Characterization of Ultra-fine Grained and Nanocrystalline Materials Using Transmission Kikuchi Diffraction

This paper provides a detailed method to characterize the microstructure of ultra-fine grained and nanocrystalline materials using a scanning electron microscope equipped with a standard electron backscatter diffraction system. Metal alloys and minerals presenting refined microstructures are analyzed using this technique, showing the diversity of its possible applications.

Transcript

The overall goal of this technique is to characterize the microstructure of crystalline materials presenting submicron size grains or that have been subjected to large plastic deformation by using the principle of electron defraction within a scanning electron microscope. This technique can help answer questions in the field of severe plastic deformation and can help determining mechanisms at play during eye strain deformation. The main advantage of this technique is that it can be used on a wide range of materials including those materials that are not conductive which would be particularly difficult with conventional electron back scatter defraction.

Use a scanning electron microscope or SEM equipped with an EBSD detector to carry out the experiment. After verifying that the specimen is thin enough for TKD analysis, place it on a specimen holder that allows it to be at 20 degrees from the horizontal once inside the SEM chamber to avoid shadowing effects during data acquisition with the EBSD camera. The most important aspect of this technique is the sample preparation.

If the sample is too thick, there will be insufficient electron transmission and the defraction patterns will be of poor quality or nonexistent. In these cases, the sample will need to be prepared again. Place the sample holder in the SEM chamber and close the chamber.

Then, start the vacuum pumping by clicking on pump in the vacuum tab. Next, tilt the SEM stage by 20 degrees clockwise, such that the specimen is now at a horizontal position and normal to the electron beam. For optimum data acquisition, set the accelerating voltage at 30 kiloelectron Volts by clicking EHT.

Select EHT on to turn on the accelerating voltage. Now, click on the aperture tab of the SEM control panel and choose a high aperture. Then, choose the high current mode.

Now, move the stage to locate the specimen and verify that the beam is hitting the specimen at the position of interest on the specimen using secondary electron imaging. Make sure that the sample holder is parallel to the x axis of the stage to avoid any possible damage to the equipment while moving the specimen and to get the optimum signal. Following this, bring the specimen at a working distance of 6 to 6.5 millimeters by changing the z position of the sample.

Turn on the EBSD software and insert the EBSD camera by entering the distance the camera needs to move so that it is at a distance of 15 to 20 millimeters from the specimen and then press the move in button. If required for the analysis, insert the EDS detector within the chamber by clicking the In button on the EDS camera control panel. To determine the optimal position for the experimental setup, check the signal count and ensure that the dead time is between 20%and 50%for optimum data collection.

Once all the detectors are positioned and the specimen has been located, perform beam alignment by selecting the focus wobble check box in the aperture tab of the SEM control panel and adjusting the horizontal and vertical knobs for the aperture alignment on the control board. Then, perform focus adjustment in the astigmatism correction by adjusting the horizontal and vertical knobs for the stigmation on the control board. Make sure that the specimen geometry in the EBSD software reflects the fact that the specimen is in the horizontal position.

Ensure that the total tilt value is zero degrees and if not, add a 20 degrees pretilt value in the specimen geometry tab. Select the phases to be analyzed as for a normal EBSD experiment in the phase tab. Following this, capture an image using the EBSD software in the scan image tab by clicking on start.

In the optimization tab, adjust the settings of the EBSD camera for an optimal data acquisition by optimizing the gain and exposure values until the images is bright but not oversaturated. Next, collect the background in the optimization pattern tab by clicking on collect. Ensure that enough grains are present for the background collection by adjusting the magnification.

Although it is important to scan across a region with similar thickness to the area to be analyzed. Check the quality of the patterns once the background has been subtracted by ensuring that the static background and auto background options have been checked. Although they will look distorted due to the special geometry of the setup, ensure that defraction bands are clearly visible.

Integrate successive frames in the EBSD camera to improve the signal to noise ratio in the image since the luminous intensity of the defraction pattern on the phosphor screen is low. Now, optimize the solver for pattern recognition and to improve the indexing rate by going to the optimize solver tab. After adjusting the focus and correcting the astigmatism of the SEM and capturing a new image in scan image, set the parameters for map acquisition in the acquire map data tab.

Finally, start the map acquisition by pressing the start button in the acquire map data tab. Microstructure characterization using TKD demonstrates that subjecting a stainless steel specimen to surface mechanical attrition treatment, or SMAT created a region of equiaxed nanograins and slightly elongated nanograins. Below the first region, an ultrafine grained region of elongated submicron sized grains can be seen.

Another stainless steel specimen subjected to SMAT was analyzed using traditional EBSD. Both the band contrast and inverse pull figure map show the presence of an ultrafine grained region at the surface, but the level of indexing is not as good as with TKD and the microstructure just below the surface that has been subjected to SMAT has not been properly characterized due to the lower spatial resolution of normal EBSD. TKD characterization of a cobalt chromium molybdenum alloy sample subjected to SMAT shows that a refinement in the microstructure took place via phase transformation.

After deformation, hexagonal close packed lathes are seen inside the phase centered cubic grains. TKD analysis of iron nickel sulfide inclusions and a polycrystalline diamond aggregate revealed the distribution of the different phases in the specimen and showed the nanostructures of the magnetite. By coupling TKD with EDS, the distribution of the different elements within the different phases was determined.

An impact diamond characterized by TKD is shown here. The plastic deformation seen by the specimen explains the presence of submicron sized grains, a high proportion of twins and gradients of crystallographic orientations within the grains. When attempting this procedure, it's important to remember that sample preparation is paramount to the success of the experiment.

After watching this video, you should have a good understanding of how to set up your sample, EBSD camera and the EDS detector to successfully perform a TKD experiment. The data analysis is exactly the same as for traditional EBSD scans.

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Journal (Engineering)

Journal (Engineering)

Characterization of Ultra-fine Grained and Nanocrystalline Materials Using Transmission Kikuchi Diffraction

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