Scanning Electron Microscopy - Gbhenterprises.com

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Test M604: Scanning electron microscopy Fig. 14: Interrelationship between depth of focus, point resolution, and magnification: Light-optical microscope and scanning electron microscope. 2.5 Fractographic analysis Any fracture of a body starts with the formation and propagation of cracks in submicroscopic, microscopic, and eventually macroscopic dimensions. The structure of the fracture surface varies depending on the composition and microstructure of the material in question as well as on other conditions given during the process of breaking, such as temperature and stress state. Thus an analysis of the fracture surface can provide essential information on the cause of fracture. 2.5.1 Transgranular and intercrystalline fracture Metals are composed of a multitude of small crystallites formed when the melt is cooling down. Atoms are very regularly arranged in the crystallites. At the boundary between two crystallites the order of the crystal lattice is disarranged. These crystal boundaries show two-dimensional lattice defects. As the atoms at the crystal boundaries are not in an equilibrium state, the crystal boundaries in engineering materials are in general of higher strength than those of regular crystallites. They form a barrier to the propagation of small cracks so that - at room temperature and at lower temperatures - cracks normally run through the grains. This process is referred to as transgranular fracture (Figs. 15 a and c). 12
Test M604: Scanning electron microscopy Fig. 15: a) Transgranular cleavage fracture, b) intercrystalline cleavage fracture c) dimple fracture (transgranular), d) fatigue fracture Various types of separation occur in brittle and tough material. In the case of transgranular brittle fracture, crystallites are split without deformation (Fig. 15a). If the material is tough, sliding processes occur in crystallographically preferred planes; microvoids and cavities form themselves. The cavities widen, any metal remaining in between propagates and narrow edges are formed. The resulting microstructure is called dimple fracture, see Fig.15c). Cyclic straining (cf. Test M512) leads to transgranular cracks showing fracture paths and fatigue striation (Fig. 15d). At higher temperatures atoms move more easily, and the strength of crystal boundaries is reduced. The path of fractures that have occurred after a long time of load at high temperatures runs along crystal boundaries. Such fractures are referred to as intercrystalline fractures (Fig. 15b); they do not occur at room temperature unless crystal boundaries have been weakened or embrittled due to precipitation or impurities. In particular the influence of hydrogen can also lead to intercrystalline fractures. 3 Technological significance 3.1 Assessment of damage As has been mentioned in the introduction, scanning electron microscopy is essential to an assessment of causes of damage due to fracture. Microscopic analysis has made it possible to distinguish between material defects and processing defects. Thus, considerable legal consequences may result with regard to liability for damage. Slag inclusion in welding seams, e.g., cannot be clearly identified using a light-optical microscope whereas, owing to the fact that the conductivity of metal differs considerably from that of slag, 13
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