1. Crystal Lattice

Crystal Lattice Overview

  • Discussion focuses on the three-dimensional arrangement of atoms in metals

  • More than 90% of solids are crystalline with regular atomic structure

  • Non-crystalline solids (amorphous) lack long-range atomic ordering, often referred to as glasses

Types of Solids

Crystalline Solids

  • Atoms arranged in a long-range periodic structure

  • Metals and ceramics are examples of crystalline materials

  • Polymers can also exhibit crystalline structures, but definitions vary

Amorphous Solids

  • Lack a long-range order in atomic arrangement

  • Common examples include glass

Bravais Lattices

  • There are 14 distinct ways to arrange atoms into metal lattices, known as Bravais lattices

  • Lattices can be categorized by symmetry:

    • High symmetry lattices: Many directions appear identical

    • Low symmetry lattices: Directions look different

  • Most metals bond into high symmetry lattice types

Common Lattice Types in Metals

Primary Lattice Types

  1. Face Centered Cubic (FCC)

    • Atoms at corners and faces of the cube

    • Represents a closed packed structure, filling 74% of the volume

  2. Body Centered Cubic (BCC)

    • Atoms at cube corners with one atom at the center

    • Not a closed packed structure, filling 68% of the volume

  3. Hexagonal Close Packed (HCP)

    • Atoms form a hexagonal arrangement with an offset central layer

    • Also a closed packed structure, filling 74% of the volume

Engineering Metals

  • Dominant metals in engineering: iron-based alloys (ferrous), aluminum and its alloys

  • Other notable metals: magnesium alloys, titanium alloys, nickel alloys, copper, zinc, molybdenum, tungsten, beryllium, cobalt, niobium, hafnium, tantalum, rhenium, iridium

Structural Characteristics of FCC, BCC, and HCP

Face Centered Cubic (FCC)

  • Each unit cell includes an atom at each corner and one at the center of each face

  • Close packed structure: maximal packing efficiency

Hexagonal Close Packed (HCP)

  • Begin with hexagonal lattice at the top and bottom, with an offset layer in the center

  • Stacking sequence: ABA pattern leading to lower symmetry and clear paths (windows)

Body Centered Cubic (BCC)

  • Contains more open volume compared to FCC

  • Packing efficiency of 68% and lacks close packed planes but has close packed directions

Growth of Grain Structures

  • Initial stage involves cooling a liquid to form crystallite regions

  • Crystallites grow and contact each other, forming grain boundaries

  • Each grain behaves like a single crystal with consistent crystallographic orientation

  • Misaligned lattice boundaries occur at the interfaces of grains

Visual Representation of Grain Structures

  • Examples include images of vanadium metal grains and aluminum ingots

  • Electron Backscatter Diffraction (EBSD) maps show crystal orientation in both metal and ceramic materials

Conclusion

  • Importance of understanding crystallographic lattices in relation to engineering materials and their respective properties.