Midterm 1(materials science)

CH2: 

  • Electron energy states

Electrons tend to occupy lowest available energy state

Each electron state can hold no more than 2 electrons that have opposite signs 

- the smaller the quantum number the lower the energy shell. 

- within each shell, larger Lnumbers, the higher the energy levels 

- Overlap may happen between the energy level of two adjacent shells. (especially for d and f) 


  • Valence electrons determine all of the following properties:
    - Chemical, electrical, thermal, and optical 

  • electropositive elements readily give up electrons to become positive ions. 

  • Electronegative elements readily acquire electrons to become negative ions 


  • Forces and Energies

  • Coefficient of thermal expansion.α is larger if E0 is smaller

  • Melting temperature Tm is larger if E0 is larger 

  • Modulus of Elasticity (E) is larger if E0 is larger 


  • Interatomic Bonding 

  • Primary: Generally associated with formation of molecules and characterized by strong atom-to-atom attractions that involve exchange of valence electrons (Ex: Ionic, covalent, metallic) 

  • Secondary: Generally associated with attraction between molecules and there is no transfer or sharing of electrons. (ex: induced dipole bonds, polar molecule bonds, and permanent dipole bonds.)



  • CH3

Crystalline materials: 

  • Atoms pack in periodic, 3d arrays 

  • typical of metals, many cermics, some polymers


Noncrystalline (Amorphous) materials:

  • Atoms have no periodic packing 

  • occurs for complex structures, rapid cooling


Metals tend to be densely packed. Why?

  • Metallic bonding is not directional 

  • Nearest neighbor distances tend to be small in order to lower bond energy

  • Electron cloud shields cores from each other 


Simple Cubic structures (SC) 

  • Rare due to low packing density ( ONLY Po Has this structure)

  • One complete atom in a unit cell 


Body-centered cubic (BCC) 

  • Atoms touch each other along cube diagonals 

  • 2 full volume of atoms/unit cell: 1 center+ (8 corners)*1/8 

  • Examples: Cr, W, Fe, Ta, Mo


Face centered Cubic (FCC) 

  • Atoms touch eacher along face diagonals 

  • Ex: Al, Cu, Au, Pb, Ni, Pt, Ag


  • Single crystals

Properties vary with direction: Anisotropic-the property of having physical properties that differ depending on the direction they are measured in


  • Polycrystals 

Properties may or may not very with direction 

If grains are randomly oriented: Isotropic 

If grains are textured then its anisotropic 


  • CH4


Solidification-result of casting of molten material happens in 2 steps:

1) Nuclei form 

2) Nuclei grow to form crystals (grain structure) 


Polycrystalline Materials

  • Low density in grain boundaries (High mobility, diffusity, and chemical reactivity) 

  • Grains can be:

- Equaixed- roughly the same size

- Columnar (elongated grains)


THERE NO IS NO SUCH THING AS A PERFECT CRYSTAL!

1) Point defects: Vacancy, interstitial, and substitutional atoms 

2) Line defects (1D): Dislocations 

3) Area defects (2D): grain boundaries 

4) Volume defects (3D): 3D pores and voids 


Vacancies: Vacant atomic sites in a structure 

Self-interstitials: “Extra” atoms positioned between atomic sites 


  • Dislocations: 

are line deftcs

slip between crystal planes result when dislocationsmove

produce permanent (plastic) deformation 


  • Linear Defects 

Are one-dimensional defects around which atoms are misaligned 

ex: Burger’s vector, Edge dislocation, Screw dislocation, and Mixed 


  • Edge dislocation: Extra half plane of atoms inserted in a crystal structure 

- b is perpendicular to dislocation line 

- Under shear, dislocation moves in the direction of the shear: Perpendicular to dislocation line and parallel to b


  • Screw dislocation: Spiral ramp resulting from shear deformation 

- b parallel to the dislocation line 

-Under shear stress, dislocation moves perpendicular to dislocation line and b


  • Burger’s vector: Measure of lattice distortion 


  • Optical microscopy: 

- Useful up to 2,000x magnification 

- Polishing removes surface features 

- Etching changes reflectance, depending on crystal orientation 


  • CH5

  • Diffusion: mass transport by atomic motion 

  • Mechanisms 

Gases and liquids- random (Brownian motion) 

Solids-vacancy diffusion or interstitial diffusion 


  • Self-diffusion: In an elemental solid, atoms migrate 

  • Interdiffusion: In alloys, tend to migrate from regions of high concentration to low concentration 


1) Vacancy diffusion: Atoms exchange w/ vacancies, applies to substitutional impurities atoms, and depends on number of vacancies and activation energy to exchange 


2) Interstitial diffusion- smaller atom can diffuse between atoms 


  • Case hardening: 

Diffuse carbon atoms into the host iron atoms at the surface