dental materials - alloys
DESCRIPTION
Dental MaterialsTRANSCRIPT
26.10.2007 Dental Materials - Graham Cross 1
3rd Year Dental Materials Science
Dr. Graham CrossSchool of Physics and CRANN
SFI Nanoscience Building, Rm 1.5
http://www.tcd.ie/Physics/People/Graham.Cross/
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Oct. 26: Basic metallurgy and alloysNov. 2: Properties of materials, thermalsTBA: Mechanics of solids and fluids
Topics
Textbooks – Further Reading
Applied Dental Materials – 8 th Edition 1998, John F. McCabe, Angus W. G. Walls, Blackwell, Oxford, UK.• Restorative Dental Materials – 10th Edition 1997 Editor Robert G. Craig, Mosby – Year Book, Inc, St. Louis, USA• Notes on Dental Materials – 6th Edition 1992 Editor E.C. Combe, Churchill Livingstone, Edinburgh, UK• Phillip’s Science of Dental Materials – 10th Edition 1996, Editor Kenneth J. Arusavice, W.B. Saunders Company Philadelphia, USA• Dental Materials, Properties and Manipulation – 6th Edition 1996 Editors Robert G. Craig, William J. O’Brien, John M. Power, Mosby – Year Book, Inc, St. Louis, USA
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• What is a metal, what is an alloy?
• Production and structure of metals and alloys
• Mechanical properties
Metals and Alloys
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Characteristic properties of metals at room temperature:
• Dense• Hard (exceptions include Mercury)• Ductile and Malleable• Good conductors of heat (and electricity)• Shiny, opaque
What is a metal?
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Crystal structure
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Metals and Alloys
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What is an alloy or alloy system?Mixture of two or more metals
Alloys used in dentistry:
Steel alloy (iron and carbon)- used for construction of instruments and wires in orthodontics
Gold alloys and Chromium alloys (Ni/Cr, Co/Cr)- used in crowns, inlays and dental bases
Amalgam (mercury alloy)- used in filling material since mid 18th Century
Alloys
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Cooling of a molten metal
Liquid
Solid
Solidifying
Thermodynamic phase change
Latent heat
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Structure on solidification
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Grain size and structure
The strength of materials is governed by propagation of defects.A grain boundary is a barrier to defect propagation.
Why?
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• Irreversible failurebehaviour of a solid that occurs above a special shear stress threshold called the “yield stress”: τyield
Plasticity and structural strength
Shear Stressτ
Strain ε
τyield
Ductility
“Remember”: Shear stress causes plasticity leading to a change of shape of an objectFor structural integrity, the yield (plastic) strength must be as high as possible.
Plastic yield:
Limit of plasticity: Ductility…followed by fracture
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Shearing a solid: Plastic flowEnergy Position
Energy Position
solid
sheared solid
One line of atoms changes neighboursStress, not temperature, increases the energy level
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Shearing a cubic crystal
Cubic atomic latticePlastic shear strain
(Simplified schematic)
Imagine this is a small grainin the metal:
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Creating a dislocation
Cubic atomic lattice Dislocation with core and distorted lattice
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Motion of a crystal dislocationOne bond is broken, and then one bond is traded as the dislocation “glides”through the material…
This is much easier than breaking all the bonds at once.
Dislocation glide direction:
DislocationNucleation
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Why small grains are stronger
Dislocations cannot glide across grain boundaries: they are blockedMore grain boundaries in small grain metals è higher strength.
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Factors affecting grain size
EquiaxedFibrous
Large grains
Small grains
, “swaging”
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3. Recrystallization (softening heat treatment)- Heat cold worked material above its recrystallizationtemperature to alter grain structure.- Reverts to equiaxed form.
Factors affecting grain size (cont.)
5. Stress relief by mild annealing- Internal stresses can cause cracking or distortion.- Lower temperature heat treatment than recrystallization.
4. Grain growth- If overheated or heated for very long grains begin to grow.- Coarse grain structure.
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Heat treatment and annealing
Critical temperature, the recrystallization temperature, above which the structure of the metal changes by thermal energy.
Heat
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Work hardening: A property change resulting from cold working- Crystals become fibrous, and full of “inter-locking” dislocations- Increase hardness, strength (increased yield stress)- Reduces ductility- This will eventually lead to fracture
Cold working a metal
In dentistry:• Formation of wires by pushing metal or alloy through die holes• Bending of wires and clasps during appliance construction or alteration• “Swaging” of stainless steel denture bases
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Cooling should be carried out quickly if:- Fine grain structure strength and hardness is important
Cold working also increases strength and hardness- But, it reduces ductility i.e. metal becomes brittle
Can remove internal stresses of cold worked material: Annealing
Heating to recrystallization temp. undoes work hardening- Initially produces fine grain structure- Extended heating time, however, results in grain growth- Slow cooling promotes grain growth- Use to lower strength and hardness (ie. soften, malleable)
Practical considerations
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Properties of metals depend critically upon: CompositionThermal treatmentMechanical treatment
Recap of metal properties
All of this applies to alloys, but…
Mechanical properties of alloys can be very different from properties of component metals:
Eg. Consider an alloy containing 75% Au and 25% Cu:
Alloy has higher tensile strength, greater than either Au or Cu!
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Mechanical strength:Solution hardening in Gold–Copper system
Mechanical strength of alloys
Cu or Au (random)Au Cu
(fcc)
Either way, dislocation glide under stress is inhibited:Increase strength, hardness, reduce ductility
Cooled rapidly from 450°C:Disordered substitutional solid solution
Slow coolingOrdered substitutional solid solution
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• General properties of materials including thermal properties
• Mechanical properties
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