In this article, I’m going to take a step back to consider the basic science of metallurgy. On a microscopic level, there are many things going on inside of a metal. Metals consist of numerous microscopic structures that have a direct and large influence on the properties of metals. Through composition, mechanical treatment, and thermal treatment these microscopic structures can be modified to impart specific properties. Whether the desired structures, and resulting properties, are obtained in a completed component or joint between components depends on the knowledge and skill of designers and manufacturers.
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One set of major structures within a metal are the crystal lattice, grains, and phases. The crystal lattice is the arrangement of the atoms within the metal. Grains are individual crystals within a metal. Figure 1 shows grains in a brass alloy. Phases are different combinations of the elements present in an alloy. Figure 2 shows pearlite in steel. The light colored material is the ferrite phase, which is comprised of iron with a little bit of carbon mixed in. The dark colored phase is cementite, which is comprised of the compound Fe3C. It is also referred to as iron carbide. The properties of a metal are affected by the size of the grains and the phases present.
Defects in the metal crystal lattice make it possible to form alloys and deform metals with the metals cracking. These defects are not the same as manufacturing defects such as voids, inclusions, seams, and cracks. Instead, without crystal lattice defects we would only have pure, brittle metals.
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Various mechanical (e.g. cold rolling) and thermal (e.g. through hardening and precipitation strengthening) processes take advantage of these crystal defects in order to bring about modification of the grains and phases present in a metal, to obtain the desired properties.
Also, the number of crystal defects in the metal can be modified to obtain desired properties. For example, cold rolling results in an increase in the number of dislocations in a metal, resulting in increased strength. The, annealing a cold-rolled metal results in a reduction in the number of dislocations and modification of the grains, resulting in a decrease in the metal strength.
A common representation of the relationship between properties, composition, microscopic structures, and manufacturing defects is shown in the image below. When the effects of the manufacturing processes on the microscopic structures are properly understood, it is possible to consistently produce metal components and joints that have the desired properties. Essentially, the people in charge of the manufacturing processes are responsible for making sure that during the processes the atoms in metals move to where they need to be. And designers are responsible for specifying where the atoms should be.
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For more information about the microscopic structures in metals take our online, on-demand Principles of Metallurgy course or read Metallurgy for the Non-Metallurgist, A.C Reardon, editor or Materials Science and Engineering, W.D. Callister.