Metals And Alloys 1 Flashcards
List 5 applications of metals and alloys and examples of what ones are used
- Partial denture framework (CoCr, type IV gold)
- Crowns (stainless steel)
- Denture base (stainless steel)
- Orthodontic appliance (NiTi)
- Restorations (amalgam)
Why are metals used for such a wide range of appliances despite their poor aesthetics?
1. Superior mechanical properties •strength •rigidity •ductility •hardness •elastic limit
What is the definition of an alloy?
Combination of metal atoms in a crystalline structure
How is rigidity (elastic modulus) calculated?
Gradient of stress strain graph
What type of stress shows the ductility of a metal?
What type of stress shows the malleability of a metal?
- Tensile stress
2. Compressive stress
What do the following terms on a stress strain curve mean:
FS
El
UTS
ductility
- FS - fracture strength
- EL - maximum stress without plastic deformation
- UTS - ultimate tensile strength
- Ductility - amount of plastic deformation prior to fracture (y-x)%
Name 3 factors that affect mechanical properties of a metal
- Crystalline structure
- Grain size
- Grain imperfections
Describe 3 different crystal/lattice metal structures
- Cubic
- simple cubic and related structures - Face-centred cubic
- cubic close packed and related structures - Body centred cubic
- body centred cubic and related structures
Describe crystal growth during cooling of molten metals
- As the first atoms drop down to melting point temperature, they crystallise at certain sites
- atoms at these sites are known as nuclei of crystallisation - Crystals grow to form dendrites (3D branched lattice network)
- Crystals (or Grains) grow until they impinge on other crystals
- Region where grains make contact is called the grain boundary
What is an EQUI-AXED grain structure?
- Grains formed from crystal growth that is of equal dimensions in each direction
What causes a radial grain structure?
Molten metal cooled quickly in a cylindrical mould
What causes a fibrous grain structure?
Wire pulled through die (cold worked metal alloy)
How does cooling speed affect crystal growth?
- Fast cooling
- more nuclei
- small fine grains - Slow cooling
- few nuclei
- large coarse grains
What are nucleating agents?
How do they affect crystal growth?
Impurities or additives that act as foci for crystal growth
More buck eating agents results in more fine grains
Define what a grain is
A single crystal (lattice) with atoms orientated in given directions (dendrites)
What is a grain boundary?
A change in the orientation of the crystal planes
- impurities concentrate here
What are advantages/disadvantages of small fine grains?
Advantages:
- high elastic limit
- increased FS and UTS, hardness
Disadvantages:
- decreases ductility
What factors help ensure rapid cooling (quenching)?
- Small bulk
- Heat metal/allow just above melting temperature (Tm)
- Use a mould with high thermal conduction
- Quench
What are dislocations?
- Imperfections/defects in the crystal lattice
Describe slip
- Slip is due to propagation of dislocations and involves rupture of only a few bonds at a time
How does impeding the movement of dislocations in metals/alloys affect the metals properties?
- Increases:
- elastic limit
- fracture stress/UTS
- hardness - Decreases:
- ductility
- impact resistance
What factors impede dislocation movement?
- Grain boundaries (hence fine grains)
- Alloys: different atom sizes
- Cold working - dislocations stopped at grain boundaries
What is cold work?
- Work done on a metal/alloy (e.g. bending, rolling, swaging)
- done at a low temperature (below recrystallisation temperature)
What effects does cold work have on metals/alloys?
- Causes slip - so dislocations collect at grain boundaries
- results in stronger, harder material
What effect does cold work have on metal/alloy properties?
- Higher
- elastic limit
- fracture stress/UTS
- hardness - Lower
- ductility
- impact strength
- lower corrosion resistance
How does cold work effect residual stress in metals and alloys?
Increased cold work results in increased residual stress
How does residual stress effect metal?
How is residual stress relieved?
- Causes instability in lattice
- Results in distortion over time
• relieved by annealing process
What is annealing?
Heating metal or alloy so that greater thermal vibrations allows migration of atoms (rearrangement)
How does annealing effect cold worked metals?
In this scenario referred to as stress relief annealing
- eliminates stresses by allowing atoms to re-arrange within grains
- grain structure and mechanical properties unchanged
- some further cold work possible (final shaping)
What is recrystallisation and how does it effect the structure of metals and alloys?
- Heating of metals and alloys which causes
- new smaller equiaxed grains
- lower EL, UTS, hardness
- increased ductility
How does recrystallisation effect cold working?
- Spoils benefits of cold work
- Allows further cold work
- Cold work/recrystallisation repeated until correct shape obtained
What factor influences recrystallisation temperature?
- The amount of cold work carried out
- increased cold work = decreased recrystallisation temperature
How does an excessive temperature rise effect grain growth in metals and alloys?
Causes large grains to replace smaller coarse grains, yielding poorer mechanical properties
