, and different extents of coupling effect are involved in different types of GBs. The results reveal that the basic mechanism for low-angle GBs can be attributed to dislocation climb, while that for high-angle GBs can be attributed to GB diffusion. The previously puzzling behaviors (apparent high activation energy and broad peak-width) of GB internal friction in polycrystals can also be explained in the light of the coupling model.

Atomic structures of symmetrical and asymmetrical facets in a near Σ = 9{2 2 1} tilt grain boundary in copper

Atomic structures in a Σ = 9{2 2 1} tilt grain boundary (GB) grown by Bridgman solidification of a tricrystal are determined through high-resolution transmission electron microscopy and numerical simulation. Atomic models are simulated via molecular dynamics annealing using an n-body potential fitted on copper properties including its stacking fault energy. Symmetrical and asymmetrical facets are thus identified. Mainly asymmetrical facets are observed, namely Σ = 9{11, 11, 1}||{1 1 1} and also small parts of incommensurate {1 1 0}||{1 1 1}. The symmetrical facets are described by a quasi-mirror plane atomic structure. A specific GB structural unit is recognized as a Lomer unit. Its GB Burgers vector depends on the GB structure itself. Further analyses of these models and of accommodating dislocations are successfully carried out at the atomic level within the framework of the continuous structural unit approach.

Ab initio modeling of clean and Y-doped grain boundaries in alumina and intergranular glassy films (IGF) in β-Si3N4

Based on large and relaxed grain boundary (GB) models in alumina and intergranular glassy film (IGF) models in polycrystalline β-Si3N4, ab initio modeling and theoretical tensile experiments were carried out for both clean and Y-doped models. It is shown that the increased covalent bonding between Y and O or N through the participation of the Y-4d and Y-3p orbitals is the mechanism by which Y ions enhance the mechanical and elastic properties of the Y-doped GB and IGF models. In alumina, this explains the improved creep behavior in the presence of Y doping. Preliminary results on the electronic structure and bonding of a specific GB model (Σ37) in α-Al2O3 is presented. For the IGF models, the distribution patterns of Y ions in the glassy region were investigated by total energy calculations. Y ions prefer to be at the interfacial region between the IGF and bulk crystal. Defect-like states of different origin can be identified near the valence band and the conduction band edges. These theoretical predictions obtained from the calculation of the fundamental electronic structure of the materials can be used to derive local strain fields of dissimilar “particles” that may be linked to continuum level theories via finite element methods.

The effect of rejuvenation heat treatments on the repair weldability of wrought Alloy 718

Extensive precipitation of needle- and plate-shaped δ-phase in the γ-nickel matrix of wrought Alloy 718 is the major microstructural change resulting from multiple weld repair/post weld heat treatment (PWHT) cycles. Isothermal heat treatments at 954 °C for times up to 100 h were used to simulate the multiple PWHTs in laboratory samples. Grain size did not change appreciably during these heat treatments owing to δ-phase pinning of the grain boundaries (GBs). The susceptibility of Alloy 718 to heat-affected-zone (HAZ) liquation cracking degraded as a result of these heat treatments. This degradation is due to the short time, high temperature GB liquation caused by the combined effects of δ-phase dissolution, boron carbide constitutional liquation, and GB segregation. A rejuvenation treatment (1010 °C/2 h) effectively restored the degraded weldability by removing the adverse influence of δ-phase through dissolution above the δ-phase solvus. This heat treatment also promoted a spontaneous grain refinement and increase in the fraction of special GBs owing to the elimination of δ-phase pinning of GBs. The combined effects of δ-phase, grain size and fraction of special GBs are discussed in the context of HAZ liquation cracking that occurs during repair welding of Alloy 718.

Grain boundary character distribution in B2 intermetallics

This paper reports the results of the experimental studies of the effect of chemical composition, texture and processing methods on the grain boundary character distribution evaluated by the electron back-scatter diffraction pattern (EBSD or EBSP) technique, in B2 FeAl and NiAl intermetallic compounds. An alternative method based on the grain boundary surface area for calculating the unbiased fraction of grain boundaries of a given character (ΣCSL) is proposed. It is shown that the proposed method gives different results than the classical method based on the counting of grain boundary segments. It is found for both FeAl and NiAl that the fraction of low-angle boundaries (Σ1 LABs) increases with increasing percentage of the <100> and to a lesser extent the <111> texture, up to the upper limit of ∼20%. In B2 FeAl the fraction of so-called “special” grain boundaries (SGBs) (Σ3-29) seems to be independent of the percentage of the <100> texture. In B2 NiAl and NiAl+2wt.%HfC alloy the fraction of SGBs decreases continuously with increasing percentage of the <111> texture. The <110> texture does not have strong effect on LABs. Processing of B2 FeAl by shock-loading and subsequent annealing can increase the fraction of LABs to 90–97%. This effect is not observed in B2 NiAl. Instead, the premature abnormal grain growth occurs, accelerated by the accumulated shock strain energy. In B2 compounds the fraction of LABs seems to increase nearly linearly up to the limit of ∼20% with decreasing grain size from ∼400 to ∼100 μm. For grain sizes smaller than ∼100 μm the fraction of LABs seems to be independent of grain size. The fraction of SGBs does not exhibit any dependence on grain size.

Determination of the Grain Boundary Misorientation for the Analysis of the Discontinuous Ordering Reaction in Fe-50 at. % Co

The discontinuous ordering (DO) reaction represents the mechanism of atomic ordering in alloys at such low temperatures, at which the mobility of atoms in the bulk of the alloy can be neglected and the ordering process occurs only in the regions swept by migrating grain boundaries (GBs). In order to study the relationship between the grain boundary crystallography and the kinetics of the DO reaction in the Fe-50 at.% Co alloy, the orientations of all the grains in a 8x4 mm 2 sample have been determined in the scanning electron microscope by an automated analysis of the backscattered Kikuchi patterns, and the crystallographic orientation image of the sample has been constructed. With the help of this image, completed by a light microscopy map of the sample, all GBs have been numbered sequentially and the maximal width of the ordered phase has been determined in the light microscope (LM) for each individual GB. Afterwards, the misorientation parameters for all GBs investigated in the LM have been determined (totally 226 GBs). Such a procedure allowed us to establish a relationship between the rate of the DO reaction at individual GBs and their misorientation parameters. The data are presented as the dependencies of the GB migration rate on the misorientation angles around the axis close to the low-index , and axes. It is shown that the GBs close to the special Σ3 , Σ19 , Σ9 , Σ11 , Σ5 , Σ13 and Σ17 misorientations (here Σ is the inverse density ofthe coincidence sites) exhibit a low DO rate. This is interpreted in terms of a decreased diffusivity in the special GBs. At the same time, GBs with a misorientation angle slightly below the value of 46.6° for the special Σ19 boundary exhibit the highest DO rate. A model is suggested which establishes a relationship between DO rate, diffusivity and mobility of the GB.

Further statistical analysis of the composition of a grain boundary in a Mo(Re) alloy studied by atom-probe field-ion microscopy

We have recently performed a combined transmission electron microscope CIEM) and atom-probe field-ion microscope (APFIM) investigation of Re segregation to a single grain boundary (GB) that is within ---0.4 of a E = 9 coincident site lattice (CSL) orientation in a Mo-5.58_+0.31 at.% Re alloy [ 1,2]; specifically the misorientation is 0 =38.5 about the c 1 = (1/'-,/2)[ 1,1,0l i direction, and the plane of the GB is -(-1,-1,4); this GB is mainly a tilt GB with a twist component. The primary conclusion of our paper [1,2] with respect to GB segregation is that this specific GB studied has an Re concentration which is = 1.27 or 27% greater than the Re concentration of the matrix -this value represents the minimum segregation enhancement factor, 3 as it includes some solute atoms from the matrix.[3,4] In this letter we recapitulate briefly our experimental procedure, the results obtained, and we then consider in detail the question of the statistical uncertainties involved. This detailed statistical analysis confirms the original conclusions made in reference 1 with respect to the uncorrected Re concentration at the GB, as well as the corrected Re concentration.

Grain boundary structure and electrical activity in shaped silicon

Crystalline structure, recombination activity, and electron transport properties have been investigated in boron-doped, thin-walled crystals of silicon obtained from hexagons grown by the EFG process (a modification of Stepanov's method). All grain boundaries (GBs) can be divided into special (Σ3, Σ9), near coincidence, highly deviated (Σ3, Σ9, Σ13), and general (i.e., non-special) GBs. Special GBs are electrically inactive to both minority and majority carriers. Near coincidence GBs are active to minority but inactive to majority carriers. General and highly deviated GBs are very active to both minority and majority carriers. The observed classification of GB activity is explained on the basis of a phenomenological model.

Grain-boundaries: Criteria of specialness and deviation from CSL misorientation

The analytical and geometrical methods used to determine, at grain boundaries, deviations from exact CSL misorientations have been reviewed, established when necessary to make them understandable, and criticized, together with criteria of specialness of grain boundaries. One method was found erroneous, and some of the conclusions of published statistical studies relative to special grain-boundaries should be reexamined.