Porosity and Crystalline Defects

Porosity

• Volume fraction of a solid that is empty

• Has implications on material properties

  • Obviously, a solid with many pores would have a weaker yield strength compared to one without any pores

N value for porosity calculations

• Can be anything from 1 to 3

Defect

• In materials, it does not mean anything bad

• It only refers to when a deviation is present in a crystal, that deviates away from a perfect crystal in structure

• Many material properties depend on defects, it is as important to understand this as it is to understand a perfect crystal

Types of Defects

• Point Defects (0D)

• Line Defects (1D)

• Planar Defects (2D)

• Bulk Defects (3D)

Point Defects

• Affects 1 Atom

• Types of Point Defects:

  • Vacancy:

    • A lattice position is vacant because an atom is missing

  • Intersitial

    • An atom that occupies a space that is outside the normal lattice position

      • Can be self-interstitial, meaning this atom is of the same element of the lattice

      • Can be an alloying element or impurity, meaning the atom is of a different element than of the lattice

Vacancy, and how they appear

• A small number is favorable because it increases entropy otherwise

• As the temperature of the material increases, vacancies appear in the interior of the material and the ejected atoms go to the surface

Vacancies effects on Material Properties

• Conductivity of ionic compounds depends on concentrations of vacancies

• High temperature strength of material (creep performance)

• Substitutional Diffusion in solids occurs by a vacancy mechanism

  • Diffusion is slow at low T because there are very few vacancies

  • Diffusion is fast at high T because there are very many vacancies

Line Defects

• Dislocations:

  • An extra or missing plane

  • Lattice distortion produced in the vicinity of the dislocation

  • Dislocations increase the energy of the crystal

Effect of Dislocations on Material Properties

Materials deform (change shape) through the motion of dislocations

The strength of the material is determined by how easy it is for dislocations to move

Dislocations also affect the electrical conductivity of metals

Motion of Dislocations

Egon Orowan conceived of dislocations in metals when attempting to move a hallway carpet runner

Unlike vacancies, dislocations are always thermodynamically unstable

Most dislocations disappear at high temperatures

Planar Defects- Surfaces

• Surface atoms contain a higher energy compared to the atoms in the bulk, as they have dangling bonds

• Depending on which crystallographic plane makes the surface, crystals can have different surface energies and different reactivity

Planar Defects - Polycrystalline

• The solid contains many grains (a single crystal region within a polycrystalline material) separated by transition regions

• There is an atomic mismatch in a transition region where two grains meet

• These transition regions are called grain boundaries

Grain Boundaries affect on mechanical properties

• influence several mechanical properties including strength, toughness and creep rate

• Grain boundaries can be more (or less) reactive than the rest of the material (e.g. corrosion)

• e.g

  • Silicon wafers and turbine engine blades are amongst a very small number of applications for which single crystals are used (no grain boundaries).

Bulk Defects

• Voids (e.g. casting)

• Porosity (e.g. 3D printing)

• Cracks

• Fracture toughness is very sensitive to bulk defects.