Accurate electron channeling contrast analysis of a low angle sub-grain boundary

Abstract

a b s t r a c tHigh resolution selected area channeling pattern (HR-SACP) assisted accurate electron channeling contrast imaging (A-ECCI) was used to unambiguously characterize the structure of a low angle grain boundary in an interstitial-free-steel. The boundary dislocations were characterized using TEM-style contrast analysis. The boundary was determined to be tilt in nature with a misorientation angle of 0.13°consistent with the HR-SACP measurements. The results were verified using high accuracy electron backscatter diffraction (EBSD), confirming the approach as a discriminating tool for assessing low angle boundaries.The development and structure of low angle grain boundaries play critical roles in a wide range of materials processes and behaviors. Examples of where such boundaries are inexorably linked to behavior include the development of low energy dislocation structures during static and dynamic recovery, creation of dislocation cell structures during fatigue deformation, and many creep processes.In order to fully understand many of these phenomena, it is necessary to characterize low angle boundaries in terms of their misorientations and dislocation structure. In many cases such studies have been carried out using transmission electron microscopy (TEM) in conjunction with electron diffraction, which offers direct imaging combined with high accuracy orientation measurements [1-9]. Nevertheless, TEM is limited by the need to create, and the potential artifacts of, thin foils [10,11].Over the past two decades electron backscattered diffraction (EBSD) and associated orientation mapping has been used to rapidly map crystal orientations at the surfaces of bulk materials, allowing sub grain structures to be readily observed over large areas [12,13]. However, this approach is limited in that it does not allow direct imaging and characterization of the defects associated with low angle boundaries and, in most cases, is hampered by the inherent limits in precision of the Hough transform based approach to solving EBSD patterns [14][15][16].Electron channeling contrast imaging (ECCI) is a powerful technique for observing and characterizing crystallographic defect such as dislocations, stacking faults, and grain boundaries in scanning electron microscopy (SEM) [17][18][19][20][21][22][23]. In order to detect defects, ECCI uses the fact that the backscattered electron yield is very sensitive to the angle between the incident beam and the crystal lattice. ECC can be optimized by controlling the channeling conditions to so called two-beam conditions, allowing the characterization of crystallographic planes responsible for the channeling effect and facilitating TEM-style contrast analysis [17,24,25].A novel approach to control the channeling conditions for performing ECCI with an accuracy of 0.04°has recently been developed. This approach is based on an innovative procedure for collecting high angular and spatial resolution (about 500 nm) selected area channeling patterns (HR-SACPs) on the GEMINI-type electron column [...

A new characterization approach for studying relationships between microstructure and creep damage mechanisms of uranium dioxide

“…6 gives an example of a SEM image acquired in electrons channeling conditions (ECCI mode), on the crept sample from LG1 batch, according to the method described in Refs. [19,21,22]. On this image, small clear lines can be clearly distinguished within the grains: these lines reveal free dislocations (pointed by black arrows), as well as sub-grain boundaries (pointed by white arrows).…”

“…The sample deformed up to 8% (mean value) from the batch LG1 also underwent additional examinations by electron channeling contrast imaging (ECCI), using the methodology described by Mansour et al, which is based on a suitable orientation of the grain in order to be in two-beam conditions, and highlight the contrast associated with the presence of crystallographic defects [21,22].…”

Toward Predictive Understanding of Fatigue Crack Nucleation in Ni-Based Superalloys

“…To address this problem, ECCI is a promising technique to be used as an alternative dislocation characterization method to determine the total dislocation density. 32,33 The initial dislocation density was low and homogeneously distributed through the microstructure, and the presence of residual stress gradients was low. 31 After two cycles, the GND density and residual stress gradient ''hot spot'' (red) maps are shown in Fig.…”

Influence of strain conditions on the grain sub-structuration in crept uranium dioxide pellets

“…It is based on an optimized protocol for EBSD data acquisition and processing with the potential to detect and quantify low angle sub-boundaries disoriented down to 0.1 in UO 2 . In addition, Accurate-ECCI experiments were also performed to image the dislocations induced by creep and their arrangement in very low sub-grain boundaries [22,23]. Moreover, an advanced sub-grain completion procedure was applied to the EBSD data to quantify the sub-grain size evolution [24].…”