Defect in a Defect Hierarchy of Defects Part
- Slides: 7
Defect in a Defect & Hierarchy of Defects Part of MATERIALS SCIENCE & A Learner’s Guide ENGINEERING AN INTRODUCTORY E-BOOK Anandh Subramaniam & Kantesh Balani Materials Science and Engineering (MSE) Indian Institute of Technology, Kanpur- 208016 Email: anandh@iitk. ac. in, URL: home. iitk. ac. in/~anandh http: //home. iitk. ac. in/~anandh/E-book. htm
Defect in a Defect! This is an interesting concept to explore. q We will consider some examples to explore this concept. q 0 D ◘ Vacancy is a point defect. Vacancies can get ordered to form Vacancy Ordered Phases. ◘ There could be point defect in the Vacancy sublattice → an atom present where a vacancy should have been there (Atom is a defect in the vacancy sublattice!) → this is a defect in a defect! q 1 D ◘ Dislocation is a defect in the crystal. ◘ A kink or a jog is a defect in the dislocation line → a defect in a defect. q 2 D ◘ Surface is a defect in the crystal. ◘ Defect in a defect → Steps (ledges) in a surface is a defect in the surface. ◘ Defect in a defect → Steps in these steps (called kinks) is a defect in the steps. q There are other defects possible on the surface, such as: surface vacancies and adatoms. q A vacancy in a ledge is a defect in a defect. [Ledge vacancy is a defect in the ledge, which is a defect in the surface, which is a defect of the crystal].
Click here to know more about Defect Association q Defect association can also be viewed as a defect in a defect. E. g. a carbon atom ‘segregating’ to the core of an edge dislocation can be construed as a defect in the dislocation line (as the ‘perfection’ of a dislocation line has been disrupted). In this example, the strain energy of the system (enthalpy) is lowered and hence favours' the association. However, the entropy of the system decreases and this aspect opposes the association. It is to be noted that, there is some residual entropy even when the carbon atom is present at the dislocation core (as there is some degeneracy of positions along the core).
0 D In a Vacancy Ordered Phase (VOP) there could be a vacancy in the ‘vacancy sublattice’ → this is an atom! we can conceive such a hierarchy of ‘higher order’ defects! Vacancy in the vacancy sublattice → i. e. an atom! Perfect Ordered Crystal Vacancy Ordered Phase Schematics
1 D Jog or kink is a defect in the dislocation line Dislocation is a defect in a crystal We can even further visualize things like a kink in the jog!
2 D D ef ec ti n th es ur fa ce A ledge (step) is a defect in the surface Defect in the Ledges The surface is a defect in a crystal A kink (which is a step in the ledge) is a defect in the ledge
Hierarchy of Defects q q This is another interesting concept. We will try to understand this using the example of vacancy ordered phases (VOP). We have seen that an atom is a defect in the vacancy sublattice of the VOP. These atoms could be randomly present in the vacancy sublattice or be ordered in the sublattice to give a sub-sublattice. q A defect in this atom sub-sublattice of the vacancy sublattice is again a vacancy! q This process of formation of defects in ‘sub’, ‘sub-sub’ … can be carried forward and we can conceptualize this hierarchy of defects!
