Automated Serial Sectioning as a Method to Map Inclusions in Structural Metals

Metallic and non-metallic inclusions have significant influence on the mechanical properties of materials. Quantitative characterization of inclusions for amount, size and shape distribution, chemical composition is of great interest for accurate determination of properties and aid in the quality control of materials manufacturing processes. Automated serial sectioning is a practical and direct method of characterizing material feature size and shape distributions accurately in statistically relevant volumes of materials. Two nickel- (Samples 1,2), two aluminum 6xxx series (Samples 3,4) and one steel (Sample 5) alloy sample were analyzed for this study. Samples were excised from actual parts, and were conventionally mounted in metallographic mounts for automated serial sectioning. Nickel and steel samples were analyzed for oxide/nitride/sulfide-based inclusions while AlFeSi intermetallics were studied in aluminum sample. In this study, we used Robo-Met.3D, an automated system that metallographically generates optical microstructural data for 3D reconstruction in solid materials, with correlated SEM-EDS analyses to confirm the chemical composition of inclusions. Binary images for 2D analysis were created by selecting pixel intensity threshold using Fiji/ImageJ, and were stacked and aligned using Fiji. Next, datasets with inclusions were reconstructed and visualized in 3D using Image-Pro Premier 3D software, version 9.3. The technique successfully produced 3D maps of inclusions. Automated immersion etching produced additional phase information. For the steel sample, an inclusion with elongated morphology matching two classifications as defined by ASTM E45 (Type A-Sulfides and Type C-Silicates) was identified in a white etching region volume. The feature was traced for over 400 μ and had caused an in-service failure. The automated serial sectioning technique is being extended to study graphite clusters and carbide inclusions in cast iron and bearing steels, and can be a useful methodology in metallography.