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Deformation phenomenon is very important subject in Mechanical Engineering. Mechanical engineers generally deal with the mechanical deformation itself, not the deformation characteristics in atomic scale. But the understanding of the deformation characteristic of materials in atomic basis, we can understand the plastic and elastic deformation characteristics of metallic materials. In this article, we will explain the deformation mechanisms of metallic materials.
You know that the deformation has two types; elastic deformation and plastic deformation. In elastic deformation, applied stress to materik does not excess the interatomic bond forces. So after taking out the stress from the material, material turns it first shape, because there is no change in microstructure inside it.
But in plastic deformation, there is a shape change on material. So there is a microstructural change in material that leads to plastic deformation. The plastic deformation occurs because of dislocations in the atomic level. These dislocations are two types in metallic materials generally; Slip dislocation and Twinning dislocation.
Plastic Deformation With Slip Dislocation: Basically, slip is the movement of atoms into reverse directions each other in planar basis. These planes are called slip planes. This dislocation phenomenon is illustrated as below.
For example in general, HCP crystal structured metals has fewer slip dislocations inside their mictostructure. This means that the HCP metals are not ductile as others These metal are much more brittle. BCC microstructured metals can have much more slip dislocations inside them. Which means they are more ductile compared with others.
When the stress is increased so the plastic deformation is increased, the slip dislocations are also incresed inside the microstructure of metallic material.
Plastic Deformation With Twinning: This is the other kind of mechanism that when plastic deformation occurs in metallic metals. When a shear stress is applied to deform a metallic material, atom planes are aligned around a ‘mirror’ plane as depicted in figure.
These two of dislocation mechanisms generally occur in metallic materials when they undergo plastic deformations.
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Source of images: Fundamentals of modern manufacturing