Flashcards in Dental Materials Deck (293)
What are the 5 types of corrosion?
1. Galvanism: between 2 metals; amalgam and gold
2. Localised galvanism: between metals within same alloy
3. Crevice: differences in surface O2 levels; plaque coated and clean surface
4. Pitting: similar to crevice; damage to passive oxide layer
5. Stress: sustained force in corrosive environment
Why are metals more easily deformed than expected?
Due to defects in crystal structure
What are the 2 types of crystal defects?
What are the 3 types of point defects?
1. Vacancy: atom removed
2. Substitutional: atom replaced
3. Interstitial: atom between metal atoms
What are the 2 types of line defects?
1. Edge dislocation: extra plane of atoms
2. Screw dislocation: edge in 3D
How do metals deform?
By movements of dislocations
How do dislocations lead to permanent deformations?
Force above yield stress causes atoms in single row to break bonds w/ existing atoms and form new bonds 1 along
Causes dislocation to move along slip plane causing permanent deformation
Describe the effect of grain size on deformations of metals
Deformations halted by grain boundaries, another dislocation, impurities and point defects
Smaller grain size: less distance to travel, less possible deformation, more rigid
Explain how metals can be both ductile and brittle
Ductile: dislocations move more easily than cracks grow, deform plastically
Brittle: solid has dislocations but cracks grow at lower stress than that required for dislocation movement, will deform elastically
Define ductility and malleability
Ductile: withstand permanent deformation under tensile load w/o rupture; draw into wire
Malleable: withstand permanent deformation w/o rupture under compression; hammer into thin sheet w/o cracking
Describe work (strain) hardening/cold working
Repeated deformation (strain) moves existing dislocations and produces new dislocations
Inc. dislocation density hinders movement
Dislocations stack up @ grain boundary
Inc: yield stress, hardness
What is annealing?
Process by which effect of work hardening can be overcome by heating
Describe the 3 stages of annealing
Recovery: effects begin to disappear, stress relief
Recrystallisation: old crystal structure disappears, forms new crystal structure, occurs at temp ~50% MP
Mix of 2+ metals
Have better properties compared to pure metals
Have no single MP: melt/solidify over range of temps.
What are the liquidus and solidus temps. of alloys?
Liquidus (Tl): above all liquid, below liquid + solid
Solidus (Ts): above liquid + solid, below all solid
What are the 4 types of binary alloys?
1. Solid solution: metals soluble in each other; form single solid containing atoms of both
2. Completely insoluble: solidify as 2 separate metals; eutectic alloys
3. Partially soluble: eutectic + solid solution
4. Metals w/ particular affinity: intermetallic compounds; cementite, amalgam
Describe the 3 stages in the construction of a phase diagram for a binary alloy
1. Plot cooling curve of various ratios of 2 metals; measure Tl and Ts for each ratio
2. Plot Tl and Ts against % composition
3. Join all Tl and all Ts together to from liquidus and solidus line
What is alloy coring?
Between Ts and Tl composition of liquid and solid varies w/ temp
Cooling rapidly causes formation of layers of solid of different composition
How can coring be rectified?
Reheating to allow diffusion of atoms to give homogenous composition
Define solution and order hardening
Solution: differing atomic radii hinder movement of planes of atoms (along slip plane) relative to 1 and other
Order: rapid cooling to retain random solid solution (soft) structure then reheat and cool slowly to form superlattice (harder) - ordered solid solution
Explain precipitation hardening
Supersaturate 1 metal in the other then quench (fast cool), reheat to below Ts/within insoluble region then allow to cool slowly to cause precipitation of fine particles of other metal within metal
How does precipitation Harding work?
As presence of impurities halts deformation movements