Analysis of microstructural strain-fields in grey cast iron

Abstract

With increasing engine capacity the demand on the material increases. To meet the requirements new more advanced test methods are invented. One of these test methods is the digital image correlation technique (DIC). This method analyses deformations in materials by correlating the displacement in a set of sequentially acquired images. To be able to measure the deformations the material needs to have a distinct pattern. Some materials have a “natural” pattern, e.g. the etched microstructure of grey cast iron, other materials needs to get an artificial pattern which is done by spraying with a substance that disperses onto the surface and results in different contrasts. A new way to utilize this technique is to use a microscope viewing the surface, and collect images and analyze them with a DIC software. This gives information about the local strain-field of the material. In metals different patterns can be achieved depending on what etchant is used. This means that it is possible to differentiate local strains between different phases of the studied material. For a gray cast iron etched with alkaline sodium picrate (a color etchant which highlights eutectic cells) it is possible to detect local strains in and around these cells. Analyzes showed that the highest strains could be found in the graphite followed by pearlite. For a microstructure with revealed eutectic cells the highest strains could be seen inside the eutectic cells for larger cells. Materials with large cells showed worse global mechanical properties than materials with small cells. The difficulties with the DIC-method is that it is sensitive to contrasts and since contrast are affected by e.g. vibrations, shiny surfaces and surfaces with small contrast difference it can be hard to get quantitative answers. Digital image correlations techniques have shown to be an instrument of the future in analyzing microscopic strains. By knowing how the different phases accommodate strains and how changes in the microstructure, due to melt treatment and alloying, affect the strain-field the DIC-method can be used as a powerful tool in future material modeling.