Metals and Alloys

Question 1

1. Define metal, metalloid and alloy: METAL

Answer 1

any of a class of chemical elements … generally characterized by ductility, luster, conductivity of heat and electricity

Question 2

2. Recognize the relevance of the relationship between structure and properties

Answer 2

see pic

Question 3

4. List the methods of forming and shaping metals

Answer 3

Casting Machining Cold Working Powder Metallurgy

Question 4

5. Describe metallic bonding

Answer 4

• Primary/Strong bonding - Metallic Bonding
• Positive ions + surrounded by ‘cloud’ of electrons
• Electrons can move in the cloud

Question 5

6. List the consequences of metallic bonding (4)

Answer 5

High Conductivity(thermal and electrical) -movement of electrons

• Opacity to light -electrons absorb electromagnetic and photon energy
• Easy ability to form alloys
• High plasticity

Question 6

7. Define crystallinity

Answer 6

shows a long-range regular arrangement of atoms

–Contrast this to an amorphous –glassy-material which has no such regularity. All metals are crystalline

Question 7

8. Define unit cell

Answer 7

smallest repeating unit in a crystal (consider 3 out of a possible 14)

Question 8

10. List the various types of crystalline imperfection

Answer 8

–point defects

–line defects

–[plane defects]

Question 9

9. Identify bcc, fcc and cph (or hcp) unit cells

Answer 9

see pic

Question 10

15. Describe the mechanism of plastic strain of metals in terms of movement of dislocations

Answer 10

Plastic strain (permanent) -slip of layers of atoms over each other. A metal is soft if the dislocations can be easily moved through the crystal structure while applying force to the material.

Question 11

11. Identify point, line and planar defects

Question 12

12. Draw a typical temperature-time curve for the cooling of a metal

Answer 12

see slides

Question 13

13. Draw and describe grain structures of the following types: equiaxed coarse grain, equiaxed fine grain and fibrous structure

Answer 13

fine grain leads to harder, stronger more dense.

not really sure what else i was supposed to take away from this

Question 14

20. Illustrate the structure of an interstitial solid solution, and quote an example of this

Answer 14

Steel C atoms in Fe crystal structure

Question 15

21. Describe the phenomena that occur in the solidification of an alloy where there is partial solid solubility

Answer 15

On cooling further under specific conditions, ‘precipitation’ of a second and different solid phase may occur

Question 16

16. Describe the effect of cold working, stress relief anneal, recrystallization and grain growth on the structure of a metal

Answer 16

fibrous grain

Question 17

25. List the dental applications of alloys

Answer 17

Metals are strong and tough. Polymers are not so strong and ceramics are too brittle. Metals in mouth must be biocompatible - resist corrosion and no release of metallic ions.

Question 18

1. Define metal, metalloid and alloy: METALLOID

Answer 18

exhibit both metallic and non-metallic behavior (Si, As, B)

Question 19

1. Define metal, metalloid and alloy: ALLOY

Answer 19

a mixture of metals - almost everything we use in dentistry is an alloy, we dony use metals

Question 20

4. List the methods of forming and shaping metals: CASTING

Answer 20

Pouring liquid metal into mold

Question 21

4. List the methods of forming and shaping metals: MACHINING

Answer 21

Cutting metal: drilling, turning, …

Question 22

4. List the methods of forming and shaping metals: COLD WORKING

Answer 22

Deforming metal in solid state: forging, extruding

Question 23

23. List the three principal factors that determine the hardness of an alloy

Answer 23

see slides

Question 24

24. Describe the effect of chemical composition, mechanical history and thermal history on the hardness of an alloy

Answer 24

see slides

Question 25

List the four possibilities when a molten alloy is solidified

Answer 25

Solid Solution - Interstitial (1)

Question 26

List the four possibilities when a molten alloy is solidified

Answer 26

Complete Solid Insolubility (2)

Question 27

List the four possibilities when a molten alloy is solidified

Answer 27

Partial Solid Insolubility (3) Once the initial solid solution is formed, on further cooling the alloy under specific conditions, precipitation of a second phase (same metals mixed in different quantities and with different crystal unit cell) occurs. The resulting alloy has grains of both phases. The alloy is stronger because the second phase pins the movement of the dislocations

Question 28

List the four possibilities when a molten alloy is solidified (4)

Answer 28

Intermetallic Compound formation

Question 29

11. Identify point, line and planar defects: POINT

Answer 29

see pic

Question 30

11. Identify point, line and planar defects: LINE

Answer 30

see pic

Question 31

11. Identify point, line and planar defects: PLANAR

Answer 31

see pic

Question 32

Properties of metal

Answer 32

Usually strong (hard) and stiff Tough - not brittle Lustrous - shine Dense Good conductors of electricity

Question 34

Point defects

Answer 34

instead of metal A atom in one position, metal B atom is there - substitiutional element b smaller than metal a, put in a cavity in between metal A's with ease - interstitial vacancies - missing atom

Question 35

line defects

Answer 35

dislocation in a crystal - see pic

Question 36

plane defects

Answer 36

as you cool the metal, orientation of the crystals is different and have different orientations leading to discontinuity in the boundaries

Question 37

Distinguish between elastic and plastic properties - metals ELASTIC

Answer 37

Elastic strain (recoverable) - stretch metal and interatomic bonds, after you remove force, returns to its structure Elasticity of metals depends mainly on the resistance of the metallic bond to be stretched. The metallic bond acts as a spring

Question 38

Modulus of elasticity mainly depends on the _____ nature of each metal

Answer 38

chemical

Question 39

distinguish between elastic and plastic properties of metals PLASTIC (slip of layers of atoms over each other)

Answer 39

Plasticity, i.e. permanent deformation of metals is due to the formation and movement of dislocations.

Question 40

The easier the dislocations move with no impediments in the crystal structure, the softer the metal is.

Answer 40

In order words, to strengthen a metal the movement of the dislocations should be made difficult.

Question 41

Metals are very tough because they can develop significant plasticity, i.e., metals can form and move dislocations. If dislocations are difficult to move,

Answer 41

the material will be more brittle, less tough. Ceramics are brittle because they can not form dislocations, they break bonds instead.

Question 42

Polymers have permanent deformation because they are viscoelastic, they have viscous component (disentanglementof molecules). Metals are not viscoelastic, \_\_\_

Answer 42

heir plasticity does not depend on the time you apply a load, but on the magnitude of the load.

Question 43

Pure metals are often too ___ and \_\_\_for dental use

Answer 43

soft and ductile

Question 44

Dental Significance of Metals Point #1 - Pure, untreated metals are usually too \_\_\_\_

Answer 44

soft/weak - gold, alumin, titanium, iron

Question 45

Dental Significance of Metals Point #2 - The strength of an alloy mostly depends on: (3 things)

Answer 45

composition, mechanical history and thermal history

Question 46

Dental Significance of Metals Point #3 The elasticity of a metal mostly depends on

Answer 46

the composition of the unalloyed metal

Question 47

Principles for selection of alloys

Answer 47

Question 48

Metallic Alloys used in dentistry (3)

Answer 48

Question 49

Applications of Alloys in Dentistry

Answer 49