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BDS2 DMS Flashcards

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1
Q

Types of error in PMMA dentures (3)

A

Flawed initial impression
Error during curing process
Usage

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2
Q

Potential usage problems in PMMA denture (5)

A

Fractures
Uncomfortable
PMMA becomes warped
Surface wear
Patient anatomy changes

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3
Q

3 requirements of ideal denture

A

Replaces function of natural dentition
Fits comfortable
Good aesthetics

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4
Q

Ideal properties for denture base material 7

A

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

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5
Q

What does high Young’s Modulus mean?

A

High stress causes small strain

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6
Q

What does high elastic limit mean?

A

Only large stresses will cause permanent deformation

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7
Q

Free radical addition polymerisation

A

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)

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8
Q

4 steps of acrylic polymerisation reaction

A

Activation
Initiation
Propagation
Termination

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9
Q

What happens in activation stage of acrylic polymerisation?

A

Free radicals are formed

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10
Q

What happens in initiation stage of acrylic polymerisation?

A

Free radicals break C=C bonds in each monomer, and transfer free radicals

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11
Q

What happens in propagation stage of acrylic polymerisation?

A

Polymer chain grows

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12
Q

What must be done to enamel surface before bonding a composite resin?

A

Acid etch

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13
Q

Advantages and disadvantages of porcelain

A

Aesthetically good
Hard and rigid - more easily fractured

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14
Q

What is stress?

A

Force/ unit area in N/metres squares - PASCALS

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15
Q

Difference in retention between amalgam and composite

A

Amalgam uses mechanical retention - undercuts
Composite uses adhesive retention - minimal cavity prep

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16
Q

Creep

A

Gradual dimensional change due to repetitive small force

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17
Q

Fatigue

A

Repetitive small stresses cause fracture

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18
Q

Deformation

A

Repetitive small stresses cause permanent change in material dimensions without fracture

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19
Q

Elasticity

A

Ability of a material to recover its dimensions follow application of stress

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20
Q

Properties of ideal dental adhesive (6)

A

High bond strength
Immediate bond
Durable bond
Impermeable bone
Easy to use
Safe

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21
Q

Why is enamel easy to bond to?

A

Heterogenous - densely packed prismatic structure

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22
Q

Acid etch process

A

Roughens the surface, long enamel prisms are filled with imperfectly packed hydroxyapatite crystals

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23
Q

Why acid etch?

A

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

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24
Q

Why must enamel be dry for composite?

A

Moisture contamination prevents flow of resin into etched surface

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25
What is used for etch?
30-50% aqueous phosphoric acid
26
Dentine composition
20% inorganic (mostly collagen) 70% inorganic (mostly hydroxyapatite) 10% water
27
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
28
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
29
Describe mechanical bonding to dentine
Dentine bonding agent and dentine surface mesh and interlock together with minimal gaps
30
Chemical adhesion of dentine
Mineralised dentine forms ionic bonds Organic dentine forms covalent bonds
31
What happens to dentine as it ages?
Becomes more mineralised
32
Relevance of surface energy in bonding
A liquid will only spread on a surface with higher surface energy than it
33
What is amalgam?
An alloy formed by the reaction of mercury (liquid) and silver, tin, copper and other metals (powder)
34
Purpose of copper in amalgam
Increases strength and hardness
35
Amalgam particle types
Lathe cut Spherical/spheroidal
36
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
37
Setting reaction for amalgam
Ag3Sn +Hg -> Ag2Hg3 +Sn7Hg9
38
Set structure of gamma particles in amaglam
Gamma 1 holds together particles of Gamma and Gamma 2
39
In modern amalgam what are 2 setting dimensional changes?
Small contraction (<0.2%) Solid solution of Hg in Ag3Sn
40
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
41
4 factors affecting amalgam's properties
Handling Cavity design Corrosion Variation between products
42
5 advantages of spherical amalgam particles
Earlier high tensile strength Higher tensile strength Less sensitive to condensation Easier to carve Less mercury required
43
Describe early (1hr) and late (>24hr) strength of amalgam
Early - poor Late - fairly good
44
Abrasion resistance of amalgam
High, suitable for posterior teeth Too high for deciduous
45
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
46
3 factors affecting marginal integrity of amalgam
Creep Cavity design Corrosion
47
What is creep?
When a material flows slightly as a result of repeated low level stresses. This is a kind of permanent deformation
48
Thermal conductivity of amalgam and relevance
High thermal conductivity so in deep cavities it may be necessary to use a liner
49
How is amalgam bonded to tooth?
It is not bonded. It uses mechanical retention
50
What is added to amalgam to resist corrosion?
Copper
51
Is it easier to bond a material to enamel or dentine?
Enamel
52
What acts as a "wetting agent" when carrying out a restoration?
Bonding agent
53
4 benefits of copper enriched amalgam
Higher early strength Less creep Higher corrosion resistance Increased durability of margins
54
Advantages of amalgam (5)
Strong Hard Durable Radiopaque User friendly
55
Disadvantages of amalgam (4)
Corrosion Leakage Poor aesthetics Mercury - potential toxicity and environmental impact
56
5 potential restorative materials
Composite GIC Amalgam Precious metal Ceramic
57
Why is release of chemicals an undesirable property in restorative materials?
Could be pulpal irritants and lead to pain or pulpal damage
58
What is the purpose of intermediate restorative materials (cavity liners)?
Prevent gaps Act as protective barrier
59
What is a cavity base?
Thick mix placed in bulk Replaces dentine to minimise restoration material used or block out undercuts
60
Which type of restorative material more often requires a cavity base?
Metal
61
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
62
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
63
Therapeutic effect of lining materials
Calm down pulp inflammation and promote healing
64
3 properties to make cavity liner easy to use
Easy to mix Long working time Short setting time/command set
65
3 thermal properties of ideal lining material
Low thermal conductivity Thermal expansion coefficient similar to dentine Thermal diffusivity similar or lower than dentine
66
Ideal thermal conductivity of denture base and reasoning
High, otherwise pt may ingest too hot liquid/food, causing burns to oesophagus
67
What is thermal expansion coefficient?
Change in length per unit length for temp rise of +1 celsius measured in ppmC-1
68
What is thermal diffusivity?
Similar to conductivity, measured in cm2/sec
69
Thermal conductivity units
W/m-1/(degrees)C-1
70
Benefit of radiopaque cavity lining material
Easy to see difference between lining and tooth, makes it easier to see secondary caries or leakage
71
Why is it important that lining materials are cariostatic?
Prevent secondary caries
72
Two potential cariostatic properties of cavity liners
Fluoride releasing Antibacterial
73
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
74
Examples of cavity lining materials
Setting calcium hydroxide - liner Zinc oxide based cements - base GlC/RMGIC - base or liner
75
Two components of setting calcium hydroxide
Base Catalyst
76
Setting reaction of calcium hydroxide
Chelation reaction between ZnO and butylene glycol disalicylate
77
What is the reactive element in the catalyst used in calcium hydroxide?
Butylene glycol disalicylate
78
Initial pH of calcium hydroxide cement
around 12
79
What effect does CaOH have on dentine?
It stimulates production of reparative tertiary dentine by causing irritation to odontoblast layer
80
Relevance of CaOH pH
pH 12 Bactericidal to cariogenic bacteria as they survive in acidic conditions
81
3 useful properties of CaOH
Quick setting Radiopaque Easy to use
82
Before mixing how does CaOH appear?
Two pastes
83
2 undesirable properties of CaOH liner
Low compressive strength Soluble
84
Zinc oxide based cement examples (5)
Zinc phosphate Zinc polycarboxylate Zinc oxide eugenol ZOE Resin modified ZOE Ethoxybezoic acid (EBA)ZOE
85
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
86
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
87
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
88
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
89
Difference between zinc phosphate cement and zinc polycarboxylate cement
Polyacrylic acid instead of phosphoric acid
90
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
91
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
92
Uses for ZOE
Linings Temporary restorations (resin modified or EBA) Root canal sealer (slow setting 24hours) Periodontal dressings (fast setting 5 min)
93
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
94
5 properties of ZOE
Adequate working time Relatively fast setting Low thermal conductivity Low strength (unsuitable under amalgam) Radiopaque High solubility
95
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)
96
Resins added to ZOE
Polymethylmethacrylate Polystyrene
97
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
98
Properties of EBA ZOE
Stronger than ZOE and resin modified ZOE, around 60MPa Less soluble
99
Most commonly used lining material
Glass ionomer
100
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
101
What is the only type of material to seal dentinal tubules?
Glass ionomer
102
Least soluble dental cement
RMGIC
103
Cytotoxic material released during RMGIC polymerisation reaction, and their effect
Benzoyl iodides and benzoyl bromides, can destroy residual cavity bacteria
104
Why is it important that RMGIC is completely cured?
Unreacted HEMA could damage pulp
105
What type of bonding occurs between tooth surface and GIC, and RMGIC?
Micromechanical and chemical bonding to the resin in RMGIC
106
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
107
4 reasons a direct restoration would be required
New dental caries Abrasion/erosion Failed restoration/secondary caries Trauma
108
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
109
5 components of composite resin
Filler particles Resin Camphorquinone Low weight dimethacrylates Silane coupling agent
110
Monomers used in the resin component of composite resin
BIS-GMA Urethane dimethacrylates
111
2 key characteristics for resin monomer in composite resin
Contains C=C Undergoes free radical addition polymerisation
112
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
113
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
114
What type of material is used in composite resin as filler particles?
Glass
115
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
116
Advantages of light curing
Extended working time Less finishing Immediate finishing Less waste Higher filler levels Less porosity
117
Typical depth of cure of composite resin
2mm
118
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
119
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
120
Abrasion definition
Removal of surface layers when two surfaces make friction contact
121
3 effects of surface roughness caused by abrasion
Appearance Plaque retention Sensation when in contact with tongue
122
Material factors affecting the wear of a composite resin (5)
Filler material Particle size distribution Filler loading Resin formulation Coupling agent
123
Clinical factors affecting wear of a composite resin (6)
Cavity size and design Tooth position Occlusion Placement technique Cure efficiency Finishing methods
124
How long to acid etch enamel and with what?
20 secs 30% phosphoric acid
125
Why is bonding to tooth surface necessary for a well placed composite resin? (2)
Reduce microleakage Counteract polymerisation shrinkage
126
Thermal properties of composite
Thermal conductivity - low - good Thermal diffusivity - low, similar to dentine - good Thermal expansion coefficient - high - poor
127
How does self cure composite appear before use?
Two pastes to be mixed
128
Elastomeric impression material types (2)
Polyether Addition silicone
129
Advantages of alginate as impression material
Fast setting for patient comfort Easy to mix Accurate impressions High elasticity for severe undercuts etc
130
Important properties in impression material
Flow Surface detail Wettability Elastic recovery Stiffness Tear strength Mixing time Working time
131
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
132
What is gypsum used for?
Dental study casts
133
Manufacture equation for dental stone or plaster
CaSO4(calcium sulphate dihydrate) + 2H2O -(heat)-> (CaSO4)2H2O (calcium sulphate hemihydrate) + H2O
134
Types of gypsum (3)
Plaster (beta-hemihydrate) Dental stone (alpha-hemihydrate) Densite (improved stone)
135
Gyspum plaster structure and production
Large porous irregular crystals, created when heated in open vessel
136
Dental stone structure and production
Non-porous, regular crystals, less water molecules produced when heated in an autoclave
137
Structure and production of densite
Compact smoother particles produced when heated in presence of Ca and Mg chloride
138
Compare gypsum setting reaction to it's manufacture
Reverse reaction (CaSO4)2H2O (hemihydrate) + 3H2O ----> 2CaSO4 (dihydrate) + 2H2O
139
Mixing ratios of dental plaster and stone
Plaster - 50-60ml/100g Stone - 20-35ml/100g
140
Strengths compared of dental plaster, stone and densite
Densite - strongest Stone - middle Plaster - least strong
141
What is the effect of increasing powder in a gypsum mix?
Faster set and greater expansion
142
Advantages of gypsum (3)
Dimensionally accurate and stable Low expansion Good colour contrast
143
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
144
Two types of glass ionomer cement
Resin modified GIC Conventional GIC
145
Two types of RMGIC
Self cure Light cure
146
4 uses of GIC
Restoratione Core build up Lining Luting
147
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
148
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.
149
What is the main advantage of anhydrous GIC?
Easier mixing/handling
150
What are the main advantages of using encapsulated GIC?
Consistent powder/liquid ratio, easier to use, more consistent properties of mixed material
151
What are the effects of having smaller particles in a GIC?
Faster setting More opaque
152
Powder particle size of GIC for luting cement?
<20micrometres
153
Acid base reaction involved in GIC
MO.SiO2 (glass) + H2A (acid) --> MA (salt) +SiO2 + H2O (Silica gel)
154
3 phases of the GIC acid base reaction
Dissolution Gelation Hardening
155
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
156
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
157
What is the gelation stage of GIC setting reaction?
The initial set (first few minutes) caused by the formation of calcium polyacrylate
158
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)
159
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
160
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
161
Disadvantages of GIC
Poor aesthetics (lack translucency) Poor tensile strength Lower compressive strength than composite (80-110MPa) Higher solubility than composite
162
Advantages of GIC
Good thermal properties No contraction on setting Fluoride release Resistant to staining Stable chemical bond to enamel and dentine
163
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
164
What makes up the powder for resin modified GIC?
Fluro-alumino-silicate glass Barium glass Vacuum dried polyacrylic acid Potassium persulphate Ascorbic acid Pigments
165
What is in RMGIC to increase readiopacity?
Barium glass
166
What is the liquid in RMGIC?
HEMA Polyacrylic acid with pendant methacrylate groups Tartaric acid Water Photo-inhibitors
167
What are the three desired advantages to RMGIC over GIC?
Light curing Improved physical properties Better aesthetics
168
What is HEMA?
2-hydroxyethylmethacrylate, a resin monomer
169
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
170
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
171
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
172
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
173
Advantages of RMGIC
Good bond to enamel and dentine Better physical properties than GIC Lower solubility Fluoride release Better translucency and aesthetics Better handing
174
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
175
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
176
Uses of metal alloys
Partial denture framework - CoCr Crowns - stainless steel Denture base - stainless steel Orthodontic appliance - NiTi Restorations - amalgam
177
What are the good mechanical properties of metals?
Strength Rigidity Elastic limit Ductility Hardness
178
What is the major disadvantage to using metals in dentistry?
Aesthetics
179
Metal defintion
Aggregate of atoms in crystalline structure
180
Metal alloy definition
Combination of metal atoms in a crystalline structure
181
What is elastic limit?
Maximum stress without plastic deformation
182
What is ductility?
Amount of plastic deformation prior to fracture
183
Factors affecting metal alloy mechanical properties (3)
Crystalline structure Grain size Grain imperfections
184
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)
185
What is the effect of fast cooling on metal alloys?
More nuclei Small fine grains
186
What is the effect of slow cooling metal alloys?
Few nuclei Large coarse grains
187
Which atoms act as nuclei of crystallisation?
Those at the edge of grains
188
What are nucleating agents for alloy crystallisation?
Impurities or additives that act as a foci for crystal growth
189
What is a grain in metal alloys?
Single crystal (lattice) with atoms orientated in given directions (dendrites)
190
Grain boundary
Change in orientation of the crystal planes (impurities concentrate here)
191
Advantages/disadvantages of small fine grains
+ high elastic limit + increased fracture strength, hardness and ultimate tensile strength - decreased ductility
192
What are dislocations in metal alloys?
Imperfections/defects in the crystal lattice. Propagation of dislocations leads to slip
193
Effects of preventing movement of dislocations in metals
Increase - elastic limit, fracture stress/UTS, hardness Decrease - ductility, impact resistance
194
Factors that impede dislocation movement
Grain boundaries Different atom sizes in atoms Cold working
195
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
196
Effects of cold working
Higher EL, FS/UTS, hardness Lower ductility, impact strength, corrosion resistance
197
What is the effect of residual stress from cold working metal or alloys?
Causes instability in the lattice which results in distortion over time.
198
How can residual stress on a metal or alloy be relieved?
Annealing
199
Describe annealing process
Heating metal (or alloy) so that greater thermal vibrations allows migration of atoms
200
What is recrystallisation?
The effect of heat on a metal/alloy, leading to new smaller equiaxed grains, lower EL, UTS, hardness, and increased ductility
201
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
202
What effect does cold working have on recrystallisation temperature?
More cold working -> lower recrystallisation temperature
203
Effect of temperature increase on grain size
Larger grains
204
What is a PHASE (metal alloys)
Physically distinct homogenous structure
205
What is a SOLUTION (metal alloys)
Homogenous mixture at an atomic scale
206
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
207
2 types of solid solution
1. substitutional - atoms of one metal replace the other metal in the crystal lattice/grain 2. interstitial - atoms markedly different in size, smaller atoms located in spaces in lattice/grain structure of larger atom
208
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
209
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
210
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
211
Eutectic alloys
Where liquidus and solidus coincide (crystallisation takes place at a single temperature)
212
What is an alloy in solid solution?
Both metals co-existing in each grain

BDS3 (20 decks)

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