solidification and cry

• Solidification


  Process of melted metals being cast (solidified in a mould) to produce finished and semi-finished shapes
• Solidification products


  • Semi finished cast aluminum alloy ingot
  • Cast finished product
• Solidification of metals


  1. Melted

  2. Cast (solidified in a mould)

  3. Converted to useful shapes and products using other metal forming operations such as rolling or forging
• Homogenous nucleation


  Metal itself will provide atoms to form nuclei, molten metal when undercooled will have slow moving atoms bond to form a small cluster (embryo), if cluster reaches critical size it grows into a crystal
• Homogenous nucleation


  • More nucleation sites results in more grains
  • Grains interact and form grain boundaries
• Volume free energy (ΔGv)


  Released by liquid to solid transformation
• Surface energy (ΔGs)


  Energy needed to create a new solid surface, opposes formation of nuclei

  
• Total free energy (ΔGT)


  Sum of volume free energy and surface energy, has a critical radius (r*) where it is minimised

  
• As undercooling (ΔT) increases


  Critical nucleus size decreases, smaller nuclei can become stable

  
• Heterogeneous nucleation


  Occurs on surfaces of container or insoluble impurities, lowers free energy required to form stable nucleus and critical size

  
• S(θ)


  Function of contact angle θ between nucleus and substrate
• Types of grains in heterogeneous nucleation

  • Equiaxed grains (smaller, grow equally in all directions)
  • Columnar grains (long, thin, grow predominantly in one direction)
• Grain refiners (Ti, B, Zr) added to aluminum alloys to produce cast ingots with fine grain size
• Single crystals


  Grown slowly so solid-liquid interface is slightly below melting point, have high temperature creep resistance and predictable electrical behavior
• Czochralski process


  Used to produce single crystal silicon for wafers, seed crystal dipped in molten silicon and slowly withdrawn as single crystal grows
• Alloy


  Metal containing two or more metals and sometimes nonmetals, can exist as single phase solid solution or multi phase mixture
• Point defects


  Zero dimensional crystal lattice imperfections including vacancies and interstitials
• Vacancy


  Missing atom, formed during solidification, plastic deformation or rapid cooling, allows for solid state diffusion
• Impurities in solids

  Foreign atoms in pure metals, intentionally added in alloys to impart specific characteristics
• Solid solution


  Solvent (parent element/compound) and solute (alloying element/compound) dispersed in a single phase
• Substitutional solid solution


  Solute atoms replace solvent atoms in crystal lattice
• Interstitial solid solution


  Solute atoms occupy spaces between solvent atoms, causing local strain
• Schotky imperfection


  Cation-anion di-vacancy in ionic crystals
• Frenkel imperfection


  Cation moves to interstitial site in ionic crystals
• Dislocations


  Line defects that create lattice distortions, formed during solidification, plastic deformation or vacancy condensation
• Types of dislocations


  • Edge (extra half plane of atoms)
  • Screw (spiral ramp distortion)
  • Mixed (combination of edge and screw)
• Interfacial (planar) defects


  Free surfaces, grain boundaries, twins, low/high angle boundaries, stacking faults, phase boundaries

  
• Volume defects

  Clusters of point defects forming 3D voids like pores, cracks, inclusions, other phases
• Atomic vibrations in solids can be considered imperfections or defects