Interpretation of Metallographic Structures

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Light microscopy has been used for many decades to provide insight into the microstructure of materials. Brightfield BF illumination is the most common illumination technique for metallographic analysis. In incident BF, the light path comes from the light source, passes through the objective lens, is reflected off the surface of the specimen, returns through the objective, and finally reaches the eyepiece or camera for observation.

Flat surfaces produce a bright background due to reflection of a large amount of the incident light into the objective lens, while non-flat features, such as cracks, pores, etched grain boundaries or features with distinct reflectivity, such as precipitate and second phase inclusions on the surface appear darker as incident light is scattered and reflected at a variety of angles or even partially absorbed.

Darkfield DF is a less known but powerful illumination technique.

The light path for DF illumination passes through an outer hollow ring of the objective, falls onto the specimen at a high angle of incidence, reflects off the surface, then passes through the interior of the objective lens, and finally reaches the eyepiece or camera. This type of illumination causes flat surfaces to appear dark, as the vast majority of the light reflected at the high incident angle misses the interior of the objective lens. For samples having a flat surface with occasional non-flat features — cracks, pores, etched grain boundaries, etc.

Differential Interference Contrast DIC , also known as Nomarski Contrast, helps to visualize small height differences on the specimen surface, thus enhancing feature contrast. The two light waves split by the prism are made to interfere after reflection from the specimen surface, rendering height differences visible as variations in color and texture. For the majority of cases, incident light microscopy provides most of the required information, but for some cases, in particular polymers and composite materials, transmitted light microscopy for transparent materials and the use of stains or dyes can provide insights into the microstructure that would remain hidden when using standard bulk sample preparation and normal incident illumination.

Polarization: Natural light consists of light waves with any number of vibration directions. Polarization filters only let light waves through that vibrate parallel to the direction of transmission. If the sample between the polarizers changes the vibration direction of the light, characteristic birefringence colors appear.

The natural color of microstructures is usually of very limited use in metallographic applications, but color can reveal useful information when exploiting certain optical methods, such as polarized light or DIC , or sample preparation methods, like color etching. Polarized light microscopy is very useful for the examination of metals with a non-cubic crystallographic structure, such as Ti, Be, U and Zr. Unfortunately, the main commercial alloys Fe, Cu, Al are not sensitive to polarized light, so color or tint etching provides an extra method which can reveal and discriminate features in the microstructure.

Color tint etchants are generally applied chemically by immersion in solution or electrochemically immersed in solution with electrodes and applied potential , producing a thin film on the surface of the specimen which is usually feature-dependent. Additionally, heat tinting or vapor deposition are alternative methods for creating interference films.

In steel alloys, the so-called "second phase" constituents can be selectively colored by etching, which provides a method to identify and quantify them separately.

Interpretation of Metallographic Structures - 2nd Edition

Discriminating ferrite and carbides in steel by color etching is a common procedure. The growth of interference films can be a function of the crystal orientation of features, e. For alloys where etching with standard reagents to attack grain boundaries yields an incomplete network of boundaries and thus prevents digital image reconstruction, the color coding of the microstructure due to different grain orientations allows the analysis of grain size to be performed.

The origin of quantitative metallography lies in the application of light microscopy to the study of metallic alloy microstructure.

Interpretation Of Metallographic Structures 1990

It can be more precisely defined as the scientific discipline of observing and determining the chemical and atomic structure and spatial distribution of the constituents, inclusions or phases in metallic alloys. By extension, these same principles can be applied to the characterization of any material. Different techniques are used to reveal the microstructural features of metals.

Most investigations are carried out with incident light microscopy in brightfield mode, but other less common contrasting techniques, like darkfield or differential interference contrast DIC , and the use of color tint etching are expanding the scope of light microscopy for metallographic applications. Many important macroscopic properties of metallic materials are highly sensitive to the microstructure. Critical mechanical properties, like tensile strength or elongation, as well as other thermal or electrical properties, are directly related to the microstructure. The understanding of the relationship between the microstructure and macroscopic properties plays a key role in the development and manufacture of materials and is the ultimate aim of metallography.

Metallography, as we know it today, owes much to the contribution of the 19 th century scientist Henry Clifton Sorby. His pioneering work with modern manufactured iron and steel in Sheffield UK highlighted this intimate bond between the microstructure and macroscopic properties. As he stated towards the end of his life: "In those early days, if a railway accident had occurred and I had suggested that the company should take up a rail and have it examined with the microscope, I would have been looked upon as a fit man to send to an asylum.

But that is what is now being done Together with new developments in microscopy technology and, more recently, with the aid of computing, metallography has been an invaluable tool for the advancement of science and industry over the last hundred years. Some of the earliest correlations between microstructure and macroscopic properties established in metallography using light microscopes include:. Thus, metallography is used in almost all stages during the lifetime of a component: from the initial materials development to inspection, production, manufacturing process control, and even failure analysis if needed.

The principles of metallography help to ensure product reliability.

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The basic steps for proper metallographic examination include: sampling, specimen preparation sectioning and cutting, mounting, planar grinding, rough and final polishing, etching , microscopic observation, digital imaging and documentation, and quantitative data extraction through stereological or image analysis methods. The first step of metallographic analysis — sampling — is critical to the success of any subsequent study: the specimen to be analyzed has to be representative of the material being evaluated.

The second, equally important, step is to correctly prepare a metallographic specimen, and here there is no unique way to achieve the desired results. Metallography has been traditionally described as both a science and an art, and the reason for this statement lies in the fact that experience and intuition are equally important for exposing the true structure of the material without causing significant change or damage, in order to reveal and make measurable the features of interest. Etching is probably the most variable step, so careful selection of the best etch composition and control of etchant temperature and etch time are mandatory to obtain confident and repeatable results. Very often a trial and error experimental method is required to find the optimal parameters for this step.

Metals and their alloys still play a prominent role in many forms of technological development, because they offer a broader range of properties than any other materials group. The number of standardized metallic materials extends to several thousand and is continuously increasing to meet new requirements.

Metallography Part II - Microscopic Techniques

However, as specifications have evolved, ceramics, polymers or natural materials have been added to cover a wider spectrum of applications, and metallography has expanded to incorporate new materials ranging from electronics to composites. The study of the structure of metals and alloys, especially by optical and electron microscopy and x-ray diffraction. Mentioned in?

References in periodicals archive? Tenders are invited for Procurement of automatic hot compression mounting system for metallographic specimen preparation Procurement Of Automatic Hot Compression Mounting System For Metallographic Specimen Preparation.

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A dyeing process was conducted, followed by metallographic analysis. This fact is due to the dependency of fatigue life on the surface roughness but also to the metallographic analysis of the fracture surface that follows each test, and that requires a surface free from scratches.

Newage Testing Instruments' HMV-G Series is designed for measuring the hardness of small parts and metallic structures used in precision equipment, processed surface layers and metal plating layers, making it an ideal choice for metallographic research and product quality control.