Crystal forms a mirror image of itself page 1 12

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Crystal forms a mirror image of itself Page 1-12 Imperfections in the Crystal Structure of Metals ¾ Strength of metals is lower than the theoretical value Defects and imperfections Line defects (disslocations) Vacancy, interstial atoms (extra atom in the lattice) Impurity (foreign atom) Voids or inclusions (volume or bulk defects) ¾ Structure sensitive properties Adversely affected by imperfections yield and fracture strength, electrical conductivity ¾ Structure-insensitive properties Melting point, Specific heat, Coefficient of thermal expansion, elastic constants ¾ Dislocations: generally reduces the shear force needed (line and screw)
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Page 1-13 Work Hardening ¾ Dislocations can sometimes be advantageous Entangled and interfere with each other ¾ Causes an increase in shear stress Æ increased shear strength: Strain hardening or work hardening ¾ Examples Rolling sheet metal Forging the head of a bolt Drawing a wire through a die to reduce its cross section Page 1-14 Grain and grain boundaries ¾ Metals are polycrystals (consists of many crystals) Molten metal begins to form in numerous locations Nucleation: initial stage of formation of a crystal ¾ Grain size Quick cooling: smaller grains ¾ Pure metals: each grain consists of only one crystal ¾ Alloys: Polycrystalline arrangement ¾ A single crystal or grain is anisotropic ¾ Polycrystalline metal is orthotropic
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