BIOM_metals_alloys

Question 1

Define an alloy

Answer 1

Mixture of metals

Question 2

Properties of metals

Answer 2

Strong (hard) 
Tough not brittle
Lustrous 
Dense
Conduct heat
Conduct electricity
Opaque - (bc electrons absorb electromagnetic energy)
Ductile 
Malleable

Question 3

What type of bonds exist in metals

Answer 3

Metallic bonds - free electrons shared by all metallic ions in structure

Question 4

Why are metals good conductors of heat and electricity

Answer 4

It is a direct consequence of the freedom of movement of the electrons

Question 5

Metals are made of crystals and have three common patterns of basic repeating units. What are these 3?

Answer 5

Bcc- body centered cubic
FCC - face centered cubic
HCP - hexagonal close packed.

Question 6

What are the 3 diff types of imperfections that can occur in the crystalline structure of metals

Answer 6

Point defects
Line defects
Plane defects

Question 7

With a point defect, impurities can be present in 2 different “fashions” or “positions” - what are they

Answer 7

Substitutional

Interstitial

Question 8

The ease of having impurities incorporated into the crystal lattice structure justifies that metals can be easily turned into _________ .

Answer 8

Alloys

Question 9

Line defects appear as dislocations in the structure - describe this phenomenon. by the formation of an extra semiplane of atoms in the metal structure. Below the extra semiplane there is a line of defect where the semiplane does not continue.

Answer 9

Slide 12. by the formation of an extra semiplane of atoms in the metal structure. Below the extra semiplane there is a line of defect where the semiplane does not continue.

Question 10

when a metal solidifies - the crystals orient and join together forming a conglomeration of “grains” - this multi-granular “situation” creates a specific crystal imperfection called a _________ defect.

Answer 10

plane

Question 11

a metal with smaller grains (crystals) is stronger or weaker????

Answer 11

stronger - bc has more grain boundries per volume - i.e. more plane defects per volume

Question 12

what 2 mechanical properties of metals did we discuss?

Answer 12

elastic properties

plasticity/toughness

Question 13

elasticity (modulus of elasticity) of metals depends on:

Answer 13

how stretchable the metallic bond is (or in other words how much resistance it gives) - the metallic bond creates resistance and acts like a spring. Strong spring = lots resistance

Question 14

define crystalline material

Answer 14

material that has a LONG_RANGE regular arrangement of atoms. in contrast to amorphous where there is no regularity

Question 15

why are metals considered “polycrystalline”

Answer 15

bc they are an agglomerate of crystals (“grains”).

Question 16

what type of arrangement or “structure” is obtained when you “cold work” a metal

Answer 16

fibrous grain structure is obtained by cold working

Question 17

what is happening on an atomic level when metals undergo “plastic strain”

Answer 17

layers of atoms are slipping past each other.
“moving” / caterpillar analogy
- the easier it is to create a displacement then the easier it will be to perform a dislocation.
- metal plasticity = movement of dislocations
- soft metal = easy dislocation = easy plastic deformation
- hard metal = hard to dislocate = brittle / less tough

Question 18

we can treat or process metals that make if difficult to perform movements or dislocations. the result will be a (stronger or weaker) material.

Answer 18

stronger

Question 19

4 types of “processing” metals and a few details about the products you get when using each method

Answer 19

1) casting - “equiaxed structure”
2) cold working - intros more dislocations - leads to fibrous structure - too much can lead to a brittle alloy
3) recrystallization - erases all deformations - leads to a softer material
4) grain growth - larger grains formed within the same volume of material which makes dislocations easier…. softer material

Question 20

metals are very tough bc they can…..

Answer 20

form and move dislocations

Question 21

why are ceramics brittle (with respect to dislocations)

Answer 21

bc they can NOT form dislocations - they break their covalent bonds instead

Question 22

polymers have permanent deformation bc they are ___________ (special “property”).

Answer 22

viscoelastic - the viscous component creates disentanglement of molecules. metals are NOT viscoelastic - their plasticity does not depend on the time you apply the load, but on the magnitude of the load.

Question 23

alloying a metal makes the metal (stronger or weaker)…. WHY????

Answer 23

stronger bc the “impurities” make it more difficult to have a movement of dislocation (but not too difficult because then it becomes too brittle). This is why we don’t use PURE metals.

Question 24

There are 4 types of solid alloys…. one of which is not important to dentistry (complete solid insolubility). SO, what are the other 3.

Answer 24

1) solid solution
2) partial solid insolubility
3) intermetallic compounds

Question 25

solid solution alloy - details

Answer 25

perfect solubility
only 1 phase present
two types: substitutional & interstitial

Question 26

substitution solid solution is formed when the two metals in the alloy have the same ________ ________ and similar ________ ______.

Answer 26

same crystal lattice (fcc, bcc, hcp) / similar atomic size ( — see slide 24 for a picture

Question 27

whats a dental example of substitutional solid alloy solution

Answer 27

mixtures of gold, copper, silver, platinum, palladium

Question 28

interstitial solid solutions - details

Answer 28

example: steel (has carbon atoms jammed btw the atoms of iron - interstitially. photo slide 26

Question 29

partial solid insolubility

Answer 29

chocolate chip cookie analogy
different phases with same metals mixed in different quantities and different crystal units. the resulting alloy has grains of both phases - the alloy is stronger bc the 2nd phase impedes the movement of dislocations. creates a harder and more brittle alloy

Question 30

intermetallic compound alloys

Answer 30

compounds of defined SPECIFIC composition/stoichiometry - example: mercury amalgam

Question 31

the strength of an alloy mostly depends on 3 things

Answer 31

1) compositon (alloying strengthens metals)
2) mechanical history (review slide 20)
3) thermal history ( review slide 20)

Question 32

the ____________ of a metal mostly depends of the composition of the unalloyed metal

Answer 32

elasticity

Question 33

metals used in the mouth must be biocompatible. the must be resistant to ____________ and must not release any __________ _________.

Answer 33

resistant to corrosion / not release metallic ions.