Metals and Alloys Flashcards by Roisin Mousa

What are metals and alloys used for in dentistry

RPD framework 
crowns
denture base
orthodontic appliance
restorations

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What metal/alloy is used in partial denture framework

cobalt chrome

type IV gold

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What metal/alloy is used in crowns

stainless steel

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What metal/alloy is used in denture base

stainless steel

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What metal/alloy is used in orthodontic appliance

NiTi

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What metal/alloy is used in restorations

amalgam

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Why do we use metals

They have superior mechanical properties

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What are the superior mechanical properties of metals

strength
rigidity 
elastic limit 
ductility
hardness

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What are the disadvantages of metals

poor aesthetics

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What is the definition of a metal

aggregate of atoms in a crystalline structure

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What is an alloy

combination of metal atoms in a crystalline structure

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What are the building blocks of alloys

metals

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What is ductility

Ductility is a measure of a metal’s ability to withstand tensile stress—any force that pulls the two ends of an object away from each other

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What is malleability

Malleability is a substance’s ability to deform under compressive stress

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What is the elastic limit

maximum stress without plastic deformation

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What is the UTS

ultimate tensile strength

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What is ductility defined as

amount of PLASTIC DEFORMATION prior to fracture

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What do mechanical properties depend on

choice of metal

crystalline structure

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What does the crystalline structure depend on

history - method of production

shaping - crucial for dental applications

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What are the factors that affect mechanical properties

crystalline structure
grain size
grain imperfections

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What is a grain

a single crystal

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What is a crystal

a lattice arrangement

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What are different crystal/lattice structures

cubic
face centre cubic
body centre cubic

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In the cooling curve of a pure metal what is the straight line

plateau phase

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What does the plateau phase mean in a pure metal

the melting point | changes from liquid to solid

Describe the process crystal growth

1. atoms act as nuclei of crystallization 2. crystals grow to form dendrites 3. crystals (or GRAINS) grow until they impinge on other crystals

What is the nuclei of crystallization

the centre of around which other atoms will crystallite

What are dendrites

3D branched lattice network

What is the region where grains make contact called

the GRAIN BOUNDARY

What parameters can effect the size and shape of grains

how you process it | which container you use to cool it

If you change the size and shape what else do you effect

the properties

What are EQUI-AXED grains

crystal growth of equal direction

What is a radial grain structure

molten metal cooled quickly in cylindrical mould

What is a fibrous grain structure

wire pulled through die (cold worked metal/alloy)

What are the different types of cooling

``` Fast cooling (quenching) slow cooling ```

How does fast cooling effect the crystal growth

more nuclei small fine grains MECHANICAL PROPERTIES ENHANCED

How does slow cooling effect the crystal growth

few nuclei large coarse grains WEAK

What are nucleating agents

impurities or additives act as foci for crystal growth | helps the crystallization process and improves the product

What is each grain

a single crystal (lattice) with atoms orientation in given directions (dendrites)

What is the grain boundary a result of

change in orientation of the crystal planes - impurities concentrate here

why are small fine grains advantageous

high elastic limit | increased UTS, hardness

What are the disadvantages of small fine grains

have a decreased ductility

What are the factors for rapid cooling

small bulk required heat metal/alloy just above its melting point pull the heat away from the metal quickly and quench

What does a defect in the crystalline structure represent

a misalignment of atoms in that lattice network

What is dislocation

imperfections/defects in crystal lattice

What happens to a defect when force is applied

it moves which in the end results in a lattice that has no defect but is a different shape

What is slip due to

propagation of dislocations and involves rupture of only a few bonds at a time this is crucial as it is a cascade effect and doesn't require supreme large forces to do it

If you impede the movement of dislocations what does this increase

elastic limit UTS hardness

If you impede the movement of dislocations what does this decrease

impact resistance

What are the factors that impede dislocation movement

grain boundaries (hence fine grains) alloys: different atom sizes cold working

In cold working where do the dislocations build up at

grain boundary

How do grain boundaries impede dislocation movement

they stop the defect moving to another grain

How is cold working achieved

bending, rolling, swaging | push the effect within the metal alloy to the grain boundary and improve the mechanical properties of that object

What temperature is cold working done

at a low one (below recrystallization temperature)

What does cold working cause

slip - hence stronger harder material

What properties are higher after cold work

elastic limit UTS hardness

What properties are lower after cold working

ductility impact strength low corrosion resistance

What is the effect of cold working on residual stress

increases it

What does residual stress cause

instability in lattice resulting in distortion over time

What is residual stress relieved by

annealing process

What is annealing

heating the metal/alloy so that greater thermal vibrations allow migrations of atoms

What does stress relief annealing do

ELIMINATE stresses by allowing atoms to REARRANGE within grains

How are grain structure and mechanical properties changed in annealing

they are not

When does recrustallisation happen

when meta alloy is heated causing new smaller equiaxed grains

What is the effect of recrystallisaton on the properties

lower EL, UTS, hardness | increased ductility

What does recrystallization spoil

spoils benefit of cold work allows further cold work cold work/recrystallisation repeated until correct shape obtained

What does the recrystallization depend on

amount of cold work | greater the cold work, lower the recrystallization temperature

What does excessive temperature rise cause

large grains to replace smaller coarse grains yielding poor mechanical properties so have to be careful when annealing

What is the definition of an alloy

two metals that form a COMMON LATTICE structure, are SOLUBLE in one another and form a SOLID SOLUTION

What does metal alloys forming a solid solution mean

it means two metals co-existing in a common lattice structure NOTHING TO DO WITH TEMP

Metals and Alloys Flashcards

(70 cards)