Imperfections in Solids

• 0D Point defects

  atoms missing or in irregular places in the lattice
• defects

  have a profound impact on the properties of materials
• thermomechanical processing

  a metallurgical process that combines mechanical or plastic deformation process like compression or forging, rolling etc. with thermal processes like heat treatment, water quenching, heating and cooling at various rates into a single process
• self-interstitials (metals)

  introduce large distortions in the surrounding lattice
• equilibrium concentration of self-interstitials is very low; less than one self-interstitial per cm3 at room temperature
• electroneutrality

  state that exists when there are equal numbers of positive and negative charges from the ions; defects in ceramics do not occur alone
• nonstoichiometric (ceramic) happens when there is any deviation from the exact ratio of cations to anions; occurs for some ceramic materials in which two valence (or ionic) states exist for one of the ion types
• solute (alloy)

  element or compound present in lesser amount
• weight percent (wt %)

  weight of a particular element relative to the total alloy weight
• "partial“ dislocations have their cores spread out over a larger area
• solid surfaces

  “reconstruct” to satisfy atomic bonds at surfaces
• deformation of materials

  produces low-energy twin boundaries with mirrored atomic positions across boundary
• cracks

  greatly affect mechanical properties
• melting

  vibrations are sufficient to rupture bonds
• atomic vibration

  caused by heat; amplitude increases with temperature

• atomic defects

  responsible for reductions of gas pollutant emissions
• catalytic converter

  pollutant-reducing device located in automobile's exhaust system
• high-resolution transmission electron micrograph

  shows surface defects on single crystals used in catalytic converters
• ceramic monolith

  metallic catalyst substrate is deposited
• alloying

  adding impurity atoms
• crystalline defect

  lattice irregularity having one or more of its dimensions on the order of an atomic diameter
• classification of crystalline imperfection

  according to geometry or dimensionality of defect
• presence of vacancy

  increase entropy of crystal
• vacancy

  a lattice position that is vacant because the atom is missing
• processing

  determines the defects
• number of vacancies increases exponentially with temperature
• self-interstitial has very small concentrations which is significantly lower than vacacies
• pure metal consisting of only one type of atom is impossible
• alloying in metals

  improve mechanical strength and corrosion resistance
• formation of solid solution

  addition of impurity atoms to a metal
• solid solution


  compositionally homogeneous; impurity atoms randomly and uniformly dispersed within the solid
• interstitial solid solutions

  impurity atoms fill the voids and interstices among the host atoms
• metallic materials have relatively high atomic packing factor and relatively small interstitial positions
• atomic diameter of an interstitial impurity must be smaller than host atoms
• express composition or concentration of an alloy in terms of its constituent elements
• dislocation

  a linear or one-dimensional defect around which some of the atoms are misaligned
• edge dislocation

  linear defect that centers on the line
• screw dislocation


  formed by shear stress
• dislocation line

  defined along the end of extra half-plane of atoms
• screw dislocation

  linear and along dislocation line