Optical microscope structural examination
Successful structure contrasting is followed by the examination of the polished specimen under the optical microscope. Here the object being examined is magnified in several stages using a lens system. The magnification of the lens and the eyepiece are multiplied for the total magnification.
With optical microscope examination, magnification of up to 1000 : 1 is common. When an immersion oil is used that is applied between the polished surface and lens, a maximum magnification of 1500 : 1 is possible. The smallest object size that can be differentiated – the lateral resolution – is determined by the wavelength of the light and the numerical aperture of the chosen lens. It lies in the range of 0.2 to 0.5 μm.
To make smaller objects visible, topographic photos of the fracture areas are required or one needs additional information about the local chemical composition of the specimens. This is where scanning electron microscopy (SEM) or transmission electron microscopy (TEM) is used.
Metallographic polished sections are examined using incident light mode under the optical microscope, since the specimens are impermeable for visible light. Figure 6 shows the four most common microscopy techniques for the structural examination of micrographs using a Ti-Al-V alloy as an example. Figure 7 explains in more detail the mechanisms of action for the optical microscopy working techniques illustrated in Figure 6.