1. Introduction to dental metallurgy

2. METALS: INTRODUCTION Metals are one of the mainstays of dentistry
They are a group of ‘structural materials’ and are best suited for stress bearing applications

3. INTRODUCTION
Esthetic restorations are ideal for single tooth restoration
Metals are ideal for more than one tooth replacement FPDs, RPDs

4. Outline of the lecture
Cast and wrought metals
Alloys

5. STRUCTURE OF METAL
In metals, atoms are tightly packed in a CRYSTAL STRUCTURE (GRAIN)
(A regular arrangement of atoms that repeats itself many times)
The repeating entity is called the UNIT CELL. The unit cell is the basic building block of a metal
Collection of many crystals in a metal is termed polycrystalline

6. STRUCTURE OF METAL

7. STRUCTURE OF METAL

8. STRUCTURE OF METAL

9. GRAIN BOUNDARIES (GB) The structure of the GB is more non-crystalline
GB is a higher-energy-region than the interior of the grain
Impurities in metal may be found in greater concentrations at the GB
GB is more readily attacked by chemicals

10. Grains & Grain Boundaries
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11. Solidification of metals
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12. Solidification of metals
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13. Solidification of metals
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14. Mechanical properties of metals
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15. SPACE LATTICE
Simple cubic structure
Lattice is a 3-dimensional array of points that coincides with the positions of atom in a unit cell
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16. SPACE LATTICE
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17. LATTICE ARRANGEMENTS IN METAL
Different lattice arrangements are seen in different metals
Body-centered cubic (BCC)
Face-centered cubic (FCC)
Hexagonal close-packed (HCP)
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18. Lattice imperfections
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19. Lattice imperfections
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20. Dislocation and slip plane
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21. SLIP PLANE
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22. Dislocations
Dislocation is crucial for the plastic, or permanent deformation of a materials
Without dislocation very high amount of mechanical stress is required for deformation
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23. Dislocations
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24. CAST & WROUGHT METAL
Cast metal is produced when molten metal is allowed to cool in a mold
Wrought metal is produced by deforming (process of working) the cast metal
Wrought alloy exhibits properties and micro-structure that is not associate with the same alloy when cast

25. WROUGHT METAL
Stainless steel (orthodontic wires, crowns, clasps, root canal reamers & surgical instruments)
Cobalt-chromium nickel
Nickel titanium
Commercially pure (CP) titanium

26. WROUGHT METAL: 2 TYPES OF WORKING
Hot working (elevated temperature)
Cold working
(plastic deformation at room temp)
rolling, drawing, pressing etc.
Also called work hardening

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28. WROUGHT METAL

29. COLD WORKING Strengthening mechanism
Cold working increases the physical properties such as yield strength, tensile strength, surface hardness, decreases ductility (% elongation at failure)
Cold working takes place in metal by the motion of defects through the material (SLIP)

30. CLINICAL APPLICATION OF WORK HARDENING
Compaction of direct filling gold (cohesive gold/ gold foil) is an example for clinical application of work hardening

31. Heat treatment of metals: ANNEALING
Annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended period of time and then slowly cooled (3 stages).
Annealing eliminates or reduces the effects off cold working
It relives stresses, increases ductility and toughness

32. STAGES IN ANNEALING
Recovery
Recrystallization
Grain growth

33. RECOVERY: (STRESS-RELIEF ANNEAL)
 The cold worked properties begin to disappear before any significant change is observed microscopically.
Very little decrease in tensile strength
Very little change in ductility
Pronounced change in electrical conductivity

34. RECRYSTALLIZATION
This occurs when further heat is applied to the cold worked material.
The previous grains are replaced by new, smaller, stress-free ones (refines grain structure)
Decreases hardness & tensile strength
Increases ductility, malleability & corrosion resistance

35. GRAIN GROWTH It follows recrystallization when the metal is overheated
This process causes migration of the grain boundary, whereby large grain replaces many small ones
Grain growth occurs only in wrought metals
Mechanical properties are lowered by grain growth

36. Effects of annealing on tensile strength and ductility

37. PRECAUTION Wires should NEVER BE RECRYSTALLIZED, ONLY RECOVERY
This is to preserve the fibrous grain structure

38. SUMMARY-III Cast metal and Wrought metal
Hot working & Cold working (work hardening)
Annealing
Recovery
Recrystallization
Grain growth