Metallography and materialography explained

What is metallography?

In short, metallography can be defined as the science and art of studying the microstructure of different metals and metal alloys. In metallography, the surface of a metal sample is prepared through various methods – including cutting, mounting, polishing and etching – to reveal the metal’s microstructure. This microstructure is then analyzed, often using optical or electron microscopy, to identify the metal’s microscopic properties. This enables to metallographer to draw conclusions about the material’s macroscopic properties.

Is materialography different from metallography?

The techniques used in metallography can be applied to a range of materials, including ceramics and polymeric materials, hence the terms ceramography and plastography. Collectively, metallography, ceramography and plastography are known as materialography. Although materialography is the overarching term, the term metallography is more widely used.

Metallographic techniques are most commonly applied to:

• Metals and metal alloys, including titanium, stainless steel, iron, brass, and other types of metal
• Ceramics, including ceramic coatings
• Polymers, including both natural and synthetic polymers
• Coatings, including thermal spray coatings, nitrided coatings and galvanized zinc coatings
• Powder metallurgy parts
• Microelectronics
• Fasteners, including screws, bolts and pins
• Additive manufacturing or 3D printing

What are the main applications of metallography?

In a metal or other material, the microstructure determines many important macroscopic properties, including tensile strength, ductility, thermal conduction and corrosion resistance. Understanding the material’s microstructure therefore enables the metallographer to better determine how it will perform in different situations. As a result, metallography is commonly used in a variety of industrial segments, including the automotive, aerospace, electronics, manufacturing and metal processing industries, as well as academic situations.

Key applications of metallography include:

• Development, analysis and testing of new alloys, materials and products
• Development of new manufacturing techniques
• Component quality assurance and failure analysis of produced components

When was metallography invented?

The first metallurgy has been dated to about 7000 BC, when human beings first discovered that fire could help shape metal. The early metallurgists knew of seven metals: They could find gold, copper and silver in the ground and they created lead, iron, tin and mercury through smelting. Despite this, metallography was not developed as an area of study until the 1800s, when the British scientist Henry Clifton Sorby began exploring manufactured iron and steel. His work demonstrated the connection between a metal’s microstructure and its macroscopic properties.

This may seem obvious now, but at the time it was a revolutionary thought. As Henry Clifton Sorby 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 ..."

Since then, metallography has developed rapidly. Modern metallography owes a great deal to companies like DISA and Struers, who introduced the first mechanical equipment for metallography, including the DISA Electropol and the Knuth-Rotor grinding machine. Today, fully automated materialographic machines are available, such as Xmatic, the world’s first fully automated, end-to-end grinding and polishing solution.

How is metallographic sample preparation carried out?

In metallography, a material sample is prepared in order to make its microstructure visible. As the goal of the metallographic processes is to reveal the material’s true microstructure, it is essential that the process itself does not damage or change the microstructure in any way. The specific metallographic process varies, depending on the material. The process required on a stainless steel car component, for example, will necessarily be different from the process required for a titanium bolt, copper wire, iron weld or microchip. However, most metallographic processes follow the same broad steps.

Metallographic cutting

The metallographic process normally begins with metallographic cutting. The component is sectioned using a specifically designed metallographic cutting machine and cut-off wheel to reveal a cross-section of the material. On large components, such as steel drive axels, metallographic cutting is also used to create small samples of the material that are easier to handle.
The cut-off wheel is selected for the material: alloyed white cast irons, for example, usual require a cubic boron nitride cut-off wheel; while coppers are usually cut using hard silicon carbide.

Metallographic mounting

Once cut, most metallographic samples are mounted in hot or cold resin using a mounting press. Metallographic mounting helps preserve the layers within the material and protects fragile or coated samples during preparation. It also enables safer and more convenient handling of small, sharp, or irregularly shaped objects.
There are two metallographic mounting techniques: hot compression mounting (also called hot mounting) and cold mounting. Both mounting techniques offer certain advantages and the metallographer will decide which to use based on the sample’s properties, the number of specimens and the quality required.

Metallographic grinding and polishing (mechanical preparation)

Most commonly, the surface of the mounted metallographic sample is then prepared for microscopic examination through grinding and polishing. These two metallographic steps are collectively known as mechanical preparation.
Metallographic grinding and polishing are usually performed on specially designed grinding and polishing machines, either manually or automatically. On these machines, the sample is held against a quickly rotating wheel, which grinds or polishes the surface to reveal the microstructure beneath. The surface of the wheel is changed to suit the specific material. Soft and ductile materials, such as aluminum and aluminum alloys, for example, will require a very different grinding or polishing surface than cast iron.

Metallographic etching

Many metallographic preparation processes may stop after grinding and polishing. However, some materials and applications require an additional metallographic process step: etching. Metallographic etching is a chemical process in which the surface of the sample is treated with an etchant chemical mix to reveal or enhance the optical properties of the grain boundaries, phases or grain surfaces within the material. The goal is to make these properties more visible during microscopic inspection.
In addition to metallographic etching, special optical filters may be used in the microscope to further highlight contrast between the material’s different microstructural properties.

Electrolytic preparation

Mechanical grinding and polishing can deform the material’s surface. Therefore, metallographer’s may prefer to use electrolytic preparation. However, this is only possible on certain materials and in certain applications. In electrolytic preparation, the sample is configured as the anode in a suitable electrolyte. The material’s surface is removed by controlled dissolution, revealing the microstructure beneath. This can be followed by electrolytic etching to bring out contrasts in the microstructure under the microscope.

How is a sample analyzed after metallographic preparation?

After metallographic preparation, the microstructure of the sample material is inspected and analyzed. The information derived from this analysis enables the metallographer to draw conclusions about the material’s properties, including its strength, ductility and conductivity.

Metallographic inspection: Microscopy and macroscopy

Depending on the application and material, it may be enough to inspect the specimen with the naked eye (macroscopy). However, in most situations, metallographers use microscopy (optical or digital magnification) to enhance the appearance of the microstructure.

• Optical microscopy can magnify the microstructure up to 1,000 times.
• Electron microscopy can magnify the microstructure up to 500,000 times. This is mainly used for failure analysis and in educational scenarios.

What is metallographic hardness testing?

Hardness testing is a key element in many metallographic laboratories as it enables the metallographer to establish the exact hardness of the material, which gives important information on its strength, wear resistance and ductility. Hardness testing is done by pressing a specifically dimensioned object (indenter) into the material’s surface and measuring the size and/or depth of the impression made.
There are a number of metallographic hardness testing standards, including Vickers, Knoop and Brinell. All require specific hardness testing machines and equipment. These machines must be calibrated to ensure accurate hardness testing results.