MSE200 Lecture 5 CH 4 3 4 4 Crystal Imperfections Defects Instructor Yuntian Zhu Objectives outcomes You will learn the following Describe point defects solid solutions vacancies and interstilialcies Describe line defects dislocations Burger s vector edge and screw dislocations Describe grain boundaries and grain size Understand metallography http www mse ncsu edu zhu Materials Science Engineering Metallic Solid Solutions Solid solution is a simple type of alloy in which elements are dispersed in a single phase The crystal structure of the solvent is maintained Why do we care There are two types of solid solutions Example http www mse ncsu edu zhu Materials Science Engineering Substitutional Solid Solution Solute atoms substitute for solvent atom in a crystal lattice The structure remains unchanged Lattice might get slightly distorted Solvent atoms Solute atoms http www mse ncsu edu zhu Materials Science Engineering Substitutional Solid Solution Cont The solubility of solids is greater if Similar diameter difference 15 Same crystal structures Similar electronegativity else compounds will be formed Same valence Examples System http www mse ncsu edu zhu Atomic Electroradius negativity Difference difference Solid Solibility Cu Zn 3 9 0 1 38 3 Cu Pb 36 7 0 2 0 17 Cu Ni 2 3 0 100 Materials Science Engineering Interstitial Solid Solution Solute atoms fit in between the voids interstices of solvent atoms Solute atoms is much smaller than solvent atoms Calculate the radius of the biggest interstitial void Example problem 4 3 Iron atoms r 0 129nm Carbon atoms r 0 075nm http www mse ncsu edu zhu Materials Science Engineering Crystalline Imperfections defects No crystal is perfect Why do we care about defects Imperfections can be classified as Zero dimensional point defects One dimensional line defects dislocations Two dimensional plane defects grain boundaries twin boundaries http www mse ncsu edu zhu Materials Science Engineering Point Defects Vacancy Vacancy is formed due to a missing atom Vacancies are equilibrium defects http www mse ncsu edu zhu Materials Science Engineering Point Defects Interstitials Atom in a crystal sometimes occupies interstitial site How can this happen Effect on crystal lattice http www mse ncsu edu zhu Materials Science Engineering Point Defects in Ionic Crystals Complex as electric neutrality has to be maintained If two appositely charged particles are missing cationanion divacancy is created This is Schottky imperfection Frenkel imperfection is created when cation moves to interstitial site http www mse ncsu edu zhu Materials Science Engineering Line Defects Dislocations Lattice distortions are centered around a line Different types of line defects are Edge dislocation Screw dislocation Mixed dislocation http www mse ncsu edu zhu Materials Science Engineering Edge Dislocation http video google com videosearch q dislocation hl en emb 0 aq f emb 0 hl en q dislocation slip view 3 Created by insertion of extra half planes of atoms Positive edge dislocation Negative edge dislocation Burgers vector Burgers vector Shows displacement of atoms slip The Burgers Vector is perpendicular to the dislocation line http www mse ncsu edu zhu Materials Science Engineering Screw Dislocation Created due to shear stresses applied to regions of a perfect crystal separated by cutting plane Distortion of lattice in form of a spiral ramp The Burgers Vector is parallel to the dislocation line http www mse ncsu edu zhu Materials Science Engineering Mixed Dislocation Most crystal have components of both edge and screw dislocation Each dislocation has only one Burgers vector b When a dislocation glide across a crystal it produce a plastic deformation by b Examples of edge screw and mixed dislocations http video google com videosearch q dislocation hl en emb 0 aq f emb 0 hl en q dislocation slip view 3 http www mse ncsu edu zhu Materials Science Engineering Grain Boundaries Grain boundaries separate grains Width 2 5 atomic diameters Atoms in grain boundaries have higher energy Stone walls Grain boundary http www mse ncsu edu zhu Materials Science Engineering Twin Boundaries Twin A region in which mirror image pf structure exists across a boundary Formed during plastic deformation and recrystallization Strengthens the metal Twin Plane Twin http www mse ncsu edu zhu Materials Science Engineering Observing Grain Boundaries Metallography Sample surface is polished The surface is then etched chemically Tiny groves are produced at grain boundaries Groves do not intensely reflect light Hence observed by optical microscope http www mse ncsu edu zhu Materials Science Engineering Grain Size Why do we care Smaller grains lead to stronger materials http www mse ncsu edu zhu Materials Science Engineering Homework Example Problems Version 4 5 4 3 Regular Problems Download from website Reading assignment for the next class 5 1 5 4 http www mse ncsu edu zhu Materials Science Engineering