Defects of crystalline materials Flashcards
Point defects
Position of atoms
Vacancies
Self intersitials
Impurities in solid (substitutional and intersitial)
Linear defects
one dimensional defects
edge dislocations
Screw dislocations
Vacancies
Atom missing from lattice site
crystalline materials may contain many vacancies which increases the randomness of the crystal lattice
How do vacancies influence material properties
Controls rate of diffusion of atoms in crystals
slows down the flow of electrons in a material
controls propagation of the edge dislocations
Self interstitial defects
Atom crowded into an interstitial site
An atom at interstitial site substantially larger then space causing distortions in the surrounding lattice
rare in comparison to vacancies
Impurities in solids-> alloys
Impurities are intentionally added to modulate the properties of the material
ex. Cu added to Au and Ag,
Cu-Zn brass
Cu-Sn bronze
Cr added to Fe to make steels stainless
How does adding impurity atoms affect metal atoms
Forms a solid solution
What are solid solutions made of?
Solvent: Major Concentration
Solute: Minor Concentration
Rules of solid solubility
atoms must have a similar atomic radii (+_15%)
similar electronegativity
similar crystal structure
Same charge
solute (impurity) with higher valence dissolves more easily in solvent atoms
increased temp increases solubility
Substitutional impurities
Solute /impurity replaces or substitutes host atom
Interstitial Impurities
Solute/Impurity fills voids or interstices among host atoms
Interstitial atoms smaller than host atoms
What happens if the interstitial atom is larger than the interstitial space?
Lattice strains on adjacent host atoms
What are linear defects or dislocations?
-One-dimensional defects where some atoms are misaligned
What happens when load is applied to linear defects
movement of dislocations causes plastic deformations
How does the type movement of dislocations effect a material
Free movement- Ductile (metals)
Restricted- Brittle (Ceramic)
What happens when dislocations move to the grain boundaries
material breaks
Edge dislocations
Extra half-plane of atoms which distorts the nearby planes of atoms
Where are edge dislocations located
perpendicular to the line of dislocation
How do edge dislocations affect grain boundaries?
Moves to grain boundaries as a result of applied shear stress
Causes plastic deformation and ultimately fracture
Screw dislocations
A shift of atoms parallel to the line of dislocation
Looks like steps in a crystal structure
moves in a spiral or helical path due to unzipping motion
Mixed dislocations
Combination of both edge and screw dislocations
How do dislocations affect metals
Dislocations move easily due to non-directional metallic bonds
Metals are more ductile and break easily
How can you heal dislocations of metals
Annealing
makes material soft and ductile
How do dislocations affect ceramics?
Ceramics are covalently or ionically bonded
highly directional strong bonds
restricted movements of dislocations
Brittle
Interfacial defects
two-dimensional defect that occurs between boundaries
boundaries: two separate regions in a material with different crystal structures
What are the areas where interfacial defects can occur?
External surfaces
grain boundaries
phase boundaries
How do impurities at a grain boundary affect a material?
Makes the material brittle
