Flashcards
Dental Materials 2
Cards in this deck (21)
- 1What is an alloy, and what may they be classified as?An alloy is a mixture of two or more metals (constituents). Alloys may be classified as Binery, Ternary, and Quaternary
- 2Name the four things that can occur when a molten mixture is cooled below its melting point.- Metals remain soluble - solid solution
- Metals may be partially soluble
- Metals may be completely insoluble
- Metals form inter-metallic compounds - 3What is a eutectic alloy?A eutectic alloy is easily melted. The metals separate from each other on solidification and are often used as solders. Metals that form eutectic alloys are dissimilar.
- 4What structure does a metal usually have when solid?Polycrystalline
- 5Where does crystallisation begin within molten metal/alloy when cooled? What do crystals grow as?Crystallisation begins at specific sites called nuclei and are believed to form impurities within the molten mass.
Crystals grow as dendrites. - 6What is each dendritic crystal called? What are the areas called where grains come in contact?Each dendritic crystal is called a grain.
The area where grains come in contact are termed the grain boundary. - 7What is the structure of the metal crystals/dental casts?Equiaxed crystals - crystals that have axes of approximately the same length.
- 8Name arrangements of some dental metals.- Face Centred cubic - Gold, Platinum, Palladium, Silver and Copper.
- Body Centred Tetragonal - Tin
- Close Packed Hexagonal - Zinc - 9What are crystal-arrangement defects usually referred to as, and how does their occurrence effect the metal?The defects are referred to as DISLOCATIONS and their occurrence effects the ductility of the metal/alloy.
- 10What happens to the dislocation when stressed?When stressed the dislocation moves through the lattice arrangement until reaching a grain boundary.
- 11Explain the cooling curve for an amorphous materialProduces a straight line, temperature over time
- 12Explain the cooling curve of a pure metal- A metal in the liquid state initially the atoms have sufficient energy to move about freely.
- As the temperature falls the atomic movement becomes more sluggish. Upon reaching a certain temperature (A) the thermal energy is no longer sufficient to prevent groups of atoms from forming the space-lattices of the metal. The change from liquid to solid is accompanied by the release of the latent heat of freezing. This heat compensates for the loss of heat to the surroundings and is usually sufficient to keep the temperature constant whilst solidification proceeds. This portion on the cooling curve is therefore a horizontal line and is called a 'plateau'
- Once solidification is complete, the temperature of the metal again falls. - 13Explain the colony curve for a binary alloyThe cooling curve of an alloy (3) shows solidification occurs over a range of temperatures.
With a binary alloy solidification begins at Ti and finishes at T2.
Above T1 ( the liquidus) the alloy is in a molten/liquid state and below T2 (the solidus) it is in a solid state. The area between T1 and T2 consists of some solid and some liquid - 14What does a phase diagram for a binary system show?Mixtures of different known compositions are prepared (varying from 100% to 0%) and values for A and B on a graph (temperature against time) are determined. All these results are plotted and a Phase Diagram is produced.
- 15Explain the plotted points on a phase diagram for a binary systemThe plotted points T1 are joined and this line is the 'LIQUIDUS'. Any composition above this temperature is molten. The lower line is obtained by plotting points T2 and this line is called the 'SOLIDUS'. Below this line the material has solidified. The areas between the two lines represent the temperatures and compositions for partially solidified alloys.
- 16What information can be gathered form a constitutional phase diagram?An alloy with composition X. Raise the temperature above T1. On cooling the first solid begins to form TL (Tie Line).
Drawing tie line T1 reveals the first solid to form has a composition of approx. 90% A and 10% B = Z.
Further cooling, more crystallisation occurs between T1+T2, a mixture of solid and liquid exists. Select temperature TmL. Within this region the composition of the solid and the liquid can be predicted by noting where the tie line intersects the SOLIDUS and LIQUIDUS. Thus at temperature TmL the composition of the solid is Y (approx. 70% A and 30% B) (vice versa for W). On further cooling the alloy is completely solid at T2, so the last liquid to crystallise has the composition V (approx. 80% B, 20% A). - 17What does a constitutional phase diagram reveal? What does it mean when the solidus and liquids are wide apart?It reveals that the first material to crystallise is rich in the higher melting point metal (A) and the last rich in the other metal (B). This is termed a CORED STRUCTURE.
The wider the SOLIDUS and LIQUIDUS lines are apart, the greater is the potential to CORING, which may seriously reduce the corrosion resistance of some alloys. - 18What does a Eutectic phase diagram show?A eutectic phase diagram includes two hypothetical metals labelled A and B. TmA and TmB show the melting points of these two metals. C is the eutectic alloy. ACE is the liquids line. ABCDE is the solidus line.
To the left of C = Pure metal A
To the right of C = Pure metal B
The Eutectic alloy crystallises at a given temperature and not over a range of temperatures, they are often used as solders. - 19Explain the shaping of dental casting alloysLost wax process - introduced by Taggart 1907. the process of turning a wax pattern of restoration into a metallic one.
- 20List the stages of shaping1. Create the wax pattern on the die.
2. Sprue the wax pattern.
3. Invest the wax pattern.
4. Burn out the invested pattern to create a mold.
5. Cast the molten metal.
6. De-investing.
7. Finishing & polishing.
8. Cementing. - 21—
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- undergraduate Dental Technology Flashcards on Dental Materials 2 , created by Matilda matthews on 30/04/2019.
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- Created by Abigail Withers · Mar 3, 2018
- Copied by Matilda matthews · Apr 30, 2019
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