MEtals 2 Flashcards
how can gold be made harder?
heat treating–cooling quickly, freezes atoms in their latice structure
describe the softening heat treatment of gold alloys (Au:cu)
- 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
describe the hardening heat tx for Au:Cu (age hardening)
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
what is a good alternative to gold casting alloys? describe
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)
what are the properties of Ag-Pd alloys?
- colour– silver white in colour
- 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?)
- castability – poor castability, due to low density of Pd
- Tarnish-without Cu it will tarnish, which is the major drawback –> Cu-containing is preferable
what are the base metal casting alloys?
- CoCr
- NiCr
- CP Titation and Titation alloys
what are the application of CoCr alloys, their composition, and properties?
Applications:
- complete denture bases & RPD frameworks
- Sub-periosteal implant
- 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
- 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
- density: 7 gm/cm3; lower than gold; therefore more preferred for construction maxillary dentures but more casting for should be applied
- 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)
- strength: similar to type IV gold alloys (high)
- mod of elasticity (E): 2-times more thna type IV gold alloys; highly rigid even when thin
- ductility (as %elongation = 2-10%); lower ductility, can fracture in small/thin section (e.g. clasp–>fracture)
what are the application of NiCr alloys? Composition? Properties?
applications:
- full metal crown and bridge
- 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
- having Ni instead of Co causes:
what are the applications of commerically pure (CP) Ti and Ti alloys? Composition? Properties?
Applications:
- all-metal and metal-ceramic restos
- 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
- Ti cast is usually porous–>special casting equipment is req’d
- can be scanned by CAD-CAM and milled into inlays or crowns
how can we achieve finer grains?
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;
what does alloy grain size depend on?
- cooling rate
- composition
- presence of grain refiners
what does grain size influence in relation to alloy properties?
- strength (increases with decreasing grain size)
- workability
- susceptibility to corrosion
smaller grain size–>better strength, better resistant to permanent deformation
ye
grain boundaries are the last to solidiy–>they have the most impurities
ye
metals with small ELONGATED grains have high strength and low ductibility
ye
