Abstract
A ferrite-pearlite two-phase steel was investigated using in situ scanning electron microscope (SEM) tensile testing combined with digital image correlation (DIC). Two different speckled patterns were used and compared. The first pattern was achieved by etching a polished surface in order to reveal the microstructural features. Second, a gold speckled pattern was obtained. Here, a continuous layer of gold was applied to a polished surface. This continuous layer was remodeled into gold nanoparticles by keeping the specimen at 180 °C for 96 h with an Ar/Styrene mixture flowing across the specimen surface. The result is randomly distributed gold nanoparticles on the surface. These particles and the etched microstructure were then used by the DIC software to correlate an image series to obtain the local strain field of the material. The differences between the two techniques are numerous. Considering the etched surface, most microstructural features were grain boundaries and pearlite lamellas. As a consequence, large areas within grains did not provide sufficient contrast for DIC, thus restricting maximum resolution. However, the technique is fast and does not expose the material to any elevated temperatures. In contrast, the gold remodeling method provides a finely dispersed gold speckle pattern on the surface, giving excellent contrast across the recorded area. DIC with gold particles achieved a spatial resolution of 0.096 µm, compared to 2.24 µm in the DIC for the etched specimen. As a result, DIC with gold speckles can resolve slip lines. Conversely, DIC with etched microstructure resolves local strains on grain level. However, it is less cumbersome and faster to perform the test on the etched specimen.
Highlights
The industry commonly uses steels with a ferritic-pearlitic microstructure
Based on the measured elongation and force during the in situ scanning electron microscope (SEM) tensile test on the etched specimen, the force–displacement curve shown in Figure 6 is plotted
The reason for the frames getting darker is that the incoming electrons from the SEM contaminates the specimen surface, while the white areas are due to the topography contrast nature of secondary electron (SE) imaging
Summary
The industry commonly uses steels with a ferritic-pearlitic microstructure. In such steels, the properties of the soft ferrite are combined with the harder pearlite to achieve a good combination of strength and ductility [1]. Knowing the heterogeneous strain field at the grain scale is an important tool in order to understand the relationship between the microstructure and the elastoplastic response. Elastic deformations are reversible, meaning that the material returns to its original shape when the applied loads are released. Plastic deformations are permanent, introducing non-reversible changes and damage to the material. Typically micro-void formation or micro-cracking is the dominating damaging mechanism leading to fracture [2]. A thorough understanding of these processes is important in the design of steel structures
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