David P. Field

A Review of Strain Analysis Using Electron Backscatter Diffraction

Since the automation of the electron backscatter diffraction (EBSD) technique, EBSD systems have become commonplace in microscopy facilities within materials science and geology research laboratories around the world. The acceptance of the technique is primarily due to the capability of EBSD to aid the research scientist in understanding the crystallographic aspects of microstructure. There has been considerable interest in using EBSD to quantify strain at the submicron scale. To apply EBSD to the characterization of strain, it is important to understand what is practically possible and the underlying assumptions and limitations. This work reviews the current state of technology in terms of strain analysis using EBSD. First, the effects of both elastic and plastic strain on individual EBSD patterns will be considered. Second, the use of EBSD maps for characterizing plastic strain will be explored. Both the potential of the technique and its limitations will be discussed along with the sensitivity of various calculation and mapping parameters.

Recent advances in the application of orientation imaging

Abstract Since the mid-1980s, electron back-scatter diffraction (EBSD, also known as back-scatter Kikuchi diffraction, BKD) has become a well-known and often used technique for interrogating the local characteristics of microstructures. The more recent development of orientation imaging microscopy (OIM) has led to the practical application of EBSD in obtaining statistically relevant information from bulk materials. Many new developments in OIM technology have evolved recently. One of these is the development of rapid and more reliable mapping of multi-phase alloys. in addition, significant work has been performed on thin film structures including patterned films and integrated circuits for investigation of texture evolution, grain growth, and circuit reliability. An additional example showing the application of OIM on rough surface specimens, such as fracture surfaces, is discussed.

Spall behavior of aluminum with varying microstructures

A series of plate-impact spall experiments was conducted to study the spall strength of seven microstructural conditions of aluminum, including three grain sizes of 6061 Al alloy, both ultrapure and commercially pure (1060) polycrystalline aluminum, and single-crystal Al with two different orientations, over the stress range of 4–22 GPa. The pullback velocity, which is a characteristic signature of spall strength, is observed to depend on initial microstructure, impact stress, pulse duration, and loading rate. The pullback velocity generally increases over the stress range of 4–14 GPa and achieves a maximum as the impact stress approaches 22 GPa. The pullback velocity of [100] single-crystal Al is higher than that for both polycrystalline samples and [111] single-crystal samples, indicating that grain orientation strongly affects material response. Experimental results also show that the spall behavior is strongly dependent on sample thickness, while the effect of shock pulse duration was observed to be l...

Rodrigues parameterization for orientation and misorientation distributions

Abstract The Rodrigues parameterization of rotations offers an intuitively simple space which enables visualization of the geometric configuration of orientations and misorientations. Recent efforts have concentrated on promoting the use of Rodrigues vectors for orientation parameterization. In the present work a procedure for determining the boundaries of the asymmetric domains for orientations and misorientations in Rodrigues space is given for all crystal symmetries, and the complete description of these zones is presented. The geometric properties of the Rodrigues space are described in detail and important relations useful in crystallographic texture analysis are derived. It is demonstrated that the special properties and geometry of the Rodrigues space facilitate analysis of texture function reproduction from pole figures and of texture evolution in plastic deformation of polycrystals.

Grain boundary misorientation angles and stress-induced voiding in oxide passivated copper interconnects

Grain boundary misorientations were determined by electron backscattering diffraction for tantalum-encapsulated, copper interconnects which contained thermal-stress-induced voids. The misorientation angles at voided and unvoided line segments were analyzed for two differently heat treated sample types, which were not equally susceptibile to stress voiding. Unvoided line segments contained a larger percentage of low misorientation angle, lower diffusivity boundaries than regions adjacent to voids. In addition, the more void resistant sample type also contained an overall higher proportion of low misorientation angle boundaries than the sample type which exhibited more voiding. The data provide further support for the importance of local variations in microstructure, which control the kinetics of stress void formation and growth.

The flow stress behavior of OFHC polycrystalline copper

Abstract The flow stress behavior of OFHC polycrystalline copper was evaluated using cold-rolled and equal channel angular extruded materials. Prior to tensile testing at room temperature, the specimens were heat treated to obtain grain sizes ranging from 3 to 60 μm. The flow stress, when correlated with the square root of true strain, is associated with four stages of hardening. These stages, in terms of increasing strain, are attributed to: (1) dislocation source activation, possibly at annealing twin boundaries during the onset of plastic flow, (2) dislocation slip and cross slip, (3) constriction of the screw dislocation partials for cross slip continuation, and (4) dislocation annihilation and saturation as interpreted through dynamic recovery. The tensile properties and analyses are supported by observations and measurements from orientation imaging and transmission electron microscopy.

Present State of Electron Backscatter Diffraction and Prospective Developments

Electron backscatter diffraction (EBSD), when employed as an additional characterization technique to a scanning electron microscope (SEM), enables individual grain orientations, local texture, point-to-point orientation correlations, and phase identification and distributions to be determined routinely on the surfaces of bulk polycrystals. The application has experienced rapid acceptance in metallurgical, materials, and geophysical laboratories within the past decade (Schwartz et al. 2000) due to the wide availability of SEMs, the ease of sample preparation from the bulk, the high speed of data acquisition, and the access to complementary information about the microstructure on a submicron scale. From the same specimen area, surface structure and morphology of the microstructure are characterized in great detail by the relief and orientation contrast in secondary and backscatter electron images, element distributions are accessed by energy dispersive spectroscopy (EDS), wavelength dispersive spectroscopy (WDS), or cathodoluminescence analysis, and the orientations of single grains and phases can now be determined, as a complement, by EBSD.

Recent studies of local texture and its influence on failure

Advances in techniques for measuring individual crystallographic orientations in polycrystals have made it possible to investigate the role of local crystallography on failure in polycrystalline materials in more detail than previously possible. In particular, the automated electron backscatter diffraction technique is particularly well suited for making spatially specific measurements of crystallographic orientation. This tool has made it practical to characterize the local distribution of orientations in polycrystalline microstructures with statistical significance. This work reviews the application of the automatic electron backscatter diffraction technique to the characterization of the various crystallographic parameters associated with failure. Several approaches for investigating the role of various crystallographic parameters on crack behavior in polycrystals are shown in the context of statistical distribution functions along with the use of the automated electron diffraction technique for acquiring the necessary orientation data.

Analysis of grain-boundary structure in Al–Cu interconnects

The role of crystallographic texture in electromigration resistance of interconnect lines is well documented. The presence of a strong (111) fiber texture results in a more reliable interconnect structure. It is also generally accepted that grain-boundary diffusion is the primary mechanism by which electromigration failures occur. It has been difficult to this point, however, to obtain statistically reliable information of grain-boundary structure in these materials as transmission electron microscopy investigations are limited by tedious specimen preparation and small, nonrepresentative, imaging regions. The present work focuses upon characterization of texture and grain-boundary structure of interconnect lines using orientation imaging microscopy, and particularly, upon the linewidth dependence of these measures. Conventionally processed Al–1%Cu lines were investigated to determine the affects of a postpatterning anneal on boundary structure as a function of linewidth. It was observed that texture tende...

Quantification of dislocation structure heterogeneity in deformed polycrystals by EBSD

Plastic deformation in polycrystalline materials involves a complex interaction of dislocations with defects in the lattice. The geometrically necessary component of the dislocation density can be quantified to some extent using data obtained from automated electron backscatter diffraction scans over planar regions or volumes using the three-dimensional imaging techniques that are currently available. Reliable measurements require that the step size of the orientation data used in determination of geometrically necessary dislocation densities be on the scale of the microstructural information. Measurements were performed in deformed Cu, Al and steel specimens. Geometrically necessary dislocation density in Cu deformed 10% in compression was about 15–30% of the overall estimated dislocation density. Measurements in Al demonstrate that three-dimensional estimates are on the order of 1.2–2 times the values obtained from 2D measurements on the same structures. Analysis of interstitial free steel specimens shows an increase in average geometrically necessary dislocation density by an order of magnitude for specimens deformed to 12% tensile deformation elongation.