Effect of Metallographic Pretreatment of TRIP Steel Specimens on Correlative Imaging and Electron Backscatter Diffraction Analysis

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Abstract
Steel is an indispensable material in many industries because of its favourable properties and low cost compared to other materials. New advanced high strength steels (AHSS) are continuously being developed and the demands on material structure characterization are becoming greater [ 1]. The mechanical properties of steels in general are highly variable and are very sensitive to even a small change in structure. There are efforts to obtain as much information as possible about a specific sample location, which requires the simultaneous use of multiple techniques. The requirements for each technique are often conflicting. In a light microscope (LOM), a mechanically polished surface is only suitable for the study of macroscopic defects or inclusions that have a significantly different light reflectance. The reason for the impossibility of observing the real structure of the material under investigation is the formation of a plastically deformed layer. The thickness of this layer is determined by the pretreatment of the metallographic sample. From the process of material cutting, when thermal and mechanical damage to the material occurs, to the final mechanical polishing. The thickness of the deformed layer after cutting with a precise metallographic saw is in the order of units to tens of μm [ 2]. The primary requirement of each processing step is to remove the damage caused by the previous processing step [ 3]. In LOM, chemical etching is generally used to highlight the structure and remove most of the deformed layer after mechanical polishing. The main motivation of this study is to demonstrate the real structure of the material, which is achieved by etching grain boundaries, their surfaces or by selectively etching different phases. The conventionally prepared sample still has a residual layer, which is not disturbing and cannot be seen by the LOM. For the high sensitivity of modern low energy scanning electron microscopy (SEM), mechanical polishing as the last polishing step becomes insufficient. For SEM techniques, including an electron backscatter diffraction (EBSD), electropolishing as the last step of the whole preparation is preferably used. The surface is too smooth and the structure is not distinguishable in the LOM. For high quality EBSD data, chemical-mechanical polishing and vibratory polishing are also recommended as the last preparation step [ 4]. After this step, the structure is not clearly resolved in the SEM or in the LOM. The EBSD technique is surface sensitive due to shallow information depth, say a few nm in the case of iron. Chemical etching is generally not recommended because the induced surface roughening becomes even more apparent after tilting the sample [ 5]. The challenge is to visualize the same area using different microscopy techniques, including the real structure of advanced high-strength transformation induced plasticity (TRIP) steel and verify the phases using the EBSD.

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