dental materials - alloys

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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/

Graham.Cross@tcd.ie

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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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