Metallic Grain Structures and Their Microscopic Analysis

Fig. 9: Ferritic steel with titanium carbide and oxide inclusions following mechanical preparation to 1 µm diamond. Fine traces of deformations can still be observed in the differential interference contrast image. The sample has not been etched.

The sample should only be etched after this step. The etching process enhances contrasts that are invisible or only visible to a limited extent under brightfield conditions and allows the microstructure to be examined with a high degree of accuracy. Typically, only weak acids are used for this step. Weak nitric acids with an alcohol content of 1–3 % (nital) are very often used for unalloyed and low-alloyed carbon steel (cf. Figs. 2–5). Corrosion-resistant steel requires special etching processes, such as color etching, which influences the interference of the microscope’s light on the surface, or electrolytic etching (Figs. 10 and 11).

Fig. 10: Corrosion-resistant austenitic steel after final polishing with OP-S and subsequent Lichtenegger and Bloech color etching. Austenite grains with twins and ghost lines in the direction of deformation become visible.

Fig. 11: Corrosion-resistant austenitic-ferritic steel (duplex) following electrolytic etching in 20 % sodium hydroxide solution. The austenite grains (light brown) are embedded in the bluish-brown ferritic matrix.

Please note: The desired structures only become visible when there is an ideally coordinated interplay between the preparation technique, the appropriate chemical or electrochemical etching process and the microscopic contrasting technique (brightfield, DIC, polarization) as well as the desired resolution (of the smallest discernible feature).

Which Contrasting Techniques Can We Now Use to Characterize the Structural Properties of Metals?

The option of using different contrasts on a light microscope enhances the analysis of metallic structures. This type of contrasting, which can take place either on its own or in conjunction with preceding chemical or electrolytic contrasting procedures, is also referred to as “optical etching.” The following section discusses the most common contrasting techniques used for routine analysis of structures and provides examples of several typical applications.

Table 1: Examples of applying contrasting techniques for examining metallic structures

The brightfield is a standard technique for all types of material analyses. Fissures and pores, nonmetal phases and oxidation products are first observed in an unetched condition, as they typically exhibit different reflective behavior than the base metal material. The location of fissures and pores in relation to other structural characteristics, on the other hand, can typically only be evaluated if appropriate chemical etching has been carried out (Fig. 12).

Fig. 12: Laser welding seam on high-alloy steels with fissures and pores following electrolytic etching. These are also visible in the unetched condition, but the intercrystalline course of the fissures can only be evaluated after etching has been completed.

The darkfield technique is used less frequently in metallurgical microscopy than it is in the microscopy of nonmetal materials. However, this contrasting technique also offers several advantages for metals, as well as when evaluating colored structures such as layers of lacquer or plastic coatings on metal substrates. This contrast can also be used to evaluate corrosion products (Fig. 13). Darkfield microscopy can be used to show very fine scratches on polished samples as a method of examining the grinding quality.

Fig. 13: Corroded area on a brass pipe, unetched. Reflective areas appear dark (metal substrate) under the darkfield microscope, while the corrosion products can be observed in their own color.

The differential interference contrast is a useful tool for analyzing very fine deformations that may still be present in the surface after polishing (cf. Fig. 9). Checking the quality of polishing prior to the etching process can prevent additional effort being required during preparation. DIC can also be used to distinguish hard and soft structural elements, as hard phases are removed to a lesser extent than softer ones during the final polishing process. They therefore “protrude” from the surface (Figs. 14 and 15).

Fig. 14: Copper alloy after final polishing. Due to their reflectivity, the various phases appear to have different colors under a brightfield microscope.

Fig. 15: Copper alloy like that in Fig. 14. Due to their ablation behavior, the phases of varying hardnesses have varying heights, which are only visible in DIC microscopy. This enables a qualitative distinction between their hardnesses. At the same time, the grain structure can already be made visible in the unetched condition.

This minimal difference is not typically visible under a brightfield microscope, but can be seen in DIC. As such, this contrast can be used to make a qualitative distinction between the hardness of different phases. A further key advantage is the possibility of making grain structures such as grain boundaries visible even in an unetched condition (Fig. 15). The structure can therefore be evaluated prior to etching, so chemicals that pose a risk to health do not need to be used on materials that are difficult to etch (corrosion-resistant materials). However, a perfect final polish is required in this case.

The polarization contrast is primarily used in the analysis of materials with a hexagonal lattice structure. This includes titanium, zinc and magnesium in particular, as well as a range of other materials (Fig. 16). However, aluminum and its alloys can also be analyzed under polarized light after being etched appropriately. This requires electrolytic etching with tetrafluoroboric acid, which is known as Barker etching (Fig 17).

Fig. 16: Technically pure titanium (Grade 1) following mechanical polishing, seen under a polarization contrast microscope. Unetched surface. The polarized light is enhanced or eliminated on the crystal faces due to the hexagonal lattice structure, which manifests itself as a contrast between light and dark. The image appears in color due to a so-called λ/4 plate.

Fig. 17: Aluminum welding seam following electrolytic etching with tetrafluoroboric acid (Barker etching), seen under a polarization contrast microscope. The etching creates a layer of oxide of varying thickness depending on the orientation of the crystals; the polarized light can interfere in this oxide layer. This also results in elimination and enhancement effects.

Fluorescence is a further contrasting technique that can be used in metal and material microscopy. This method exploits the fact that certain materials that are excited at a certain wavelength emit visible light at another wavelength. These f luorescent powders (e.g., Epodye) are mixed with the embedding agent (typically transparent epoxy resin) during the embedding process and, together with it, penetrate existing and open pores and fissures. This procedure is supported by vacuum impregnation. Following curing and preparation, the microscope’s light in the blue spectrum excites the fluorescent dye, which then emits light in the yellow-green spectrum. The filled pores or fissures are illuminated in yellow-green (Figs. 18 and 19).

Fig. 18: Pore and fissure between a WSC coating and the steel to which it is applied. This is illuminated in yellow-green in the corresponding microscope contrast because the fissure was penetrated by embedding agent with fluorescent powder. The fissure was therefore present prior to embedding and may have arisen during fabrication; however, it could theoretically also have occurred during the cutting process and be a preparation artifact.

Fig. 19: Fissures in a carbon fiber composite material

The Light Microscope Equipment

Reflected-light microscopes are used to visualize structures in the range from mm to approx. 1 μm (10x/20x/50x/100x objectives are typical for the material microscope. Fig. 20). The design can be either upright or inverted, which is advantageous for large specimens or when viewing clamped specimens in specimen holders. High-definition stereo microscopes, on the other hand, are used less frequently for structural interpretations. The image generated by the microscope must be rendered faithfully and be suitable for integration into current documentation systems.

Because the illumination and contrasting of the reflective samples is performed by the imaging optics – the microscope objective – special requirements apply to the design and light guide. Furthermore, the objective must exhibit precise field f lattening characteristics for object measurement. These types of objectives, which are specially optimized for reflected light specimens, can be recognized by the abbreviation “EPI” (e.g.,the ZEISS 50x EC EPIPLAN # 422070-9961-000).

Fig. 20: Versions of light microscopes for structural analyses. In addition to inverted and upright material microscopes (such as the A ZEISS Axiovert.A1 MAT and ZEISS AxioLab.A1 MAT), digital microscopes such as the ZEISS Smartzoom 5 are increasingly becoming an alternative.

Among other things, special antireflection coatings reduce the reflections from the object and the optical calculation does not provide for a cover glass. The digital camera requirements for metallic samples must be optimized for measuring and documentation purposes. A highly dynamic imaging camera chip displays metal surfaces and their high levels of contrast in an ideal way. Special industrial software packages (such as “ZEN core” and its material modules) are available to provide assistance in using a camera of this type (for example, the ZEISS Axiocam 305) in practical settings. Microscopes for beginners, such as the “ZEISS Primotech,” include the most common contrasting techniques and can also be used with simplified ”MATSCOPE” software and tablet solutions (”MATSCOPE” or iPads).

The rapid optical development of the “digital microscope” is also making it an increasingly interesting tool for structural analyses. These devices are becoming increasingly significant thanks to their ease of use and their combination of the advantages of stereomicroscopy with those of reflected light microscopy.

They therefore also cover a relatively broad magnification and application range. In particular, reflected-light microscopes do not offer the same opportunity to digitally post-process images with a wide range of measuring tasks. However, digital microscopes are unable to keep pace with the good resolution offered by reflected-light microscopes, which is a drawback when working with very small structural elements (cf. also Figs. 3 and 4).

Please note: Special optics must be used, as the excitation and object lighting are directed through the same optical system. These are denoted by the abbreviation “EPI.” Cameras must be capable of processing high contrast values.

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