Imperfections in Solids

Imperfections in Solids

• Solids contain imperfections, also known as defects
• These defects impact material properties
• Solidification mechanisms are crucial, influencing structure size and shape, depending on cooling rate.
• Nuclei form and grow, leading to a grain structure.
• Defects are created during material processing.
• Point defects: vacancies (missing atoms) and self-interstitials (extra atoms)
• Impurities: intentional or unintentional additions altering material properties (e.g., carbon in steel).
• Alloys: intentional impurity addition to modify properties.
• Solvent is the main component, solute is the impurity.
• A solid solution forms when solute atoms are incorporated into the host material, maintaining the crystal structure, and is compositionally homogenous.
• The addition of impurities to metals can result in solid solutions or new phases, influenced by impurity type, concentration, and temperature.
• Impurities can be substitutional (replacing atoms) or interstitial (fitting into spaces).
• The solubility of solutes in solvents depends on atomic size, crystal structure, electronegativity, and valence.
• Copper and nickel have complete solubility in each other due to similar size, structure, and electronegativity
• Dislocations are line defects visible in electron micrographs.
• Dislocations cause slip between planes, resulting in plastic deformation.
• Types of imperfections include point, line, area, and volume defects.
• Point defects include vacancies, interstitials, and impurities
• Line defects include edge and screw dislocations.
• Area/interfacial defects are grain boundaries, which create changes in crystallographic direction; impede dislocation.
• Volume defects are cracks, pores, and inclusions.
• Grain boundaries, are classified by low to high-angle between the misorientation of the grains; high-angle boundary is more susceptible to impurity segregation and lower material density than fine-grained materials
• Movement of edge dislocations requires the successive bumping of a half-plane of atoms, bonds are broken and remade in succession.
• Motion of screw dislocations results from shear stresses shifting parts of the crystal.
• Mixed dislocations are a combination of edge and screw dislocations
• Twin boundaries - Essentially a reflection of atom positions across the twin plane; region between twin boundaries called a twin
• Stacking faults occur in close-packed structures.
• Antiphase boundaries occur when one side of a boundary has an opposite phase from the other side for ordered alloys.
• Bulk (volume) defects include pores, cracks, foreign inclusions, and other phases; introduced during processing/fabrication.
• Voids are clusters of vacancies.
• Impurities cluster together to form precipitates.
• Microscopy helps examine crystallites (grains) and grain boundaries, which vary in size (diamond, aluminum garbage can).
• Several applications of microscopic examination determine properties-structure relationships, predict material properties, design alloys with new combinations, identify if a material has been heat treated correctly, determine mode of mechanical fracture.
• Optical Microscopy: useful up to 2000X, dependent on crystal orientation;polishing removes surface features;etching changes reflectance, reveal grain boundaries to etching.
• Polarized light increases contrast in metallographic scopes and transparent materials
• Electrons are used in higher resolution, atomic-level observations with high magnification
• Scanning Tunneling Microscopy reveals atomic arrangements, as in carbon monoxide molecules, or iron atoms on copper
• Point, line, and area defects exist in solids.

Summary

• Number and type of defects can be controlled.
• Defects significantly influence material properties.
• Defects may be desirable or undesirable depending on context.

Structure of Ceramics

• Ceramic structures are complex involving ionically or covalently bonded elements.
• The degree of ionic character depends on electronegativity differences.
• The ratio of cations to anions in ionic ceramics determines charge neutrality, impacting the crystal structure.
• Ionic radius impacts ceramic crystal structures.
• Common AX structures: Rock salt (NaCl, MgO), Cesium chloride (CsCl), Zinc blende (ZnS), Fluorite (CaF2), and Perovskite (BaTiO3).

Summary

• Ionic bonding in ceramics produces complex structures influenced by the relative size of ions.
• The ratio of cations to anions in an ionic ceramic determines the stoichiometry and influences the structure.
• Structures are affected by cation-anion radii.

Structure of Metals

• Crystalline metals have a periodic array of atoms.
• Five parameters quantify metallic structures: lattice parameter, coordination number, number of atoms per unit cell, atomic packing factor, and density.
• Common crystal structures for metals include simple cubic, body-centered cubic, face-centered cubic, and hexagonal close-packed.
• Metallic alloys can have simple or more complex structures.

Summary

• Crystalline structures in metals are characterized by their close packing.
• Common structures are cubic (simple, body centered, and face centered) and hexagonal.
• Metallic alloys can exhibit diverse structural arrangements.

Classifications of Solids

• Solids can be crystalline or amorphous
• Crystalline Solids: Periodic 3D arrays of atoms.
  • Molecular Solids
  • Metallic Solids
  • Covalent Solids
  • Ionic Solids
• Amorphous Solids: No periodic packing

Summary

• Solids are classified as crystalline or amorphous based on their atomic arrangement.
• Crystal structures are categorized by the arrangement of atoms and bond types.