4 - Metals and Alloys Flashcards
- 7 - When heating up a metal alloy and then rapidly cooling it, what occurs to:
- The number of nuclei per unit volume?
- Grain size?
Rapid cooling forms more nuclei per unit volume and decreases the grain size
4.7 What is grain refining?
Adding very fine, high melting metal (Ir) or oxide powder to the melt as seeds of nucleation. Produces smaller grains.
- 10 What is the relationship between finer grain size and:
- Yield strength?
- Ductility?
- Ultimate strength?
Changes in these properties with grain size are related to what kind of deformation and fracture?
Finer grain size raises yield strength, increases ductility, and raises the ultimate strength.
Changes in these properties are related to plastic deformation and fracture
4.14 What will happen to a material when a crack opens up on a grain boundary?
In what size grain metals will this be more likely to occur?
A material will fracture.
This is more likely to occur in large grain metals when the planes can not be slipped into the adjacent grains. (The plastic deformation cannot be accommodated.)
4.19 What kind of deformation is caused with cold working (wrought)?
Plastic deformation is caused by hammering, drawing, cold forging, cold rolling, or bending.
Produces many dislocations that cannot slip easily through each other as the lattice becomes more distorted.
4.24 Do alloys have a melting point or a melting range?
A melting range
Per the phase diagram, above ACB, all liquid
Below ADB, all solid
Between is a mix of both
4.25 As metals are mixed together in a molten state and cooled, the resulting chemical phase depends on the solubility of the metals in each other. If the metals are soluble, what results?
A solid solution results.
If not soluble - eutectic system results
- 26 Au and Cu are compatible in each other at any combination
- When they are rapidly cooled, what kind of solution do they form?
A solid solution with random distribution of Au and Cu atoms in the F-C cubic array
- 26 For Au and Cu - at a certain % of Au they form a ________ solution.
- These kinds of solutions tend to have higher or lower levels of hardness and strength?
They form an ordered solution. Au and Cu occupy specific positions in the F-C cubic.
Ordered solutions impart higher hardness and strength to alloys.
4.27 When does a eutectic alloy occur?
Do pure eutectics have a melting point or a melting range? How does this point/range compare to either of the pure components individually?
Occurs when two metals are soluble as liquids but nearly insoluble as solids
Pure eutectics have a melting point that is lower than either of the pure components
4.29 The ADA uses what composition classification for noble alloys?
In order to be classified as a noble alloy, the alloy must have a lower limit of 25% by weight of noble metal content
(Noble metal at least 25% and no gold stipulation)
4.34 Pure cast gold (100% gold) is not practical for dental restoration because it lacks strength and hardness. What can we do to resolve this?
- Solid solution -
We can mix two elements in the crystal lattice randomly and significantly increase the force needed to distort the lattice - a noble alloy
Ex. Add 10% Cu to Au
Increases tensile strength from 105 to 395
Increase BHN from 28 to 85 (Brinell Hardness Number test)
- Ordered solution - result depends on % content of metals and rate of cooling - specific positions occupied by specific metals
Ex. Hardness can increase by 10%, elongation decrease 30% - 12%, and yield strength increase by 25%
4.37 What are the composition requirements for a high noble metal?
Noble metal at least 60% and gold content at least 40%
