Fraction Of Special Boundaries Research Articles

EBSD investigation of intergranular corrosion attack on low interstitial stainless steel

AbstractGrain boundary effect on corrosion behaviour of low interstitial AISI 316LN austenitic stainless steel after homogenization and heat treatment at 700 and 800°C was investigated by means of Electron Backscattered Diffraction (EBSD) technique. Rapid oxalic acid etch test (ASTM A262‐ practice A) was used to determine steel susceptibility to intergranular corrosion. An attempt to quantify oxalic acid etch test results was also made.Beneficial effect of low carbon and nitrogen content on grain boundary precipitation and corrosion was observed. No changes neither in grain orientations nor in grain boundary types between sensitised and non‐sensitised steel states using orientation measurements were recorded. Twin boundaries were found in all samples with highest amount and only small occurrence changes. CSL categorizations showed only little increase of special boundary fractions with increasing time and temperature of heat treatment. Irrespective to annealing conditions a continued network of random boundaries was retained. Additionally, based on misorientation measurement of 50 grooved grain boundaries, a tendency for preferred attack of high angle boundaries (30–55°) was noted.

On the relationship between grain boundary connectivity, coincident site lattice boundaries, and intergranular stress corrosion cracking

The present work reports on refinements proposed to existing models for predicting the distribution of intergranular stress corrosion cracking based on the frequency of “special”, crack-resistant, low- Σ CSL grain boundaries in the microstructure. The accuracy of these models in predicting the extent of intergranular cracking in low-stacking fault FCC materials ranging from Alloy 600 to Admiralty Brass is improved by factors ranging from 2 to more than 6 by discounting isolated, “neutral-twins” from the measured total special boundary fraction. A stochastic model is presented for easily and accurately determining the proportion of neutral-twins, which do not disrupt connectivity in the network of random boundaries based on the relative frequency of twin-variant boundaries (i.e. Σ9s and Σ27s) in the microstructure. Improvements in the predicted susceptibility to intergranular attack achieved from the refinements described herein will enhance current abilities to identify components at high risk of failure by IGSCC based on grain boundary structure.

Roughness Scaling of Fracture Surfaces in Polycrystalline Materials

AbstractThe roughness scaling of fracture surfaces in two-dimensional grain boundary networks is stud- ied numerically. Grain boundary networks are created using a Metropolis method in order to mimic the triple junction distributions from experiments. Fracture surfaces through these grain boundary networks are predicted using a combinatorial optimization method of maximum flow — minimum cut type. We have preliminary results from system sizes up to N = 22500 grains suggesting that the roughness scaling of these surfaces follows a random elastic manifold scaling exponent ζ = 2/3. We propose a strong dependence between the energy needed to create a crack and the special boundary fraction. Also the special boundaries at the crack and elsewhere in the system can be tracked.

Grain Boundary Engineering of Copper Shaped-Charge Liners

SummaryGrain boundary engineering technology has been successfully transferred from rolling and forging to back-extrusion, and back- extruded copper shaped-charge liners with engineered microstructures have been produced. The grain-boundary engineered SCLs have special boundary fractions of 0.60 and 0.66, compared to 0.48 in a conventionally processed SCL. It is hoped that these engineered SCLs will perform better through a combination of a delayed onset of plastic instability and an improved resistance to void nucleation and coalescence.Previous work demonstrates that SCL performance is affected by microstructure, and there is significant evidence which shows that microstructure influences both the fracture and constitutive responses of a material. For SCLs, constitutive properties which affect both the strength and stability of a material are important to performance. In the work described here, the focus is on improving stability properties such as strain-hardening rate and strain-rate sensitivity through grain boundary engineering.Initial mechanical tests of grain boundary engineered copper indicate that its yield strength is nearly the same as conventionally processed material, even though the engineered sample has a larger grain size. Yield strength certainly affects the strength of an SCL, but its effect on stability, in terms of delaying the onset of plastic instability in a jet, is still under investigation. Although we can measure constitutive properties such as yield strength and strain to failure in the laboratory, the true effects of grain boundary engineering on SCL performance can only be measured in actual tests of the liners. Therefore, conventionally processed and grain boundary engineered SCLs will be fielded in real shaped-charges.

Analysis of grain boundary networks and their evolution during grain boundary engineering

The goal of grain boundary engineering is to increase the fraction of so-called special grain boundaries, while decreasing the contiguity of the remaining random boundaries which are susceptible to intergranular degradation such as cracking, cavitation, corrosion and rapid self-diffusion. In the present work, we describe a technique for the quantitative experimental study of grain boundary network topology, with an emphasis on the connectivity of special and random grain boundaries. Interconnected grain boundary networks, or “clusters”, of either entirely random or entirely special boundaries are extracted from electron backscatter diffraction data on a Ni-base alloy, and characterized according to their total normalized length (their “mass”), as well as their characteristic linear dimensions. The process of grain boundary engineering, involving cycles of straining and annealing, is found to substantially reduce the mass and size of random boundary clusters. Furthermore, quantitative assessment of the boundary network topology shows that the special grain boundary fraction is a poor predictor of network topology, but that the higher-order correlation derived from triple junction distributions can successfully describe the length scales of random boundary clusters.

Microstructural evolution during grain boundary engineering of low to medium stacking fault energy fcc materials

Grain boundary engineering comprises processes by which the relative fractions of so-called special and random grain boundaries in microstructures are manipulated with the objective of improving materials properties such as corrosion, creep resistance, and weldability. One such process also referred to as sequential thermomechanical processing (TMP), consists of moderate strains followed by annealing at relatively high temperatures for short periods of time. These thermomechanical treatments on fcc metals and alloys with low to medium stacking fault energies result in microstructures with high fractions of Σ3 n and other special boundaries, as defined by the coincidence site lattice (CSL) model. More importantly, the interconnected networks of random boundaries are significantly modified as a consequence of the processing. The modifications in the grain boundary network have been correlated with post-mortem electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) observations of the deformed and annealed states of the material. The evolution of the microstructure to a high fraction of Σ3 n boundaries is correlated with the decomposition or dissociation of immobile boundaries during annealing. This is evidenced by TEM observations of the decomposition of relatively immobile boundaries into two components, one with very low energy and thus immobile, and the other a highly mobile boundary that migrates into neighboring areas of higher strain levels. The formation of low-energy grain boundaries through this mechanism and its effect on boundary network topology is discussed within the context of grain boundary engineering and linked to known microstructural evolution mechanisms.

Electromigration properties of multigrain aluminum thin film conductors as influenced by grain boundary structure

Electromigration rates in polycrystalline interconnect lines are controlled by grain-boundary diffusion. As such, reliability of such interconnects is a direct function of the grain-boundary character distribution in the lines. In the present work, drift velocity experiments were performed on multicrystalline lines of pure Al to determine the electromigration activation energy of the lines. Lines cut from films processed by partially ionized beam deposition techniques were analyzed. One set of lines was analyzed in the as-deposited condition while the other film was annealed before testing. The measured drift velocities varied dramatically between these two types of films, as did the grain-boundary character distributions measured by orientation imaging. The data were analyzed based on Borisov's equation to obtain mean grain-boundary energies. Grain-boundary energy of the film with poor electromigration performance was calculated to be that reported for random boundaries, while that for the more reliable film was calculated to be that reported for twin boundaries in Al. Percolation theory was used to aid explanation of the results based upon the fraction and connectedness of special boundaries in the films.

Combined effect of special grain boundaries and grain boundary carbides on IGSCC of Ni–16Cr–9Fe– xC alloys

Susceptibility to intergranular stress corrosion cracking in Ni–16Cr–9Fe– xC alloys in 360°C primary water is reduced with increasing fraction of special grain boundaries, i.e. coincident site lattice boundaries (CSLB) and low angle boundaries, and grain boundary carbides. Intergranular stress corrosion cracking (IGSCC) was investigated using interrupted constant extension rate tensile tests in a primary water environment at 360°C. Thermal–mechanical treatments were used to increase the fraction of special boundaries from approximately 20–25% to between 30 and 40%. In a carbon-doped heat, further heat treating was used to precipitate grain boundary carbides preferentially on high-angle boundaries (HAB). Orientation imaging microscopy was used to determine the relative grain misorientations and scanning electron microscopy (SEM) was used to identify specific grain boundaries after each interruption. After each strain increment, the same regions in each sample were examined for cracking. Results showed that irrespective of the microstructure condition, CSLBs always cracked less than HABs. Results also showed that IGSCC is reduced with increasing solution carbon content, and for the same amount of carbon in solution, the addition of grain boundary carbides reduced IGSCC still further. The best microstructure was the one consisting of an enhanced CSLB fraction and chromium carbides precipitated preferentially on high-angle boundaries.

Toward Optimization of the Grain Boundary Character Distribution in OFE Copper

The authors used a two-step (low and high temperature) strain-annealing process to evolve the grain boundary character distribution (GBCD) in fully recrystallized oxygen-free electronic (OFE) Cu bar that was forged and rolled. Orientation imaging microscopy (OIM) has been used to characterize the GBCD after each step in the processing. The fraction of special grain boundaries, special fraction, was {approximately} 70% in the starting recrystallized material. High, moderate, and low temperature processing conditions were employed. The high-temperature process resulted in a reduction in the fraction of special grain boundaries while both of the lower temperature processes resulted in an increase in special fraction up to 85%. Further, the lower temperature processes resulted in average deviation angles from exact misorientation, for special boundaries, that were significantly smaller than observed from the high temperature process. Results indicate the importance of the low temperature part of the two-step strain-annealing process in preparing the microstructure for the higher temperature anneal and commensurate increase in the special fraction. The authors approach is aimed at testing a hypothesis for the mechanism for GBCD optimization in Cu, i.e., it is possible to optimize the GBCD in Cu by selectively removing random grain boundaries through a strain-annealing process. Theymore » begin with a fully recrystallized material, deform it by a crucial amount, then apply a specific heat treatment schedule. The deformation is not intended to be sufficient to induce full recrystallization upon heating, but is intended to localize the deformation energy at random boundaries (since random boundaries are expected to be less efficient at transmitting dislocations than special boundaries). Upon heating, the stored energy near these random boundaries is expected to provide sufficient driving force to rearrange these boundaries into special types.« less

Toward Optimization of the Grain Boundary Character Distribution in Copper by Strain Annealing

ABSTRACTWe have used a two-step (low and high temperature) strain-annealing process to evolve the grain boundary character distribution (GBCD) in fully recrystallized oxygen-free electronic (OFE) Cu bar that was forged and rolled. Orientation imaging microscopy (OIM)[1–4] has been used to characterize the GBCD after each step in the processing. The fraction of special grain boundaries, “special fraction,” was ∼70% in the starting recrystallized material. Three different processing conditions were employed: high, moderate, and low temperature. The high-temperature process resulted in a reduction in the fraction of special grain boundaries while both of the lower temperature processes resulted in an increase in special fraction up to 85%. Further, the lower temperature processes resulted in average deviation angles from exact misorientation, for special boundaries, that were significantly smaller than observed from the high temperature process. Results indicate the importance of the low temperature part of the two-step strain-annealing process in preparing the microstructure for the higher temperature anneal and commensurate increase in the special fraction.

On the possible correlation between grain size distribution and distribution of CSL boundaries in polycrystals

This paper addresses the possible correlation between grain size distribution and the geometrical properties of grain boundaries. A series of functions are used to describe the grain size and the shape distribution, and considerations given to the population of grain edges and corners which result. Of particular interest are the dihedral angles at grain edges and how these are expected to vary with a change in the grain size distribution. In turn this is then correlated with fraction of special and general (random) grain boundaries. The concepts introduced here are then tested by looking at some microstructural and textural data from a grain growth experiment on high purity aluminum. In parallel with this a model of the distribution function of the grain boundary misorientation parameter is employed. The outcome of this series of investigations is the ability shown to establish a correlation between parameters describing the distribution of size and shape of the grains, and other parameters describing the fraction of grain edges which join boundaries having random and special character.

Effects of grain size and grain boundary structure on yield strength of micro- and submicrocrystalline TiAl

Mechanical properties of polycrystalline intermetallics strongly depend on their type of grain boundaries. An interesting example of the structure and properties being affected by grain boundaries is related to TiAl. By processing TiAl in certain special modes, it is possible to obtain structural states with practically the same grain size, but different content of random and special boundaries. Comparing the mechanical properties of the intermetallic compound in the above states has indicated that, with one and the same grain size, the growth of the fraction of special boundaries and, hence, the reduction of the fraction of random ones result in a considerable deterioration of the TiAl ductility above 600C and in an abrupt reduction of its strength below 750C. The latter effect is rather interesting, for it implies a question of whether TiAl obeys to the Hall-Petch equation, which is known to related yield strength to grain size. At the same time, the TiAl yield strength largely depends on the slip character which, in its turn, is determined by the grain size and grain boundary structure. As the grain size grows up to d[approximately]10 [mu]m and the number of special boundaries increases, a transition from very fine and homogeneous slipmore » to planar slip can be observed, which dramatically reduces the yield strength. This experimentally confirmed fact also causes doubt about the validity of the Hall-Petch relationship for TiAl.« less

Change in the crystallographic structure of grain boundaries in an order-disorder phase transition in Ni3Fe alloy

Electron diffraction microscopy and metallography are used to investigate the crystallographic structure of grain boundaries in Ni3Fe alloy with short-range and long-range atomic order. It is found that the fraction of special boundaries in alloys with short-range and long-range order is 0.3–0.4. Heat treatment, which leads to ordering, causes a reorientation of some of the grains with boundaries of a general type, boundary migration, and also faceting of some of the boundaries of general type.

Effect of grain boundary characteristics on mechanical behaviour of quenched aluminium

Tests concerning the mechanical properties of quenched aluminium were carried out on samples having different grain boundary characteristics. In comparison with aluminium of equilibrium structure, the quenched aluminium was found to be characterised by: a greater value of yield point in samples having a low fraction of special grain boundaries, and a lower value of yield point and a reduction of the Lüders strain in samples having a high fraction of special grain boundaries. The effects can be ascribed to the activity of dislocation loops as sources of dislocations or to a change in grain boundary characteristics as a result of annihilation of non-equilibrium vacancies on the grain boundaries.MST/965

Effect of grain boundary characteristics on mechanical behaviour of quenched aluminium

Tests concerning the mechanical properties of quenched aluminium were carried out on samples having different grain boundary characteristics. In comparison with aluminium of equilibrium structure, the quenched aluminium was found to be characterised by: a greater value of yield point in samples having a low fraction of special grain boundaries, and a lower value of yield point and a reduction of the Lüders strain in samples having a high fraction of special grain boundaries. The effects can be ascribed to the activity of dislocation loops as sources of dislocations or to a change in grain boundary characteristics as a result of annihilation of non-equilibrium vacancies on the grain boundaries.MST/965