Defects in crystal

Introduction,types

1. Defects in crysta l Dr.R.Vidhya

2. Introduction Crystalline solids – The constituent atoms or molecules are arranged in an orderly fashion throughout in a three dimensional pattern. The atoms are arranged like soldiers on a parade ground in a well defined columns and rows. Crystal defect – The irregularities in the structure of crystal. 2

3. Types of defects in crystals Volume defects (3D) Voids

4. Point defect – 0D Point defect Imperfect packing of atoms during crystallisation . Imperfect point like regions in the crystal. (a) Vacancies Simplest point defect in a crystal. Missing of atom or a vacant atom site. May be single or divacancies or trivacancies. 4 Vacancy

5. (a) Interstitial defect Extra atom occupy interstitial position. Atom is displaced from its normal position within the crystal to an interstitial position. Foreign atom substitutes the position of parent atom in the crystal lattice. 5 Interstital defect

6. (c) Schottky defect 6 A pair of positive and negative ions are missing in a crystal. Pair of ion vacancies – Schottky defect. Crystal is electrically neutral in this type of defect.

7. (d) Frenkel defect 7 An atom may leave its regular site and may occupy nearby interstitial site giving two defects – Vacancy and interstital . Two defects together – Schottky defect. Common in ionic crystals. Vacancies Interstial site

8. Line defect – 1D Line defect Due to dislocation or distortion of atoms along a line in some direction. Dislocation is a boundary between slipped and unslipped region. Types – Edge dislocation Screw dislocation 8

9. (a) Edge dislocation 9 Due to insertion of an extra half plane of atoms. Extra plane of inserted atoms is above the slip plane – Positive dislocation represented by Extra plane is inserted below the slip plane – Negative dislocation represented by T Burger vector – indicates the direction and magnitude of the shift of the lattice on the slip plane.

10. (b) Screw dislocation 10 Displacement of atoms in one part of a crystal relative to the rest of crystal forming a spiral ramp around dislocation line. Region of lattice disturbance extends in two separate planes at right angles to each other. Burger vector – indicates the direction and magnitude of the shift of the lattice on the slip plane. Burger vector is parallel to the dislocation line. Screw dislocation

11. Surface defect – 2D Surface defect Takes place on the surface of a material. Due to imperfect placing of atoms during crystallization or defective orientation of the surface. Types - Grain boundary - Twin boundary - Stacking fault 11

12. (a) Grain boundary 12 Grain boundary – Defects which separate grains of different orientation from each other in a polycrystalline material. Orientation difference between two crystals is greater than 10-15° - High angle grain boundary Orientation difference between two crystals is less than 10° - Low angle grain boundary. Grain boundary

13. (b) Twin boundary 13 Boundaries in the grains at which the atomic arrangement on one side of the boundary is the mirror image of the atoms on the other side. Volume of material which has orientation similar to the mirror image of the matrix operation – Twin Mirror – Twinning plane.

14. (c) Stacking fault 14 Stacking fault – planar surface imperfections and are caused by fault in the stacking sequence of atomic planes in crystals of FCC and HCP materials.

15. Volume defect – 3D Volume defect Cracks may arise when there is only small electrostatic dissimilarity between the stacking sequences of close packed planes in metals. Whenever a cluster of atoms are missing large vacancy or void is got. Foreign particle inclusions, large voids or non crystalline regions which have the dimensions of the order of 20 Å - volume imperfection. 15

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