MSE230 Chapter 4 Part 1 Aisling Coughlan Defects Introduction Crystals are like people it is the defects in them which tend to make them interesting Colin Humphreys Real crystals are never perfect there are always defects Defects Introduction Defects have a profound impact on the macroscopic properties of materials The processing determines the defect Control defects control properties Imperfections in Solids Solidification 2 steps nuclei liquid crystals growing grain structure Adapted from Fig 4 14 b Callister Rethwisch 8e Imperfections in Solids There is no such thing as a perfect crystal What are these imperfections Why are they important Many of the important properties of materials are due to the presence of imperfections Types of Imperfections Vacancy atoms Interstitial atoms Substitutional atoms Dislocations Grain Boundaries Pores Types of Defects Defects may be classified into four categories depending on their dimensions 0D point defects 1D linear defects 2D planar effects 3D volume defects Types of Defects Defects may be classified into four categories depending on their dimensions 0D point defects 1D linear defects 2D planar effects 3D volume defects Point Defects in Metals Vacancies vacant atomic sites in a structure Self Interstitials extra atoms positioned between atomic sites Equilibrium Concentration Point Defects Qv N v N exp kT 12 Measuring Activation Energy We can get Qv from an experiment Nv exp N Measure this Replot it Nv ln N Nv N Qv kT slope Q v k exponential dependence T defect concentration 1 T Defects in Surface Science Catalysis fig 04 10 Transmission Electron Microscopy TEM fig 04 11 Impurities in Solids A pure metal with only one type of atom does not exist In alloys impurity is added intentionally to impart specific properties e g mechanical properties and corrosion E g sterling Ag 92 5 Ag 7 5 Cu Solid Solutions When impurity is added into host material and crystal structure is maintained with no new crystals formed The impurity is randomly dispersed Substitution when substitution on host atoms take pace with impurity Solute and Solvent or Host and Guest Imperfections in Metals OR Role of Metal Atoms Size Ce Sm O Ce Zr O 20 80 60 40 20 80 40 60 40 60 60 40 80 20 80 20 100 0 100 0 20 30 40 50 60 70 80 2 degrees Ce3 vs Zr4 radius difference 30 Ce3 vs Sm3 radius difference 6 5 20 30 40 50 2 degrees 60 70 80 For substitutional defects they ll do so if Atoms should have similar properties Hume Rothery rules for solid solution formation Similar radii less than 15 difference otherwise high distortions and new phase will be formed Similar electronegativity x increases intermetallic and not solid solution Same crystalline structures FCC BCC HCP Valency compatibility Valency rule All else being identical a metal will have higher tendency to dissolve metals of higher valency Example Cu Ni FCC both x 1 9 1 8 rCu 0 128 nm And rNi 0 125 nm Impurities in Solids Specification of composition weight percent atom percent
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