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Flashcards in Dental Materials Deck (293)
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271

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

272

Why are metals more easily deformed than expected?

Due to defects in crystal structure

273

What are the 2 types of crystal defects?

1. Point
2. Line

274

What are the 3 types of point defects?

1. Vacancy: atom removed
2. Substitutional: atom replaced
3. Interstitial: atom between metal atoms

275

What are the 2 types of line defects?

1. Edge dislocation: extra plane of atoms
2. Screw dislocation: edge in 3D

276

How do metals deform?

By movements of dislocations

277

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

278

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

279

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

280

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

281

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
Dec: ductility

282

What is annealing?

Process by which effect of work hardening can be overcome by heating

283

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
Grain growth

284

Describe alloys

Mix of 2+ metals
Have better properties compared to pure metals
Have no single MP: melt/solidify over range of temps.

285

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

286

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

287

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

288

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

289

How can coring be rectified?

Homogenisation
Reheating to allow diffusion of atoms to give homogenous composition

290

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

291

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

292

How does precipitation Harding work?

As presence of impurities halts deformation movements

293

Why are constitutional phase diagrams important?

Understanding various heat treatments used for dental alloys
Controlling microstructure of alloys and properties