3D-EBSD: Cutting-edge technology to understand metallic microstructures in 3D

The microstructure of metallic materials plays a critical role in determining their performance during use. Throughout processing, the size and shape of the grains within the microstructure evolve, and careful monitoring is essential to identify any problematic steps that may require adjustment. Key processing parameters—such as temperature, time, and applied force—can be customised to alter the microstructure and improve the material's properties.

How do you prefer to study the grains of a microstructure?

• EBSD
• 3D-EBSD

Electron Backscattered Diffraction (EBSD) is a common technique to evaluate the orientations and shapes of the grains that compose the metallic microstructures. It is useful to have a wide vision of how the grains are oriented along the sample, which provides hints of how the material will behave in service. While a single slice of EBSD data provides valuable information, it doesn't offer the full picture of grain interactions in three dimensions. To study all the neighbouring grains that surround a specific grain and understand the interactions between grains, EBSD maps at one slice are not enough.

EBSD map showing the grains at the cross-section of a metallic sheet:

The last-generation microscopes can ablate the sample surface to obtain flat surfaces with extraordinary quality that contribute to excellent images and EBSD maps. These microscopes called tri-beam laser-FIB-SEM, can be set to sequentially remove slices from the surface of the sample, and if they are coupled with an EBSD detector, they can measure an EBSD map at each slice. Finally, all the maps collected can be merged to provide 3D information of the microstructure.

This is the chamber of one of these last-generation laser-FIB microscopes, showing the laser removing one slice of material:

The novelty of this method is that we can capture in the same experiment images and EBSD maps of numerous material slices with only 1 μm of separation between them. When the experiment is finished, we can merge the images and EBSD maps of each slice and create 3D volumes that contain 3D information about the grains and their specific location along the three dimensions of the sample. One powerful application is the study of the interaction of grains with their neighbours and how they affect different properties of the material.

This is an example of a 3D-EBSD where some grains can be isolated and studied apart:

With these volumes, we can threshold grains with special characteristics. For example, grains below a certain size or with specific orientations. By isolating them, we can study if these grains of interest are similar to the grains around and form clusters or, on the contrary, they are found completely dispersed in the microstructure.

Use of a 3D-EBSD on a Crystal Plasticity Model

3D-EBSD not only allows for a deeper understanding of grain interactions but also provides critical data needed for advanced modelling techniques like Crystal Plasticity simulations. To study the interaction between grains, these 3D volumes are extremely useful to feed models with 3D realistic data and simulate how the strain evolves under different load cases and load directions. For example, this is the simulation of how the strain is localised at certain grains or between specific grains of the 3D-EBSD when the volume is stretched along the x-axis.

Crystal Plasticity simulations use real 3D data to simulate how the grains deform under load, identifying weaker and stronger combinations of grains. High levels of strain localisation marked in red in the strain maps indicate where material failure is more likely to begin. To prevent or delay fractures in metallic parts, it is essential to avoid grain characteristics that concentrate strain.

Let's play with a 3D microstructure

The combination of Crystal Plasticity Modelling and 3D-EBSD is a good strategy to find undesirable grain combinations that promote strong strain localisation. Looking at this interactive 3D-EBSD and the corresponding strain map after running a stretching simulation, can you identify where the strain tends to localise?

Feel free to rotate the volume and zoom in for a closer look at the details.

Click on the strain button to visualise the strain localisation and consider how it relates to the original 3D-EBSD grains.

What do you think?

• Does the higher strain localise between grain boundaries or within the grains?

• Do you think the shape and size of the grains are affecting strain localisation?

• What about the grain orientations coloured differently in the 3D-EBSD map?