Metals

Question 1

What qualities do metals have?

Answer 1

1. elecrtopositive element
2. luster
3. good thermal and electrical conductors
4. high fracture toughness
5. stronger, more ductile, denser than non-metals

Question 2

Alloy

Answer 2

1. mixture of 2 or more metals

2. sometimes mixed with non-metals

Question 3

Classification of Metals

Answer 3

1. elemental vs. alloy (alloys most common in dentistry)
2. cast vs. wrought
3. noble vs. non-noble
4. application

Question 4

What are the major crystal systems of metals used in dentistry?

Answer 4

1. face centered cubic
2. body centered cubic
3. hexagonal close-paced

Question 5

Noble Metals in Dentistry

Answer 5

1. Palladium
2. Ruthenium (used as a grain refiner)
3. Osmium
4. Gold
5. Rhodium
6. Iridium (used as a grain refiner)
7. Platinum

**Gold, palladium, and platinum are the three MAJOR

Question 6

Base Metals

Answer 6

1. NOT noble
2. ex: titanium, cobalt, silver, zinc, gallium, tin, aluminum
3. in pure form, greater tendency to corrode in the mouth compared to noble metals

Question 7

High Noble Alloys

Answer 7

1. gold content greater or equal to 40 wt%

2. noble metal is the remainder

Question 8

Titanium and Titanium Alloys

Answer 8

titanium greater or equal to 85 wt%

Question 9

Noble Alloys

Answer 9

noble metal content greater or equal to 25 wt%

Question 10

Predominantly Base Alloys

Answer 10

noble metal content less than 25 wt%

Question 11

Should “precious” be used to describe noble?

Answer 11

NO!!

Question 12

ADA Type I Alloy

Answer 12

1. soft
2. < 140 MPa tensile strength
3. 18% elongation
4. low sustainable stress, no occlusion
5. used for inlays

Question 13

ADA Type II Alloy

Answer 13

1. medium
2. 140-200 MPa tensile strength
3. 18% elongation
4. moderate sustainable stress, light occlusion
5. used for onlays and inlays

Question 14

ADA Type III Alloy

Answer 14

1. hard
2. 201-340 MPa
3. 12% elongation
4. high sustainable stress, full occlusion
5. used for crowns, short-span fixed partial dentures

Question 15

ADA Type IV Alloy

Answer 15

1. extra-hard
2. > 340 MPa tensile yield strength
3. 12% elongation
4. very high sustainable stress
5. thin veneer crowns, long-span fixed partial dnetures, removable partial dentures

Question 16

Homogeneous Nucleation: Pure Metal

Answer 16

1. no impurities/clean container
2. solid forms from liquid, sometimes requires “super-cooling”
3. have a melting POINT

Question 17

Review Phase Diagrams

Answer 17

DO IT!!

Question 18

Heterogeneous Nucleation: Pure Metal

Answer 18

1. walls of container or impurity or impurity particles catalyze nucleation
2. have a melting POINT

Question 19

Alloys

Answer 19

1. exhibit a melting range

2. some very specific alloy compositions have a melting point

Question 20

Rapid vs. slow cooling of metals

Answer 20

1. rapid cooling=more nuclei=smaller grains

2. slow cooling=fewer nuclei=larger grains

Question 21

Grain Refiners

Answer 21

1. method to reduce grain size
2. add <1 wt% of Iridium (Ir), ruthenium (Ru), or rhenium (Re) to noble casting alloys
3. finer grain size=increased yield strength, composition uniformity, corrosion resistance

Question 22

Dendritic Metal Structure

Answer 22

1. dendrites: formed through thermal supercooling

2. dendrites growth is along specific crystallographic directions

Question 23

Solid Solutions

Answer 23

1. incorporation of “foreign” atoms (solute) into crystal structures of matrix atoms (solvent)
2. may be a wide range of composition

Question 24

Substitutional solid solution

Answer 24

one atom is replaced with another atom

Question 25

Interstitial solid solution

Answer 25

atoms are fit in amongst the other atoms

Question 26

Disordered vs. ordered solid solutions of metals

Answer 26

disordered=stronger

Question 27

What does the solubility of a solid solution of metals depend on?

Answer 27

1. size * *solubility decreases as size differences increases 2. crystal structure * *greater solubility if same crystal (ex: FCC) 3. Valence * *greater solubility if same (ex: both +2) 4. chemical reactivity

Question 28

Intermetallic Compounds

Answer 28

differ from solid solutions in that they exist as fixed atomic ratios

Question 29

Point Defects

Answer 29

1. substitution or interstitial atoms: depends upon size of atoms 2. vacancies: vacant lattice site 3. self-interstitial: matrix atom in an interstitial site

Question 30

Dislocations

Answer 30

1. crystals are rarely ideal 2. sometimes atoms are missing or extra half-planes of atoms are formed 3. ex: line imperfections in crystalline solids 4. hinder dislocation movement=increase strength

Question 31

Mechanisms for strengthening metals

Answer 31

1. solid solution strengthening 2. strain or work hardening 3. precipitation hardening 4. transformation strengthening

Question 32

Solid solution strengthening of metals

Answer 32

1. interstitial solute atoms have a greater effect in modifying material behavior 2. greater effect if solute atoms are ordered 3. important factors * *size of solute atom * *relative nodulus of the two types of atoms * *electronic, chemical, and configurational interaction

Question 33

Solid solution strengthening of metals: __strength and hardness, __ductile than either pure metal

Answer 33

1. higher strength and hardness | 2. less ductile than either pure metal

Question 34

Solid solution strengthening of metals: melt __highest melting point of pure metals

Answer 34

melt below highest melting point of the pure metals

Question 35

Solid solution strengthening of metals: __corrosion resistance than multi-phase alloys

Answer 35

higher corrosion resistance than multi-phase alloys

Question 36

Strain hardening/cold working: strength/hardness __, ductility __

Answer 36

strength and hardness increase, ductility decreases 1. plastic deformation=more difficult to deform/shape more 2. increase in number and interaction between dislocations 3. must re-heat to regain workability

Question 37

Manufacturing orthodontic wires

Answer 37

1. cast as ingot 2. series of mechanical reduction (grains become flattened, elongated) 3. series of wire-drawing steps **heat tx's are done during the stages to eliminate work hardening

Question 38

Annealing

Answer 38

1. heat tx used to soften metals and refine their grains | 2. recrystallization=new grains nucleate above a certain temperature

Question 39

Steps in annealing

Answer 39

1. original microstructure 2. cold-worked 3. new grains nucleate at recrystallization temperature 4. grain growth above recrystallization temp 5. annealing complete (equiaxed structure) 6. further grain growth if heated longer

Question 40

Precipitation hardening of metals

Answer 40

1. solute atoms not dissolved, but form separate second phase particles dispersed in matrix 2. dislocations impeded by precipitates * *have to bow between and around (if precipitate is stronger than matrix)

Question 41

Transformation strengthening of metals

Answer 41

1. change in morphology of lattice structure (occurs upon heating or cooling) 2. ex: FCC to BCC