Crystalline Defects: Point Defects and Alloys

Introduction

• Perfect solids do not exist; all contain defects or imperfections
• Crystalline defect: a lattice irregularity on the scale of an atomic diameter
• Defects include point, linear, and interfacial defects

Point Defects

• Vacancy: a vacant lattice site
• Vacancies form during solidification or from atoms displaced by vibrations
• Vacancy quantity depends on and increases with temperature
• Self-Interstitial: an atom from the crystal crowded into an interstitial site
• Interstitial causes lattice distortions because the atom is substantially larger than the interstitial space
• Interstitial formation is improbable and exists in smaller quantities than vacancies

Alloys

• Alloy: a combination of two or more metals/elements
• Impurity atoms (solute) are added to host atoms (solvent) for specific characteristics
• Adding 7.5% copper by volume to pure silver, creates sterling silver
• Increasing mechanical strength without depreciating corrosion resistance

Mechanisms of Alloying

• Insoluble Components: The component structures form within the host matrix
• Insoluble Structures are called phases
  • Example: Pearlite
  • Composed of alpha-ferrite (88%) and cementite (iron-carbide) (12%)
• Soluble Components: Forms a continuous structure, with no visible phases
  • Example: Austentite
  • A solid solution of carbon in gamma-ferrite
• Intermediate Compounds: Forms from components
  • Example: Magnesium Plumbide (Mg2Pb)

Mechanisms of Alloying

• When impurity atoms (B) are added to host atoms (A), two things can happen
• Solid Solution: Random distribution of point defects
• Formation of a new phase: More likely as the concentration of B increases
• Features Determining Solute/Solvent Dissolving Degree:
  • Atomic Size: Must be within 15%; otherwise, substantial lattice distortions occur
  • Crystal Structure: Must be the same for constituent metals
  • Electronegativity: A greater difference results in an intermetallic compound
  • Valence: The higher the metal’s valence, the higher the tendency to dissolve in another metal

Solid Solution

• Two or more elements dispersed in a single phase
  • Primary: Has the structure of the solvent material
  • Secondary: Characterized by a different crystal structure from that of the component elements
• Substitutional Solid Solution
  • Example: Copper and Nickel
    • Completely soluble in each other at all proportions
    • Atomic radii for copper and nickel are 0.128 and 0.125 nm
    • Have FCC crystal structures
    • Electronegativities are 1.9 and 1.8
    • Copper +1 (sometimes +2) and nickel +2 for common valence
• Interstitial Solid Solution
  • Interstitial atoms fill the voids of the host atoms
  • Interstitial atoms are substantially smaller than the host atoms
  • Concentration of interstitial impurity atoms is low, less than 10%
  • Example: Iron and Carbon (Steel)
    • Carbon forms an interstitial solid solution when added to iron
    • Maximum carbon concentration of ~2%
    • Atomic radius of carbon (0.071 nm) vs iron (0.124 nm)

Composition

• Amount of impurity (B) in the host system (A)
• Composition can be in wt% or at%

Linear Defects - Dislocations

• A dislocation is a linear, one-dimensional defect around which some of the atoms are misaligned
• Edge Dislocation: A half-plane of atoms terminating within the crystal
  • Distortion around the dislocation line, with tension on one side and compression on the other
• Screw Dislocation: Caused by a shearing force (formed by shear stress applied to produce distortion)
  • Looks like a spiral ramp
  • A portion of the crystal is shifted by one atom
  • Distortion is parallel to the dislocation line

Mixed Dislocation

• Features both edge and screw dislocations

Miscellaneous Interfacial Defects

• Stacking: A break in the ABC layering in an FCC crystal
• Phase Boundaries:

Bulk or Volume Defects

• Other defects in all solid materials are larger than those previously discussed
• Includes pores, cracks, foreign inclusions, and other phases
• Introduced during processing and fabrication steps

Introductory Concepts

• In service, materials are subjected to various forces/loads
• Knowing material performance limits before failure is important
• Material performance is designed for by its mechanical properties
• Mechanical properties are ascertained from laboratory tests

Geometry and Performance

• Need a material that withstands high stress, meaning it is very strong
• Add more wires to withstand the object's heavy weight

Stress and Strain

• Stress - the application of a load/force on an area
  • σ = F/A
• Strain - the amount of deformation from an applied force.
  • ε = lₓ

Deformation

Elastic Deformation

• When a force is applied to a member, it deforms proportionally to the force applied
• Piece returns to its original shape when force is released
• A non-permanent, reversible deformation
• Polymers tend to show this behavior, but metals/ceramics do not as much

Plastic Deformation

• When strain is no longer proportional to the applied force and non-recoverable deformation occurs
• A permanent deformation
• Most materials are used in plastic deformation