Introduction to dental metallurgy

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

INTRODUCTION

• Esthetic restorations are ideal for single tooth restoration

• Metals are ideal for more than one tooth replacement FPDs, RPDs

Outline of the lecture

• Cast and wrought metals• Alloys

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

STRUCTURE OF METAL

STRUCTURE OF METAL

STRUCTURE OF METAL

• 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

GRAIN BOUNDARIES (GB)

Grains & Grain Boundaries

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Solidification of metals

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Solidification of metals

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Solidification of metals

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Mechanical properties of metals

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SPACE LATTICE

• Lattice is a 3-dimensional array of points that coincides with the positions of atom in a unit cell

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Simple cubic structure

SPACE LATTICE

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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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Lattice imperfections

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Lattice imperfections

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Dislocation and slip plane

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Dislocation

SLIP PLANE

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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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Dislocations

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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

WROUGHT METAL

• Stainless steel (orthodontic wires, crowns, clasps, root canal reamers & surgical instruments)

• Cobalt-chromium nickel• Nickel titanium• Commercially pure (CP) titanium

• Hot working (elevated temperature)

• Cold working (plastic deformation at room temp)rolling, drawing, pressing etc. Also called work hardening

WROUGHT METAL: 2 TYPES OF WORKING

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WROUGHT METAL

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)

• Compaction of direct filling gold (cohesive gold/ gold foil) is an example for clinical application of work hardening

CLINICAL APPLICATION OF WORK HARDENING

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

Heat treatment of metals: ANNEALING

1. Recovery2. Recrystallization3. Grain growth

STAGES IN ANNEALING

The cold worked properties begin to disappear before any significant change is observed microscopically.ª Very little decrease in tensile strengthªVery little change in ductilityª Pronounced change in electrical

conductivity

RECOVERY: (STRESS-RELIEF ANNEAL)

• 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

RECRYSTALLIZATION

• 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

GRAIN GROWTH

Effects of annealing on tensile strength and ductility

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

PRECAUTION

SUMMARY-III

Cast metal and Wrought metal

Hot working & Cold working (work hardening)

Annealing

RecoveryRecrystallizationGrain growth