Macroscopic properties of materials can be significantly improved by gaining a better insight into the nature of a material at the microscopic level.
The formability and strength of a material are defined by the way it reacts to an external force, and also the way it deforms plastically when the force is sufficiently high. Magnesium and aluminum alloys, for instance, have high strength-to-weight ratios, but do not have the desired formability needed to develop them into useful products easily and cost-effectively.
At the microscopic scale, generation and movement of dislocations either through the material or crystal twinning events can be used to accommodate the plastic deformation. These deformation mechanisms can be detected by studying the microstructure through orientation imaging microscopy (OIM) and electron backscatter diffraction (EBSD) techniques.
Electron Backscatter Diffraction and Nanoindentation
EBSD is an important characterization method used for studying the effects of orientation on deformation behavior. It is used to determine the crystallographic orientation, which plays a major role in both these deformation modes.
Dislocation motion generally occurs along particular crystallographic directions and planes, known as slip systems. Material’s ductility can be further improved by activating more slip systems. In addition, by controlling grain size, the material’s yield strength can also be controlled.
Nanoindentation is an indentation hardness testing method, where the volume of material sampled and the size of the indenter are sufficiently small, so that local differences in microstructure and grain orientation can be examined thoroughly.
In this example, a Hysitron PI 87 SEM PicoIndenter was utilized to place a range of indents inside the microstructure of an Inconel 600 nickel-based superalloy developed for EBSD analysis.
EBSD orientation maps, obtained from a series of indents placed with loads ranging between 5 mN and 35 mN, is shown in Figure 1. These maps were collected using the EDAX TEAM EBSD Analysis System. These indents were then placed in the inner part of a grain. When the load increases, the size of the indent as well as the adjacent plastic deformation also increases.
In EBSD maps, the slight changes in color match with the small changes of misorientation which is introduced during the plastic deformation, and this deformation in turn is caused by indentation. Also, the grain orientation effect can be easily seen. For the (100) oriented grains, the deformation occurs along the side of the indent, and for the (111) oriented grains, the deformation occurs close to the corners of the indents.
Figure 1. Orientation maps from a set of indents placed with loads varying from 5 mN to 35 mN.
Results and Discussion
Using the EBSD image quality (IQ) maps (Figure 2), the spatial relationship between the crystal orientations and indenter geometry can also be observed. In these images, the spatial positioning of the active slip systems are indicated by the deformation slip bands. The related crystallographic data can be obtained by physically drawing along the trace of these slip bands. Multiple slip systems are active for the 25 mN load, indicting why there is a greater degree of deformation with respect to the 35 mN load indent. These changes in orientations and related slip systems help in showing the variation in deformation behavior.
Figure 2. EBSD map of laser crystallised SiGe thin film with Grain Aspect Ratio scalar texture plot (Sample courtesy from Balaji Rangarajan).
To this end, several measurements methods have been devised to determine and visualize the plastic strain fields, which tend to occur in materials relevant to this type of study (Wright 2011). Figure 3 shows one of these metrics, the Grain Reference Orientation Deviation (GROD) -Angle map, for the series of indents. In these maps, when the thermal intensity of the map coloring scheme increases, the plastic strain also increases. The TEAM software includes the orientation precision performance feature, which enables a clear resolution of the deformation fields inside the microstructure.
Figure 3. EBSD map with interactive grain highlighting results pane and contextual options window.
The slip transfer across grain boundaries generally relies on the orientation relationship between the two adjoining grains, and this association will ascertain whether areas of damage will suppress or nucleate (Bieler 2014). By integrating nanoindentation and EBSD techniques, the misorientation relationship between grains can be detected before selecting the regions for nanoindentation, and the related deformation distribution can be studied thoroughly. An example of this is illustrated in Figure 4.
Figure 4. The misorientation relationship between grains is identified before nanoindentation and the corresponding deformation distribution analyzed.
EDAX OIM Analysis™ Software
In the EDAX OIM Analysis™ Software, the EBSD data can be put to significant use. For instance, the entire representations of a measurement point are joined together and can be highlighted in all the displayed charts, maps and plots for sophisticated correlative examination.
Users can choose sub-sections of their data based on a number of properties such as scalar values, orientation, or grain properties to isolate particular components of their dataset. In case a portion of the microstructure is chosen, that data can be extracted for additional study.
The partition properties are powerful features as they enable users to produce subsets of a whole dataset for complete analysis. For instance, a Confidence Index filter (CI>0.1) can be applied to get rid of suspect indexed points. Besides single parameter filters, the formula tab includes Boolean and mathematical operators that enable the combination of grain and multiple point parameters to accurately define a part of a dataset for individual examination.
In the event that filter-based partitioning are not viable, subsets can be generated using the highlighting functionality. Once a highlight is produced, users can right-click in the map and choose “send points to" to create the subset. “Apply colors as highlight" is another highlighting function through which users can easily color-code distinct plots with the coloring utilized for the “source" map.
It can be seen in Figure 5 that the grain orientation spread value is emphasized in an IPF plot. The green background denotes the orientation difference due to lattice bending of the deformed grains, and the blue points represent the distinct recrystallized grains. In order to measure the occurrence of a scalar value as a virtue of orientation, users can use the scalar texture function. The grain aspect ratio as a virtue of orientation is shown in Figure 6.
Figure 5. Grain orientation spread map of partially recrystallised steel with corresponding colour-highlighted IPF plot.
Figure 6. EBSD map of laser crystallised SiGe thin film with Grain Aspect Ratio scalar texture plot (Sample courtesy from Balaji Rangarajan).
The green zones at the bottom and top of the pole figure reveal that highest grain elongation take place along the  axis within ~30 degrees to the map lines.
Furthermore, by means of the highlighting function, quantitative data can achieved. This function shows the selected data in any plot, chart and map, and also enables users to record data related to grain and orientation for the grain or point chosen. Upon right-clicking in the interactive tab, a range of properties is displayed which may be exported and recorded, if required (Figure 7).
Figure 7. EBSD map with interactive grain highlighting results pane and contextual options window.
If it is not feasible to highlight all grains to achieve specific grain properties, the partition contextual menu offers several grain property export options. However, if the visualization and quantification options in the OIM Analysis™ Software are not adequate, the software provides direct access to almost all the data, so that users can process on their own. This data can be accessed using the range of export functions available in the partition, dataset or map contextual menus, as shown in Figure 8. Generally, users can right-click in the OIM Analysis™ Software when they require something special.
Figure 8. Export data options from map and partition contextual menus.
Electron Backscatter Diffraction (EBSD) is a microstructural-crystallographic technique used to study the crystallographic orientation of many materials. This technique has become tantamount with appealing color images, such as grain size, IPF, image quality, and other types of maps, which include significant amount of data. These maps, however, do not reveal all the data that can be obtained from an EBSD dataset.
The combination of nanoindentation and EBSD techniques provides a suitable means to measure and correlate the material’s mechanical response with the crystallographic orientation. This approach not only helps in developing materials with enhanced properties, but also extends their lifetime for a wide variety of applications.
This information has been sourced, reviewed and adapted from materials provided by Edax Inc.
For more information on this source, please visit EDAX Inc.