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Ionic and Covalent Bonds

• Ionic Bond: Electrons are transferred between atoms, leading to a strong attraction between resulting charged ions.

  • Anion: Negatively charged atom with excess electrons.

  • Cation: Positively charged atom with fewer electrons.

• Covalent Bond: Electrons are shared between atoms, forming molecules.

• Metallic Bond: Electrons are shared among metal atoms creating a cloud of free-moving electrons.

Crystal Structures

• Body-centered cubic (bcc): Crystal structure with atoms located at the corners and a single atom at the center.

• Face-centered cubic (fcc): Crystal structure with atoms located at the corners and at the center of each face.

• Hexagonal close-packed (hcp): Crystal structure with atoms arranged in a hexagonal configuration.

• Primary Bonds: Strong atomic attractions that involve interactions of electrons.

• Unit Cell: The basic repeating unit in crystal structures.

Electrostatics and Stability

• Electrostatic Attraction: Force between oppositely charged ions in ionic bonds.

• Electron Cloud: Collective shared electrons that create attractive forces within metals.

• Stability of Atoms: Atoms with eight outer electrons are more chemically stable.

Bonding Types and Structure

• Single Bond: One pair of electrons shared between two atoms.

• Double Bond: Two pairs of electrons shared between two atoms.

• Crystal Structure Density: The arrangement of atoms significantly influences material density.

• Volume Assumption: The assumption for equal volumes for bcc and fcc unit cells.

Unit Cell Structures

• BCC Unit Cell: Contains 2 atoms (1 at the center and 8 at the corners).

• FCC Unit Cell: Contains 4 atoms (6 faces and 8 corners).

Defects in Crystals

• Elastic Deformation: Temporary shape change that reverts back when the applied force is removed.

• Ductile Metals: Metals that have five or more slip systems enabling them to deform plastically.

• Point Defects: Examples include missing atoms, extra atoms, or foreign atoms within a crystal structure.

  • Vacancy: A missing atom in the crystal lattice.

  • Interstitial Atom: An extra atom inserted within the lattice structure.

  • Impurity: A foreign atom that replaces a pure metal atom within the structure.

  • Dislocations: Linear defects that cause distortion in the crystal structure.

    • Edge Dislocation: A dislocation characterized by an extra half-plane of atoms.

    • Screw Dislocation: A dislocation forming a spiral arrangement of atoms.

Mechanical Properties of Materials

• Grain Boundaries: Surfaces that separate individual grains of a metal.

• Anisotropic Behavior: Properties that vary with direction in single crystals.

• Isotropic Behavior: Properties that do not vary with direction in polycrystals.

• Grain Boundary Embrittlement: Weakening of material due to the infiltration of foreign atoms at the grain boundaries.

• Recrystallization: The process of forming new grains to replace deformed ones,

• Annealing: Heating a metal to restore its original properties while facilitating recovery and grain growth.

Testing Mechanical Properties

• Tensile Test: Measures the load and displacement of materials to determine their ability to withstand tension.

  • Load: The force applied during a tensile test.

  • Displacement: The change in length that occurs during the test.

  • Stress: Defined as force per unit area acting on materials.

  • Strain: The relative deformation in relation to the original length.

• Toughness: Measure of the energy absorbed by a material before fracture.

• Modulus of Resilience: Amount of energy per unit volume up to the yield point a material can absorb.

• Bauschinger Effect: The tendency of a material to exhibit lower yield strength in compression after experiencing tension.

• True Stress: Evaluated as force divided by actual cross-sectional area during material deformation.

Connections and Effects

• Creep: Permanent deformation that occurs under constant load over time, influenced by the material's melting temperature.

  • Creep Rate: The rate at which deformation increases with temperature.

  • Stainless Steels: Notable for their creep resistance.

  • Superalloys: High-performance materials used in extreme conditions, also resistant to creep.

• Strength Factors: Actual material strength is often lower than theoretical estimates due to existing defects.

  • Fatigue: Weakening effect from repeated loading, characterized by an endurance limit below which failure will not occur.