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

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

Grains & Grain Boundaries AK-Y02-04-DM-PP

Solidification of metals AK-Y02-04-DM-PP

Solidification of metals AK-Y02-04-DM-PP

Solidification of metals AK-Y02-04-DM-PP

Mechanical properties of metals AK-Y02-04-DM-PP

SPACE LATTICE Simple cubic structure Lattice is a 3-dimensional array of points that coincides with the positions of atom in a unit cell AK-Y02-04-DM-PP

SPACE LATTICE AK-Y02-04-DM-PP

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

Lattice imperfections AK-Y02-04-DM-PP

Lattice imperfections AK-Y02-04-DM-PP

Dislocation and slip plane AK-Y02-04-DM-PP

SLIP PLANE AK-Y02-04-DM-PP

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

Dislocations AK-Y02-04-DM-PP

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

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

AK-Y02-04-DM-PP

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)

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

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

STAGES IN ANNEALING Recovery Recrystallization Grain growth

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

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

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

Effects of annealing on tensile strength and ductility

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

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