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.