Introduction to dental metallurgy. METALS: INTRODUCTION Metals are one of the mainstays of dentistry...
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Introduction to dental metallurgy. METALS: INTRODUCTION Metals are one of the mainstays of dentistry They are a group of ‘structural materials’ and are
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
Slide 3
INTRODUCTION Esthetic restorations are ideal for single tooth
restoration Metals are ideal for more than one tooth replacement
FPDs, RPDs
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Outline of the lecture Cast and wrought metals Alloys
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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
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STRUCTURE OF METAL
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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)
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Grains & Grain Boundaries AK-Y02-04-DM-PP
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Solidification of metals AK-Y02-04-DM-PP
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Solidification of metals AK-Y02-04-DM-PP
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Solidification of metals AK-Y02-04-DM-PP
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Mechanical properties of metals AK-Y02-04-DM-PP
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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 AK-Y02-04-DM-PP
Simple cubic structure
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SPACE LATTICE AK-Y02-04-DM-PP
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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)
AK-Y02-04-DM-PP
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Lattice imperfections AK-Y02-04-DM-PP
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Lattice imperfections AK-Y02-04-DM-PP
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Dislocation and slip plane AK-Y02-04-DM-PP Dislocation
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SLIP PLANE AK-Y02-04-DM-PP
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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
AK-Y02-04-DM-PP
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Dislocations AK-Y02-04-DM-PP
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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
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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AK-Y02-04-DM-PP
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WROUGHT METAL
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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)
Slide 30
Compaction of direct filling gold (cohesive gold/ gold foil) is
an example for clinical application of work hardening CLINICAL
APPLICATION OF WORK HARDENING
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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
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1.Recovery 2.Recrystallization 3.Grain growth STAGES IN
ANNEALING
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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 RECOVERY:
(STRESS-RELIEF ANNEAL )
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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
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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
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Effects of annealing on tensile strength and ductility
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Wires should NEVER BE RECRYSTALLIZED, ONLY RECOVERY This is to
preserve the fibrous grain structure PRECAUTION
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SUMMARY-III Cast metal and Wrought metal Hot working & Cold
working (work hardening) Annealing Recovery Recrystallization Grain
growth