Microstructure refers to the microscopic structures present in metals. Microscopic structures in metals include crystal structure, grains, grain boundaries, phases, and defects in the crystal structure. Though some aspects of microstructure can be seen low-lower (25x), higher magnifications are required to see almost all microstructure features of interest. Required magnification ranges from about 100x to 100,000x. Optical (light) microscopes are capable of 50x to 1,500x. For higher magnifications, electron microscopes are used.
The microstructure of metals and alloys depends on their composition and the mechanical and thermal processes to which they have been exposed. Mechanical processing includes cold rolling, drawing, bending, forging, and machining. Thermal processing includes heat treating, welding, and casting. Thermomechanical processing is a combination of thermal and mechanical processing, such as hot rolling, hot forging, and extrusion.
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The microstructure of a metal or alloy has a strong influence on its physical properties, such as strength, ductility, toughness, fatigue resistance, and corrosion resistance. Through alloying and mechanical and thermal processing it is possible to tailor the microstructure of metals and alloys to obtain the desired properties for any specific application.
The rest of this article discusses microscopic structures, how they are observed, and basics of microstructure considerations for different alloys and manufacturing processes.
Metals are crystalline, which means the atoms in a metal are arranged in a periodic manner. The specific arrangement of atoms differs from metal to metal and even within a metal. Crystal structure refers to the three-dimensional periodic arrangement of atoms within a metal. Atoms are arranged in a repeating pattern based on a unit cell, which describes the arrangement of atoms. Common crystal structures in metals are face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP). The figure below shows the unit cell arrangement of atoms for each and examples of metals with those crystal structures. The lengths of the principal axes of a unit cell and the angles between them are called lattice parameters.
In metals, the unit cell repeats in all directions as illustrated below. It is possible to create a piece of metal in which the unit cell is repeated in all directions through the entire piece. However, except for a few cases, there are disruptions in the arrangement of atoms because of how metals are produced, resulting in individual grains.
Furthermore, defects in the crystal structure upset the periodic arrangement of atoms. Crystal defects include dislocations, vacancies, and interstitials. As it turns out, these defects are actually beneficial because they enable metal plastic deformation, alloying, and heat treating.
Grains are individual crystalline areas in a metal. Each grain consists of a periodic arrangement of atoms that are arranged in a specific pattern. The interface between two adjacent grains, where the crystal orientation changes, is called a grain boundary. The areas where different grains meet are grain boundaries. Grains can vary in size, shape, and orientation. Their size, shape, and orientation significantly impact metal strength and ductility. The image below shows the grains in a brass alloy and in a low carbon steel.
Grain size and shape can be modified through alloying, cold working, and annealing. For example, cold rolling or drawing causes grains to elongate in the direction of rolling or drawing and causes the metal's strength to increase. The grain elongation results in non-uniform mechanical properties in directions parallel and perpendicular to the rolling or drawing direction. Annealing a cold worked metal results in new grains forming from the cold-worked grains, results in a decreased strength. The annealing temperature and time are selected to obtain the desired amount of new grain formation and grain growth, which affects metal strength and other mechanical properties.
Phases are physically distinct materials within metals. Phases have distinct properties, are comprised of the elements present in an alloy, and can be either a mixture or a compound,. Metals can be comprised of one phase or multiple phases. The properties of a metal are affected by the phases present, their relative amounts, and their shape and size, all of which depend on a metal’s composition and how it was processed.
Examples of phase in metals are ferrite and cementite in steel and aluminum and aluminum-copper precipitates in precipitation strengthened aluminum-copper alloys.
For example, consider 1045 carbon steel. It can be purchased in the hot-rolled condition to have a microstructure similar to the image below on the left, resulting in about 100 ksi yield strength. The same alloy can be heat treated using the quench and temper process to form the microstructure similar to the image below on the right, resulting in about 210 ksi yield strength. The hot-rolled sample's phase microstructure consists of grains of ferrite and regions of pearlite (parallel plates of cementite and ferrite between the plates). The quench and tempered sample consists of the martensite phase. Of course, the same alloy can be heat treated to obtain variations of these microstructures, resulting in different strength, toughness, hardness, fatigue resistance, and other mechanical properties.
Dislocations are defects in the arrangement of atoms in metals. An example of one type of dislocation is shown. It consists of an extra plane of atoms above the dashed line. On the right is an electron microscope image of dislocations in a metal. Dislocations are present in all metals. There are about 10 million dislocations per square centimeter in as-cast metals and up to a trillion dislocations per square centimeter in heavily cold-worked metals.
Dislocations move through a metal when the stress applied to the metal is large enough. Metal strength and other mechanical properties depend on dislocation motion. These properties can be modified through alloy composition, cold working, grain size, and phase microstructure. By understanding how dislocations move through a metal and the things that affect dislocation motion, metals can be engineered to modify metal strength and other mechanical properties.
Metallography is the term used to refer to the observation and analysis of the microscopic structure of metals using microscopy. The examination of structure may be done over a wide range of length scales or magnification levels, ranging from a visual or low-magnification (20x) examination to magnifications over 1,000,000x with electron microscopes.
The most familiar tool of metallography is the light microscope, with magnifications ranging from 50x to 1,500x and spatial resolution of microstructure features down to 0.2 mm. Scanning electron microscopes (SEM) and transmission electron microscopes (TEM) are used to observe smaller features using magnifications of up to 100,000x. Nevertheless, the light microscope is still the first and most important examination device in metallography.
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Michael Pfeifer, Ph.D., P.E. is President of Industrial Metallurgists and a metallurgical engineering consultant with more than 30 years of experience in product design, manufacturing, failure analysis, and metallurgy training. He helps clients worldwide solve design and manufacturing problems and is the author of Materials Enabled Designs.
