Large-Area EBSD Mapping 

Additive manufacturing (AM) technology is widely used in many industries, such as aerospace and medical, for its capability to produce complex 3D objects from digital models.

In this example, a Powder Bed Fusion in Selective Laser Melting process was used to produce austenitic stainless steel for implant technology. Selective Laser Melting SLM125 with a single 400W fiber laser was performed. Then, the sample was deformed according to DIN 50125 for tensile tests.

Investigating the spatial variations of twin boundaries and grain sizes within the materials allows its strength and ductility to be controlled. For this, EBSD is an essential analytical technique which can be used to study the evolution of microstructure in metals and alloys.

Here, we perform a very large-area EBSD mapping after conventional tensile testing: in one single setup, the texture evolution is measured across the entire sample using an automatic sequential measurement of 500 maps of 240 million points in total. At the end of the measurement, data was automatically saved, the EBSD detector was automatically retracted and the beam switched off.

The 500 EBSD maps were subsequently stitched in only 12 minutes using ESPRIT, Bruker's data acquisition and processing software.

The optimum procedure for successful large-area EBSD mapping requires a planar sample with the signal setup where the EBSD pattern signal is the poorest, i.e. the area with the strongest deformation.

If the sample is not perfectly planar and there is no possibility to compensate the loss of focus during measurement, we suggest to setup the EBSD signal where it is poorest and at a medium topography. This method still works elsewhere when using our system for EBSD, QUANTAX EBSD, because it can index slightly saturated patterns.

The deformation mechanisms in austenite by slip and twinning are clearly visible in the ARGUS™ orientation contrast image (Figure 1) and in the EBSD orientation maps (Figure 2 and 3). In Figure 2, an abrupt change in the grain preferential orientation is visible (from green to blue in IPFx): a  certain value of the strain is reached that induces the maximum rotation angle for the specific initial orientation.¹ The same tendancy is reported in deformed austenite for a strain of 14% during uniaxial tensile loading.²

If we zoom on the microstructure (Figure 3) we can see the boundaries distribution of a typical deformed austenite² where Low Angle Boundaries (LAB) start to form in the vicinity of grain boundaries and grow inside the grains. The amount of LABs is high due to the increased strain and as the strain increases, LABs will develop into High Angle Boundaries (HABs). Here, most HAB are Twin Boundaries (TB). Twin Boundaries are formed by shear mechanisms and can either be present in the microstructure before deformation or are deformation-induced. 

[1]: Wansong Li et. al, Materials Characterization 163 (2020) 110282

[2]: K. Yvell et. al, Materials Characterization 135 (2018) 228-237

Sample courtesy of Dr.-Ing. Kristina Roder, Research assistant, Chair of Lightweight Structures and Plastics Processing, Technical University of Chemnitz (Germany).