MEtals 2

Question 1

how can gold be made harder?

Answer 1

heat treating–cooling quickly, freezes atoms in their latice structure

Question 2

describe the softening heat treatment of gold alloys (Au:cu)

Answer 2

• heat the alloys to 700 C for 10min then quench
• decreased proportional limit, tensile strength, and hardness
• increased ductility
• modulus of elastic is not affected
• indicated for adjusting, burnishing, polishing
  microstructure: disordered face centred cubic, substitutional solid sol’n

Question 3

describe the hardening heat tx for Au:Cu (age hardening)

Answer 3

heat alloy to 424 C for 2 min then cool to 250 C over 30 min, then quench

• increase prop lim, tensile strength and hardening
• decrease ductility

mod of elasticity not affected

• indicated for service as an oral resto for pt
• microstructure: ppt of order face centred tetragonal superlatice in disordered FCC; substitutional spolid structure

Question 4

what is a good alternative to gold casting alloys? describe

Answer 4

silver-palladium

• applications for full metal crown and bridge
• composed mainly of Ag (70-72%)and paladium is >25% to provide nobility; may contain copper (<15%) and a small amount of gold

can be cast in the same temp range as gold (900-1000)

Question 5

what are the properties of Ag-Pd alloys?

Answer 5

1. colour– silver white in colour
2. physico-mechanical properties
   
   • -as type III (soft tx’d?) gold alloys
   • some alloy (60%- Ag + 25%-Pd + 15% Cu) show some properties similar to those of type IV Au alloys (hard tx’d?)
3. castability – poor castability, due to low density of Pd
4. Tarnish-without Cu it will tarnish, which is the major drawback –> Cu-containing is preferable

Question 6

what are the base metal casting alloys?

Answer 6

• CoCr
• NiCr
• CP Titation and Titation alloys

Question 7

what are the application of CoCr alloys, their composition, and properties?

Answer 7

Applications:

1. complete denture bases & RPD frameworks
2. Sub-periosteal implant
3. Bone plates and screws

Composition:

• • mainly Co (improves strength, hardness, and rigidity, and increase melting range), Cr (improves tarnishand corrosion resistance by forming passive oxide layer), Ni (same as cobalt but by a lower degree), Mb (improves hardness and strength, acts as a grain refiner–reduces size of grains), and C (impoves hardness, but may make it more brittle)
• • With traces of Fe (improves cold working of alloy, reduces strength and melting temp), Be (reduces melting temp–for every 1 gm of Be, decreases melting temp by 10 C) Mn, W, and Silicone (last three increase strength and hardness)

properties:
1. colour: silver white
2. melting temp: 1250-1450: requires either phos- or silica-bonded investmentor either oxy-acetylene gas torch or electric melting

3. Casting shrinking: shrinks by 2.3%, which is the highest amount of shrinkage among all casting alloys; therefore, mold expansion is required to compensate; not preferred for long span bridges–more proportional shrinkage
4. density: 7 gm/cm3; lower than gold; therefore more preferred for construction maxillary dentures but more casting for should be applied
5. hardness: 3 times harder than type ICV gold alloys; difficult to polish and finish, so it requires sand-blasting and electro-polishing; could cause wear of opposing teeth, so it is NOT used for crown and bridge construction (contrainditcation)
6. strength: similar to type IV gold alloys (high)
7. mod of elasticity (E): 2-times more thna type IV gold alloys; highly rigid even when thin
8. ductility (as %elongation = 2-10%); lower ductility, can fracture in small/thin section (e.g. clasp–>fracture)

Question 8

what are the application of NiCr alloys? Composition? Properties?

Answer 8

applications:

1. full metal crown and bridge
2. metal ceramic restoration

composition: same as CoCr –> except Ni is the predom. alloy (65-70%)
colour: same as CoCr – silver white

properties:

• • having Ni instead of Co causes:
    
    • decreased hardness
    • lower shrinking rate
    • lower melting temp
    • lower strenght properites (more similar to type III

Question 9

what are the applications of commerically pure (CP) Ti and Ti alloys? Composition? Properties?

Answer 9

Applications:

1. all-metal and metal-ceramic restos
2. RPD frameworks

Composition:

• CP Ti: pure Ti, can form oxide layer
• Ti alloy: tivanium (Ti6Al4V)

Color: silver-white

Characteristics:
1. low density and hgih fusion temp – difficult to cast; also has high reactivity to air

2. Ti cast is usually porous–>special casting equipment is req’d
3. can be scanned by CAD-CAM and milled into inlays or crowns

Question 10

how can we achieve finer grains?

Answer 10

1/ homogenous nucleation–enhanced by rapid cooling; creates more nuclei per volume–>thus a smaller grain size
2. heterogenous–foreign solid particle surface is added–atoms attached;

Question 11

what does alloy grain size depend on?

Answer 11

• cooling rate
• composition
• presence of grain refiners

Question 12

what does grain size influence in relation to alloy properties?

Answer 12

• strength (increases with decreasing grain size)
• workability
• susceptibility to corrosion

Question 13

smaller grain size–>better strength, better resistant to permanent deformation

Answer 13

ye

Question 14

grain boundaries are the last to solidiy–>they have the most impurities

Answer 14

ye

Question 15

metals with small ELONGATED grains have high strength and low ductibility

Answer 15

ye

Question 16

what are some properties of the protective oxide film?

Answer 16

• tightly bonded to alloy surface
• insoluble to oral environment
• very thin, few oxide ‘cules thick

–damaged by high concentrtion of Cl or F

Question 17

what are the types of corrosion?

Answer 17

• chemicalcorrosion
• electrochemical corrosion–> moveoments of e- and positive ions; require anode, cathode,circiuit, and electroluyte
  
  • galvanic corrosion: +ve ions will move into sol’n (lower energy)–>loss of ions, excess e-s; the more reactive a metal the more negative its potential
  • localgalvinc – multiphase alloy (e.g. gold and amalgam resto in same tooth)
  • crevice corrosion
  • stress corrosion
• the complete reaction happens at ne site

Question 18

what does a metal’s position on the electrochemical series indicate?

Answer 18

at top – is least reactive, positive, is oxidized–>loses electrons

at bottom – negative, anode, pick up electrons–good oxidizing, are reduced,

Question 19

study page 28

Answer 19

ye

Question 20

what is tarnish?

Answer 20

a thin layer of oxide/sulphide film -> discolouration

Question 21

the more chemically reactive a metal, the more corrosion resistant it is

Answer 21

ye

Question 22

oxygen, chlorine, sulfur chemically attacks metals –> forms metal oxides, chlorides, and sulfides

Answer 22

ye

Question 23

what does electrochemical corrosion require?

Answer 23

anode – +ve terminal from which electrons LEAVE
cathode – -ve terminal to which electrons attach
circuit
electrolyte

Question 24

describe galvanic corrosion

Answer 24

+ve metal ions will move into sol’n (lower e-)
- loss of ions = excess free e-s in the metal
-

Question 25

wht is crevice corrosion?

Answer 25

- causes pitting - plaque suppresses the local pH and O2 in comparison to other uncovered portiosn of the resto surface; the surface under the plaque behaves as an ANODE

Question 26

describe stress corrosion

Answer 26

-high stresses at points of occx contact and near margins where stresses are transferred requires three main conditions: 1. metal under tensile stress 2. dissolved oxygen 3. chloride ion brittle cracks for at sites of stress --> failure highest stress regions behave as anodes