BDS2 DMS Flashcards
Types of error in PMMA dentures (3)
Flawed initial impression
Error during curing process
Usage
Potential usage problems in PMMA denture (5)
Fractures
Uncomfortable
PMMA becomes warped
Surface wear
Patient anatomy changes
3 requirements of ideal denture
Replaces function of natural dentition
Fits comfortable
Good aesthetics
Ideal properties for denture base material 7
Can accurately create dimensions that will fit well and be stable in use
High softening temperature
Unaffected by oral fluids
High thermal conductivity
High Young’s modulus and elastic limit
Low density
Non toxic/irritant
What does high Young’s Modulus mean?
High stress causes small strain
What does high elastic limit mean?
Only large stresses will cause permanent deformation
Free radical addition polymerisation
Chemical union of two molecules either the same or different to form a larger molecule without the elimination of a smaller molecule
(involves C=C bonds)
4 steps of acrylic polymerisation reaction
Activation
Initiation
Propagation
Termination
What happens in activation stage of acrylic polymerisation?
Free radicals are formed
What happens in initiation stage of acrylic polymerisation?
Free radicals break C=C bonds in each monomer, and transfer free radicals
What happens in propagation stage of acrylic polymerisation?
Polymer chain grows
What must be done to enamel surface before bonding a composite resin?
Acid etch
Advantages and disadvantages of porcelain
Aesthetically good
Hard and rigid - more easily fractured
What is stress?
Force/ unit area in N/metres squares - PASCALS
Difference in retention between amalgam and composite
Amalgam uses mechanical retention - undercuts
Composite uses adhesive retention - minimal cavity prep
Creep
Gradual dimensional change due to repetitive small force
Fatigue
Repetitive small stresses cause fracture
Deformation
Repetitive small stresses cause permanent change in material dimensions without fracture
Elasticity
Ability of a material to recover its dimensions follow application of stress
Properties of ideal dental adhesive (6)
High bond strength
Immediate bond
Durable bond
Impermeable bone
Easy to use
Safe
Why is enamel easy to bond to?
Heterogenous - densely packed prismatic structure
Acid etch process
Roughens the surface, long enamel prisms are filled with imperfectly packed hydroxyapatite crystals
Why acid etch?
Modified roughened surface produced by etch is easier to bond to resin restorative materials, increases enamel surface energy by removing surface contaminants - better wettability, allowing the resin to adapt to the roughened enamel surface
Why must enamel be dry for composite?
Moisture contamination prevents flow of resin into etched surface
What is used for etch?
30-50% aqueous phosphoric acid
Dentine composition
20% inorganic (mostly collagen)
70% inorganic (mostly hydroxyapatite)
10% water
Why is bonding to dentine difficult?
Dentine is full of permeable tubules, fluid pumps up from pulp to dentine floor making cavity wet
Hydrophilic
Low surface energy
Smear layer
Required properties for dentine bonding agent (4)
Ability to flow
Potential for intimate contact with dentine surface
Low viscosity
Adhesion to substrate by mechanical, chemical and Van der Waals forces
Describe mechanical bonding to dentine
Dentine bonding agent and dentine surface mesh and interlock together with minimal gaps
Chemical adhesion of dentine
Mineralised dentine forms ionic bonds
Organic dentine forms covalent bonds
What happens to dentine as it ages?
Becomes more mineralised
Relevance of surface energy in bonding
A liquid will only spread on a surface with higher surface energy than it
What is amalgam?
An alloy formed by the reaction of mercury (liquid) and silver, tin, copper and other metals (powder)
Purpose of copper in amalgam
Increases strength and hardness
Amalgam particle types
Lathe cut
Spherical/spheroidal
What are the gamma, gamma1 and gamma2 phases of amalgam?
Gamma - unreacted particles Ag3Sn
Gamma 1 - Ag2Hg3 and Gamma 2 - Sn7Hg9
make up the amalgam matrix
Setting reaction for amalgam
Ag3Sn +Hg -> Ag2Hg3 +Sn7Hg9
Set structure of gamma particles in amaglam
Gamma 1 holds together particles of Gamma and Gamma 2
In modern amalgam what are 2 setting dimensional changes?
Small contraction (<0.2%)
Solid solution of Hg in Ag3Sn
Why is zinc avoided in lots of dental materials?
When zinc reacts with water it forms ZnO and H2 causing bubbles of H2, causes pressure which can cause expansion, pulpal pain or cause restorations to sit above the surface
4 factors affecting amalgam’s properties
Handling
Cavity design
Corrosion
Variation between products
5 advantages of spherical amalgam particles
Earlier high tensile strength
Higher tensile strength
Less sensitive to condensation
Easier to carve
Less mercury required
Describe early (1hr) and late (>24hr) strength of amalgam
Early - poor
Late - fairly good
Abrasion resistance of amalgam
High, suitable for posterior teeth
Too high for deciduous
5 factors decreasing strength of amalgam
Undermixing
Too high Hg content after condensation
Too low condensation pressure
Slow rate of packing - increments don’t bond
Corrosion
3 factors affecting marginal integrity of amalgam
Creep
Cavity design
Corrosion
What is creep?
When a material flows slightly as a result of repeated low level stresses. This is a kind of permanent deformation
Thermal conductivity of amalgam and relevance
High thermal conductivity so in deep cavities it may be necessary to use a liner
How is amalgam bonded to tooth?
It is not bonded. It uses mechanical retention
What is added to amalgam to resist corrosion?
Copper
Is it easier to bond a material to enamel or dentine?
Enamel
What acts as a “wetting agent” when carrying out a restoration?
Bonding agent
4 benefits of copper enriched amalgam
Higher early strength
Less creep
Higher corrosion resistance
Increased durability of margins
Advantages of amalgam (5)
Strong
Hard
Durable
Radiopaque
User friendly
Disadvantages of amalgam (4)
Corrosion
Leakage
Poor aesthetics
Mercury - potential toxicity and environmental impact
5 potential restorative materials
Composite
GIC
Amalgam
Precious metal
Ceramic
Why is release of chemicals an undesirable property in restorative materials?
Could be pulpal irritants and lead to pain or pulpal damage
What is the purpose of intermediate restorative materials (cavity liners)?
Prevent gaps
Act as protective barrier
What is a cavity base?
Thick mix placed in bulk
Replaces dentine to minimise restoration material used or block out undercuts
Which type of restorative material more often requires a cavity base?
Metal
What is a cavity lining?
Thin coating <0.5mm over exposed dentine
Able to promote pulp health by adhering to the tooth structure or by anti-bacterial action
Pulpal protection by lining material (3)
Chemical stimuli from unreacted chemicals in the filling material or initial pH of filling
Thermal stimuli e.g. composite exothermic setting reaction
Bacteria and endotoxins, microleakage of oral fluids and bacteria between restoration and cavity walls
Therapeutic effect of lining materials
Calm down pulp inflammation and promote healing
3 properties to make cavity liner easy to use
Easy to mix
Long working time
Short setting time/command set
3 thermal properties of ideal lining material
Low thermal conductivity
Thermal expansion coefficient similar to dentine
Thermal diffusivity similar or lower than dentine
Ideal thermal conductivity of denture base and reasoning
High, otherwise pt may ingest too hot liquid/food, causing burns to oesophagus
What is thermal expansion coefficient?
Change in length per unit length for temp rise of +1 celsius measured in ppmC-1
What is thermal diffusivity?
Similar to conductivity, measured in cm2/sec
Thermal conductivity units
W/m-1/(degrees)C-1
Benefit of radiopaque cavity lining material
Easy to see difference between lining and tooth, makes it easier to see secondary caries or leakage
Why is it important that lining materials are cariostatic?
Prevent secondary caries
Two potential cariostatic properties of cavity liners
Fluoride releasing
Antibacterial
3 required characteristics for biocompatibility of cavity liners
Non toxic
Not damaging to pulp (pH neutral and no excessive heat on setting)
Low thermal conductivity
Examples of cavity lining materials
Setting calcium hydroxide - liner
Zinc oxide based cements - base
GlC/RMGIC - base or liner
Two components of setting calcium hydroxide
Base
Catalyst
Setting reaction of calcium hydroxide
Chelation reaction between ZnO and butylene glycol disalicylate
What is the reactive element in the catalyst used in calcium hydroxide?
Butylene glycol disalicylate
Initial pH of calcium hydroxide cement
around 12
What effect does CaOH have on dentine?
It stimulates production of reparative tertiary dentine by causing irritation to odontoblast layer
Relevance of CaOH pH
pH 12
Bactericidal to cariogenic bacteria as they survive in acidic conditions
3 useful properties of CaOH
Quick setting
Radiopaque
Easy to use
Before mixing how does CaOH appear?
Two pastes
2 undesirable properties of CaOH liner
Low compressive strength
Soluble
Zinc oxide based cement examples (5)
Zinc phosphate
Zinc polycarboxylate
Zinc oxide eugenol ZOE
Resin modified ZOE
Ethoxybezoic acid (EBA)ZOE
What type of reaction is used to create zinc phosphate cement, between which two types of material?
Acid base, between a powder and a liquid, followed by a hydration reaction
Components of zinc phosphate cement powder and their purpose
Zinc oxide >90% - main reactive ingredient
Magnesium dioxide <10% - gives white colour and increases compressive strength
Other oxides (alumina and silica) - improve physical properties
Components of zinc phosphate cement liquid
Aqueous phosphoric acid - approx 50%
Oxides which buffer the solution - aluminium oxide ensures the consistency of set material and zinc oxide slows the reaction to give better working time
Disadvantages to zinc phosphate (7)
Low initial pH
Exothermic setting reaction
Not adhesive to tooth or restoration
Not cariostatic
Final set takes 24hours
Brittle
Opaque
Difference between zinc phosphate cement and zinc polycarboxylate cement
Polyacrylic acid instead of phosphoric acid
Advantages of zinc polycarboxylate compared with zinc phosphate
Bonds to tooth surfaces (similarly to GIC)
Less heat on reaction
pH returns to neutral more quickly
Cheap
Disadvantages of zinc polycarboxylate
Difficult to mix
Difficult to manipulate
Soluble in oral environment at lower pH
Opaque
Lower modulus and compressive strength that zinc phosphate
Uses for ZOE
Linings
Temporary restorations (resin modified or EBA)
Root canal sealer (slow setting 24hours)
Periodontal dressings (fast setting 5 min)
Reaction type ZOE and elements involved
Acid base
ZnO is base
Eugenol is acid
Base + acid -> salt and water
On setting, chelation of zinc oxide with eugenol forms zinc eugenolate matrix, which bonds the unreacted ZnO
5 properties of ZOE
Adequate working time
Relatively fast setting
Low thermal conductivity
Low strength (unsuitable under amalgam)
Radiopaque
High solubility
Effects of ZOE constantly releasing eugenol
Free eugenol has obtundant effect on pulp and can reduce pain
Free eugenol can inhibit set of resin based filling materials, softening and discolouring them (not suitable under composite)
Resins added to ZOE
Polymethylmethacrylate
Polystyrene
What is the purpose of modifying ZOE by adding resins?
Increase compressive strength to >40MPa and greatly reduce solubility, making it suitable for cavity lining
Properties of EBA ZOE
Stronger than ZOE and resin modified ZOE, around 60MPa
Less soluble
Most commonly used lining material
Glass ionomer
Advantages of GIC
Cariostatic, releases fluoride over time
Can bond to and seal dentine
Can bond to composite resin
Easy to use
Light cured - long working time short setting
Thermal conductivity lower than dentine
Thermal expansion similar to dentine
High compressive strength
Radiopaque
What is the only type of material to seal dentinal tubules?
Glass ionomer
Least soluble dental cement
RMGIC
Cytotoxic material released during RMGIC polymerisation reaction, and their effect
Benzoyl iodides and benzoyl bromides, can destroy residual cavity bacteria
Why is it important that RMGIC is completely cured?
Unreacted HEMA could damage pulp
What type of bonding occurs between tooth surface and GIC, and RMGIC?
Micromechanical and chemical bonding to the resin in RMGIC
When to use lining material, and which?
Use RMGIC for amalgam and large cavities to be filled by composite
Use CaOH only when pulp is exposed, then cover with RMGIC
4 reasons a direct restoration would be required
New dental caries
Abrasion/erosion
Failed restoration/secondary caries
Trauma
Ideal properties of a direct filling material
Mechanical - strength, rigidity, hardness
Bonding to tooth/compatible with bonding systems
Thermal properties
Aesthetics
Handling/viscosity
Smooth finish/polishable
Low setting shrinkage
Radiopaque
Anticariogenic
Biocompatible
5 components of composite resin
Filler particles
Resin
Camphorquinone
Low weight dimethacrylates
Silane coupling agent
Monomers used in the resin component of composite resin
BIS-GMA
Urethane dimethacrylates
2 key characteristics for resin monomer in composite resin
Contains C=C
Undergoes free radical addition polymerisation
What is the purpose of camphorquinone in composite resin?
Produces free radical molecules when activated by blue light, which go on to initiate free radical addition polymerisation of BIS-GMA
What is the purpose of silane coupling agent in composite resin?
Helps create good bond between filler particles and resin, stops water adhering to glass filler which would prevent resin bonding to glass
What type of material is used in composite resin as filler particles?
Glass
Effects of adding filler particles
Improved strength/hardness/rigidity/abrasion resistance
Lower thermal expansion
Lower polymerisation shrinkage
Less heat of polymerisation
Improved aesthetics
Some radiopaque
Advantages of light curing
Extended working time
Less finishing
Immediate finishing
Less waste
Higher filler levels
Less porosity
Typical depth of cure of composite resin
2mm
5 potential problems of light curing
Light/materials mismatch - overexpose
Premature polymerisation under dental lights
Optimistic depth of cure - use 2mm increments
Recommended setting times too short
Polymerisation shrinkage
Conventional v microfine v hybrid composites
Conventional - strong but problems with finishing and staining due to soft resins and hard particles
Microfine(smaller particles) - better aesthetics, smoother surface, inferior mechanical properties
Hybrid - most modern composites, improved mechanical properties
Abrasion definition
Removal of surface layers when two surfaces make friction contact
3 effects of surface roughness caused by abrasion
Appearance
Plaque retention
Sensation when in contact with tongue
Material factors affecting the wear of a composite resin (5)
Filler material
Particle size distribution
Filler loading
Resin formulation
Coupling agent
Clinical factors affecting wear of a composite resin (6)
Cavity size and design
Tooth position
Occlusion
Placement technique
Cure efficiency
Finishing methods
How long to acid etch enamel and with what?
20 secs
30% phosphoric acid
Why is bonding to tooth surface necessary for a well placed composite resin? (2)
Reduce microleakage
Counteract polymerisation shrinkage
Thermal properties of composite
Thermal conductivity - low - good
Thermal diffusivity - low, similar to dentine - good
Thermal expansion coefficient - high - poor
How does self cure composite appear before use?
Two pastes to be mixed
Elastomeric impression material types (2)
Polyether
Addition silicone
Advantages of alginate as impression material
Fast setting for patient comfort
Easy to mix
Accurate impressions
High elasticity for severe undercuts etc
Important properties in impression material
Flow
Surface detail
Wettability
Elastic recovery
Stiffness
Tear strength
Mixing time
Working time
What is a study model?
Positive replica of dentition produced from an impression, used to record position, shape and dimensions of teeth to aid assessment of dentition and enable design and manufacture of dental prostheses
What is gypsum used for?
Dental study casts
Manufacture equation for dental stone or plaster
CaSO4(calcium sulphate dihydrate) + 2H2O -(heat)-> (CaSO4)2H2O (calcium sulphate hemihydrate) + H2O
Types of gypsum (3)
Plaster (beta-hemihydrate)
Dental stone (alpha-hemihydrate)
Densite (improved stone)
Gyspum plaster structure and production
Large porous irregular crystals, created when heated in open vessel
Dental stone structure and production
Non-porous, regular crystals, less water molecules produced when heated in an autoclave
Structure and production of densite
Compact smoother particles produced when heated in presence of Ca and Mg chloride
Compare gypsum setting reaction to it’s manufacture
Reverse reaction
(CaSO4)2H2O (hemihydrate) + 3H2O —-> 2CaSO4 (dihydrate) + 2H2O
Mixing ratios of dental plaster and stone
Plaster - 50-60ml/100g
Stone - 20-35ml/100g
Strengths compared of dental plaster, stone and densite
Densite - strongest
Stone - middle
Plaster - least strong
What is the effect of increasing powder in a gypsum mix?
Faster set and greater expansion
Advantages of gypsum (3)
Dimensionally accurate and stable
Low expansion
Good colour contrast
Disadvantages of using gyspum (5)
Low tensile strength
Poor abrasion resistance
Very brittle
Surface detail less than elastomer impression
Poor wetting of some impression materials
Two types of glass ionomer cement
Resin modified GIC
Conventional GIC
Two types of RMGIC
Self cure
Light cure
4 uses of GIC
Restoratione
Core build up
Lining
Luting
What are the two components of GIC and what reaction do they undergo?
Liquid - polyacrylic acid and tartatic acid
Powder - 30-40%SiO2, 15-30%Al2O3, 15-35%CaF2, 2-10%AlF3, 4-20%AlPO4, 4-10%NaF
Undergo an acid base reaction
What are the materials for anhydrous GIC?
The acid component (polyacrylic acid and tartaric acid) is freeze dried and added to the powder (silica, aluminium oxide, calcium fluoride, aluminium fluoride, aluminium phosphate, sodium fluorider), to mix, distilled water is added.
What is the main advantage of anhydrous GIC?
Easier mixing/handling
What are the main advantages of using encapsulated GIC?
Consistent powder/liquid ratio, easier to use, more consistent properties of mixed material
What are the effects of having smaller particles in a GIC?
Faster setting
More opaque
Powder particle size of GIC for luting cement?
<20micrometres
Acid base reaction involved in GIC
MO.SiO2 (glass) + H2A (acid) –> MA (salt) +SiO2 + H2O (Silica gel)
3 phases of the GIC acid base reaction
Dissolution
Gelation
Hardening
Describe dissolution phase of GIC setting reaction
Acid releases H+ ions into solution which attack the glass surface
Ca, Al, Na and F ions released
Leaves silica gel around unreacted glass
Describe the initial set of GIC and the part calcium ions play in this
Calcium ions crosslink with the polyacid by chelation with carboxyl groups to provide the initial set
Calcium ions are bivalent so they can react with 2 molecules, joining them
Crosslinking is not perfect as the Ca can chelate with two carboxyl groups on the same molecule
What is the gelation stage of GIC setting reaction?
The initial set (first few minutes) caused by the formation of calcium polyacrylate
What ions are responsible for hardening of GIC during setting reaction and how?
Trivalent aluminium ions ensure good crosslinking, forming aluminium polyacrylate
(This does not start for at least 30 min and can take 1 week + to be complete)
What is GIC protection and why is it necessary?
Placing over a varnish, resin or grease/gel to protect from contamination with moisture or excessive drying out as this will lead to a weaker final set of the material
Describe adhesion of GIC
Can bond to enamel or dentine without intermediate material
Bond strength not high compared with composite to acid etched enamel (5MPa vs 20MPa)
Good sealing ability with little leakage around margins
Disadvantages of GIC
Poor aesthetics (lack translucency)
Poor tensile strength
Lower compressive strength than composite (80-110MPa)
Higher solubility than composite
Advantages of GIC
Good thermal properties
No contraction on setting
Fluoride release
Resistant to staining
Stable chemical bond to enamel and dentine
Disadvantages of GIC
Brittle
Poor wear resistance
Moisture susceptible when first placed
Poor aesthetics
Poor handling characteristics
Susceptible to acid attack and drying out over time
Possible problems bonding to composite
What makes up the powder for resin modified GIC?
Fluro-alumino-silicate glass
Barium glass
Vacuum dried polyacrylic acid
Potassium persulphate
Ascorbic acid
Pigments
What is in RMGIC to increase readiopacity?
Barium glass
What is the liquid in RMGIC?
HEMA
Polyacrylic acid with pendant methacrylate groups
Tartaric acid
Water
Photo-inhibitors
What are the three desired advantages to RMGIC over GIC?
Light curing
Improved physical properties
Better aesthetics
What is HEMA?
2-hydroxyethylmethacrylate, a resin monomer
Describe the dual curing of RMGIC
On mixing acid-base reaction begins same as GIC
On light activation a free radical methacrylate reaction occurs resulting in a resin matrix
Quickly light activation is complete (20s)
Acid base reaction continues within resin matrix for several hours
What is the importance of placing RMGIC in layers?
Reasonably opaque material so light does not penetrate deeply, important to use layers or it may not set
Describe the tri-curing process of RMGIC
Initial acid base reaction on mixing
REDOX reaction between methacrylate monomers
On light activation free radical methacrylate reaction forms resin matrix (complete in 20s)
REDOX continues for about 5 mins after initial mix
Acid base continues within the resin matrix for several hours
Final hardening with aluminium polyacrylate formation can take days
Bonding RMGIC tooth surface conditioning
Variation in manufacturers advice
Vitrebond - none
Fuji cement LC - optional
Fuji II LC - Yes
Vitremer - Yes
Vitremer luting - No mention
Always follow manufacturers instructions
Advantages of RMGIC
Good bond to enamel and dentine
Better physical properties than GIC
Lower solubility
Fluoride release
Better translucency and aesthetics
Better handing
Disadvantages of RMGIC
Polymerisation contraction
Exothermic setting reaction
Swelling due to water uptake (HEMA extremely hydrophilic)
Monomer leaching (HEMA toxic to the pulp so must be completely polymerised)
Reduced strength if not light cured
Uses of RMGIC (8)
Dressing
Fissure sealant
Endodontic access cavity temporary filling
Luting
Orthodontic cement
Restoration of deciduous teeth
Restoration of permanent teeth
Base or lining
Uses of metal alloys
Partial denture framework - CoCr
Crowns - stainless steel
Denture base - stainless steel
Orthodontic appliance - NiTi
Restorations - amalgam
What are the good mechanical properties of metals?
Strength
Rigidity
Elastic limit
Ductility
Hardness
What is the major disadvantage to using metals in dentistry?
Aesthetics
Metal defintion
Aggregate of atoms in crystalline structure
Metal alloy definition
Combination of metal atoms in a crystalline structure
What is elastic limit?
Maximum stress without plastic deformation
What is ductility?
Amount of plastic deformation prior to fracture
Factors affecting metal alloy mechanical properties (3)
Crystalline structure
Grain size
Grain imperfections
Crystal grain structures (3)
Equi-axed - if crystal growth is of equal dimension in each direction
Radial - molten metal cooled quickly in cylindrical mould
Fibrous - wire pulled through die (cold worked)
What is the effect of fast cooling on metal alloys?
More nuclei
Small fine grains
What is the effect of slow cooling metal alloys?
Few nuclei
Large coarse grains
Which atoms act as nuclei of crystallisation?
Those at the edge of grains
What are nucleating agents for alloy crystallisation?
Impurities or additives that act as a foci for crystal growth
What is a grain in metal alloys?
Single crystal (lattice) with atoms orientated in given directions (dendrites)
Grain boundary
Change in orientation of the crystal planes (impurities concentrate here)
Advantages/disadvantages of small fine grains
+ high elastic limit
+ increased fracture strength, hardness and ultimate tensile strength
- decreased ductility
What are dislocations in metal alloys?
Imperfections/defects in the crystal lattice. Propagation of dislocations leads to slip
Effects of preventing movement of dislocations in metals
Increase - elastic limit, fracture stress/UTS, hardness
Decrease - ductility, impact resistance
Factors that impede dislocation movement
Grain boundaries
Different atom sizes in atoms
Cold working
What is cold working?
Work done on a metal or alloy such as bending, rolling, swaging, at a low temperature. It causes slip which makes dislocations collect at grain boundaries, making a stronger, harder material
Effects of cold working
Higher EL, FS/UTS, hardness
Lower ductility, impact strength, corrosion resistance
What is the effect of residual stress from cold working metal or alloys?
Causes instability in the lattice which results in distortion over time.
How can residual stress on a metal or alloy be relieved?
Annealing
Describe annealing process
Heating metal (or alloy) so that greater thermal vibrations allows migration of atoms
What is recrystallisation?
The effect of heat on a metal/alloy, leading to new smaller equiaxed grains, lower EL, UTS, hardness, and increased ductility
How are recrystallisation and cold working related?
Recrystallisation spoils the benefits of cold work, and allows further cold work
Cold work/recrystallisation is repeated until the correct shape is obtained
What effect does cold working have on recrystallisation temperature?
More cold working -> lower recrystallisation temperature
Effect of temperature increase on grain size
Larger grains
What is a PHASE (metal alloys)
Physically distinct homogenous structure
What is a SOLUTION (metal alloys)
Homogenous mixture at an atomic scale
3 possibilities when melting together and crystallising 2 metals
Insoluble, no common lattice - exists as 2 phases
Intermetallic compound with specific formula e.g. Ag3Sn
Soluble and form a solid solution - common lattice
2 types of solid solution
- substitutional - atoms of one metal replace the other metal in the crystal lattice/grain
- interstitial - atoms markedly different in size, smaller atoms located in spaces in lattice/grain structure of larger atom
What are the effects of SLOW cooling a molten alloy?
Allows metal atoms to diffuse through lattice, ensures grain composition in homogenous, but this results in large grains
What are the effects of RAPID cooling a molten alloy?
Prevents atoms diffusing through lattice, causes CORING as composition varies throughout grain, which may reduce corrosion resistance but is otherwise undesirable
How to reduce coring but maintain small grains
Homogenising anneal -reheat to allow atoms to diffuse and so cause grain composition to become homogenous, keep below recrystallisation temp or grains will be altered
Eutectic alloys
Where liquidus and solidus coincide (crystallisation takes place at a single temperature)
What is an alloy in solid solution?
Both metals co-existing in each grain
