Physical Metallurgy Notes

DEFINITION AND OBJECTIVES OF PHYSICAL METALLURGY; THE CONCEPT OF STRUCTURE

1.1. Definition and Objectives of Physical Metallurgy

• Definition: Physical Metallurgy is an interdisciplinary field based on fundamental knowledge of mathematics and physics.

• Principal Objective: To study the relationship between chemical composition and properties, with structure acting as the link.

  $COMPOSITION \xrightarrow{} STRUCTURE \xrightarrow{} PROPERTIES$

• Objectives:

  1. Creating Traditional Materials: To create known materials with general uses, ensuring reproducible properties through established technologies.
  2. Providing Operational Forecasts: To simulate severe operating conditions through laboratory tests, predicting material behavior under stress.
  3. Creating New Materials: To establish the necessary chemical composition for materials with specific properties, using mathematical modeling.

1.2. The Concept of STRUCTURE; Levels of Definition

• Definition: STRUCTURE represents the internal organization of a material, its internal architecture.
• **Levels of Definition: **
  • A. Subatomic, Atomic & Interatomic Structure:
    • Focuses on the atom's composition and electron arrangement.
    • The study at this level primarily concerns physicists and chemists.
    • Understanding energy bands facilitates the definition of semiconductor materials.
    • Semiconductors can behave as conductors or insulators under specific conditions, revolutionizing electronics.
  • **B. Crystalline Structure
    **
  • **C. Microscopic Structure
    **
  • **D. Macroscopic Structure
    **

Interatomic Bonds

• Refers to how atoms interact.
• Types:
  • **Low energy (less stable): **
    • Van der Waals forces
    • Hydrogen bonds
  • **High energy (very stable): **
    • Metallic bond
    • Ionic bond
    • Covalent bond

Metallic Bond (specifics)

• A non-saturated, homeopolar bond where ions are in fixed positions and electrons gravitate around them, forming an electron cloud.
• Electrons ensure stability by continuously moving around any ion.
• Many metallic properties are explained by the metallic bond:
  • Ductility: Plastic deformation capacity due to less rigid bonding.
  • Opacity: Metals absorb light quanta, exciting valence electrons, which then emit visible light, giving metals their characteristic luster.
  • Poor corrosion resistance: Valence electrons easily leave the atom, contributing to oxidation reactions.
  • Electrical and thermal conductivity: High mobility of valence electrons.

Ionic Bond (specifics)

• A saturated, heteropolar bond with alternating positive and negative ions.
• Typical in ceramic materials.
• Properties are explained through this bond:
  • High hardness and brittleness due to rigid, spatially oriented bonding.
  • Thermal and electrical insulator due to lack of free electrons.
  • High chemical resistance due to structural stability.

Covalent Bond (specifics)

• Similar to ionic bonds, it is saturated, either hetero- or homopolar, formed by sharing electrons to achieve stability.
• Electron sharing creates closed or open systems.
  • The number of shared electrons follows the 8-N principle (N = group number).
    • Example: Hydrogen (H=H) shares 1 electron (the single exception).
    • Example: Oxygen (O=O), Oxygen belongs to group VI $8-6=2$
    • Example: Nitrogen (N=N), Nitrogen belongs to group V $8-5=3$
  • Closed System:
    • Example: Diamond molecule, formed from 4 carbon atoms in a regular tetrahedron.
    • Each carbon atom forms 4 covalent bonds.
  • Open System:
    • Example: Polymers, where the structural backbone is a long chain of carbon atoms.
    • This allows the molecule to grow indefinitely, with polymers potentially containing $10^3$ - $10^6$ carbon atoms.
    • A polymer’s structural unit is a monomer.
    • Polymerization forms the polymer.
    • Weaker Van der Waals forces can exist between monomers instead of covalent bonds.
    • Many polymer properties are due to these weaker bonds:
      • Electrical and thermal insulators (up to $T ≤ 200°C$, the melting temperature of most polymers).
      • Chemical stability, but less so in organic solvents.
      • Low melting points and instability in organic solvents are due to Van der Waals forces between monomers.

Crystalline Structure. Methods and Means of Investigation

• Provides information on a scale of $10^{-8}$ – $10^{-10}$ m (nanotechnologies!).
• At this level, atoms (or molecules) organize into structural units that repeat infinitely in three-dimensional space, forming a perfect, regular, CRYSTALLINE structure.
• In contrast, an AMORPHOUS structure has high disorder (similar to liquids).
• Metals generally adopt a crystalline structure, while ceramics tend to be amorphous.
• Polymeric materials usually have partially crystalline, partially amorphous structures.
• The structure adopted in the solid-state depends on the characteristics of the liquid state: a more fluid liquid state favors crystallization, while a more viscous state favors amorphization.
• Thermodynamically, the crystalline structure is considered more stable.
• Considering the crystalline structure's characteristics (perfectly ordered in space) and representing atoms by their centers of gravity gives a CRYSTALLINE NETWORK, which can be represented as above.

Interplanar Distance

• $d$ = interplanar distance, measured in Å (angstroms).

• $1 Å = 10^{-10} m$

• The interplanar distance $d$ is stable, as distance between the centers of gravity of two consecutive atoms, a material characteristic (atomic phase is specific to an element in the periodic table).

• Once established (calculated), it is the primary identifier of the crystalline network and thus the material.

• Investigation uses diffractometric methods (X-ray diffraction), sending incident radiation that penetrates the crystalline material until it intersects an atom.

• The atom diffracts the radiation ($D$).

  • $I$ = incident radiation
  • $D$ = diffracted radiation
  • $\theta$ = angle formed by the extension of the incident radiation with the diffracted radiation

Wulff-Bragg Law

• Applying the diffraction law (WULFF BRAGG), the interplanar distance $d$ can be determined:

  $nλ = 2d sin θ$

  • where:
    • $n$ = diffraction order ($n=1$)
    • $\lambda$ = wavelength of incident radiation, in Å

• Note:

  • The wavelength $\lambda$ should be comparable to $d$. In practice, a metal is excited by a current source until it emits radiation.
  • The wavelength is on the same order of magnitude as $d$.
  • Common metals:
    • $Cu oby λ_{CuKα} = 1.54 Å$
    • $Mo oby λ_{MoKα} = 0.71 Å$
  • Therefore:

  $d = \frac{λ}{2 sin θ}$

• The interplanar distance, therefore, serves as the identifying element of the crystalline network.

• The method used is DIFFRACTOMETRY, and the instrument is a DIFFRACTOMETER.

• Diffractometry is the most effective method for identifying the crystalline network of a crystalline material and thus the surest way to establish a material's nature.

C. Microscopic Structure - Methods and Means of Investigation

• Provides information on a scale (level) of $10^{-6}$ - $10^{-3}$ m.
• At this structural level, crystalline particles CRYSTALLITES associate to form a granular structure whose structural element is the crystalline grain (granule).
• The study's objective is to observe the formation, distribution, nature, and orientation of crystalline grains.
• Investigation methods are microscopic methods:
  • Optical microscopy
  • Electron microscopy

Optical Microscopy

• Studies are done via reflection (metals are opaque), and the apparatus used is the optical microscope, which uses light radiation ($\lambda = 0.5 - 10^{-6} m$).

• For a surface to reflect light radiation, it must have a mirror-like character.

• Samples of metallic materials are prepared so that the surface is suitable for analysis.

• They are pre-polished until a mirror surface is achieved and then etched with a chemical reagent to reveal the structure.

• The optical microscope consists of an optical system with two detachable lenses:

  • Objective lens: Forms the magnified image by a certain number of times
  • Ocular lens: Visualizes the magnified image, magnified by a certain number of times

• The magnification power M of the optical microscope will be:

  $M = M<em>{ob} * M</em>{oc}$

• Note: Optical microscopy investigations are performed between 100x and 1000x because there is an inverse proportionality relationship in microscopy between the magnification power and the wavelength of the incident radiation:

  $M \sim \frac{1}{λ}$

• Since $\lambda$ is fixed, M cannot be increased. This can only be achieved through electron microscopy.

Electron Microscopy

• Provides images at much higher magnifications (M = 2000 – 1,000,000x).
• Uses a more complex electron microscope than the optical microscope.
• The incident radiation is an electromagnetic radiation with much smaller $\lambda$
• Made of a wolfram filament combined with magnetic fields forms a fascicle of electrons.

Macroscopic Structure

• Refers to the overall view analyzed through macroscopic analysis

  • A. Macro-analysis

    • A method of studying a material's surface with the naked eye or a magnifying glass.

    • Surfaces analyzed via macro-analysis are analyzed from multiple points of view.

      • On natural surfaces.
        • Morphology.
        • Fractures.
        • Mechanical traces.
      • On corroded surfaces
      • On fracture surfaces
        • Determining if a material has ductile or brittle rupture.
        • On surfaces from accidental rupture
          • locating crack initiation the generated defects.

    • Macro-analysis on specially prepared surfaces

      • Surfaces are prepared similarly to microscopic analysis, but chemical reagents are more potent to observe non-homogeneities with the naked eye.
        • Structural non-homogeneity.
        • Material discontinuities.
        • Processing non-homogeneity
        • Example: welding.