(5 - 6) microstructures qwq

Chapter 5 microstructures introduction (point defects, diffusion)

3 Types of Bonds:

1. Metallic: sea of electrons + metallic cations (two metals)

2. Covalent: sharing electron (two nonmetals)

3. Ionic: transfer of electrons (nonmetal + metal)

Rare Crystal Structure:

• Simple Cubic: atom at each corner of a cube → repeating square lattice

Common Crystal Structures:

1. fcc (face centered cubic) → corners + faces (highly packed)

2. HCP (hexagonal close packed) → hexagon top + bottom + 4 faces (highly packed)

3. bcc (body centered cubic) —> corners + center

4. bct (body centered tetragonal) → corners + center (elongated)

Microstructure Approximate Size:

• Atoms: 10^-9

• Microstructure: 10^-4

Processing → Microstructure → Properties

• Defects in crystals

• Phases

Defects:

1. Point Defects

   1. Vacancy (gone) → Schottky

   2. Self interstitial atom (displaced same) → Frenkel

   3. Interstitial impurity atom (small foreign squeezes itself in)

   4. Substitutional impurity atom (foreign replaces)

2. Dislocations (line defects)

   1. Edge

   2. Screw

3. Grain Boundaries (plane defects)

What do defects do to the material?

• Elastic stress

• Increase free energy (unstable)

  • favorable to REMOVE defects but requires thermal activation (slow)

n is number of defects, N is number of atoms, A is a constant, k is Boltzmann’s constant

• k = 1.38 × 10^-23 J / K

• E_D is energy of formation for defect

Self Diffusion: only one component due to movement of vacancies

Chemical Diffusion: more than one chemical species (high concentration to low)

Chapter 6: Line Defects (Dislocation) and Planar Defects

Burger’s Vector: vector of lattice distortion bc of dislocation

• Glide (slip) bc of sheer stress ALONG slip plane

• Climb bc of atomic diffusion PERPENDICULAR slip plane

• no equilibrium concentration

• due to plastic deformation

How to get rid of line defects:

• Recovery

• Recrystallisation

Mixed Dislocation (screw + edge): Loops

Types of Planar Defects:

1. Surfaces (different phases)

2. Boundaries between solids

   1. Grain boundary: same phase, different orientation due to angle

   2. Interface (same phase, but aligned either perfectly, messy, not not at all)

      1. Coherent

      2. Semicoherent

      3. Incoherent

3. Twin boundaries (reflection in hcp metals)

Breaking or distorting atomic bonds → energy increase + inc in reactivity

Calculations:

Light Equations:

• E = hv

• c = λv

  • c = 3 × 10^8

  • h = 6.626 × 10^-34

1 Amstrong = 10^-10 m

Know this silly chart: