Metals Flashcards
Metal definition
Any of a class of chemical elements generally characterized by ductility, luster, conductivity of heat and electricity
Alloy definition
a mixture of metals
8 properties of metals
Usually strong/hard and stuff tough - not brittle lustrous dense good conductors of electricity good conductors of heat opaque ductile, malleable
casting metal
pouring liquid metal into mold
machining metal
cutting - drilling, turning…
cold working metal
deforming metal in solid state - forging, extruding…
powder metallurgy
pouring metal powder into mold and sintering
Metallic bonding
Primary/strong bonding in metals
+ ions surrounded by a cloud of electrons
Electrons can move
What are the consequences of metallic bonding?
- High conductivity (thermal and electrical) due to the movement of e-
- Opacity - electrons absorb electromagnetic and photon energy
- easy to form alloys
- High plasticity
All metals have a ______ structure
crystalline
technically polycristalline
Crystalline material definition
Shows a long-range regular arangement of atoms
eg. metals
In conrast to amorphous/glassy materials
unit cell
Smallest repeating unit in a crystal
What are the 3 common unit cells of the metals used in dentistry?
body centered cubic
face centered cubic
hexagonal close packed
Many of the properties of metals can be explained in relation to _____ in their crystalline structures
imperfections
What are the 3 types of crystal imperfections in metals
point defects
line defects
plane defects
What are the types of point defects a metal can have
impurities - substitutional, interstitial
vacancies
substitutional impurity point defect
One of the points in the crystal lattice is replaced by a different thing
interstitial impurity point defect
impurity lies between points of crystal lattice
vacancy point defect
a piece of the crystal lattice is missing
The easiness of having impurities (atoms of another nature in positions of the crystal lattice of metals, point defects) justifies that metals can be ______
easily alloyed (mixed)
Line defects
Dislocation in structure of metal
Formation of an extra semiplane of atoms in the metal structure
Below extra semiplane there is a line defect where the semiplane does not continue
Plane defect
When a metal cools after being liquid - solidifies as a multicrystalline/multigranular structure
Crystals with different crystal orientation join togethr forming grain boundaries = plane defects
metals are an agglomerate of small ______
crystals/grains
metals have a _____ structure
polycrystalline
grain size/shape influences the ______ of metals
mechanical properties
A metal with _______ grains/crystals is stronger. why?
smaller
finer - less coarse
More grain boundaries/plane defects per volume
fibrous grain strucure of metals in obtained via ____
cold working
Elastic strain of metals
recoverable
stretching of inter-atomic bonds
on removal of force atoms return to their equilibrium position
metallic bond acts as A SPRING
metallic bonds act as…
a spring
elasticity of metals depends mainy on…
the resistance of the metallic bond to be stretched
Sum of elastic deformation of metallic bonds accounts for…
the overall elastic deformation of the metallic material
Modulus of elasticity of a metal mainly depends on
the chemical nature of each metal
hardly depends on other structural features of the metal
Mechanism of plastic strain in metals
permanent
movement/slip of dislocations
The easier it is to create dislocations in a metal, the _____ it is
softer
How could you strengthen a metal?
treat/process metal to make dislocations difficult
How does cold working a metal change it?
Introduces more dislocations, more interaction between dislocations = harder to move them
STRENGTHENS
too much = brittle
Lead to fibrous structure
Casting of a metal results in
equiaxed structure
Effects of recrystallization after cold working a metal
Further heating = reformation of equiaxed structure, elimination of dislocations, erases deformation/cold working history of metal
SOFTER
Effect of grain growth during metal processing
even more heating
larger grains formed in same volume of metal
less grain/crystal boundaries, less impediment of dislocation
SOFTER
What are the steps in mechanical/thermal history (processing) of a metal
casting
cold working
recrystallization
grain growth
metals are very tough because they can develop significant ____
plasticity - form/move dislocations
if dislocations are difficult to move, the metal will be more ____, less ____
brittle
tough
Like ceramics
Polymers have permanent deformation because they are _____
viscoelastic
viscous component - disentanglement of molecules
Metals are not viscoelastic, their plasticity does not depend on the time you apply a load, but ____
the magnitude of the load
Pure metals are often not used in dentistry because
they are often too soft and ductile
Why are alloys stronger than pure metals?
The movement of dislocations is more difficult when introducing a new metal in the pure metal structure
What are the 4 types of solid alloys
solid solution
complete solid insolubility
partial solid insolubility
intermetallic compound
solid solution alloy
perfect solubility
1 phase present - no separation
What are the 2 types of solid solution alloy
substitutional
interstitial
substitutional solid solution alloy
2 metals in alloy have same crystal lattice (unit cell) and similar atomic size
Similar chemical valency (same # electrons to share)
no rxn to form intermetallic compounds
Which metals are used to alloy in dentistry? which kind of alloy do they form? why?
gold, copper, silver, platinum, palladium
substitutional solid solution alloy
All have fcc crystal structures and similar atom size
interstitial solid solution alloy
atoms of one metal between crystal lattice of other metal
Example of interstitial solid solution (dental)
Stainless steel - instruments, ortho wires
carbon atoms inside of iron crystal structure
complete solid insolubility alloy
total incompatibility of 2 metals
like oil and water
rare
partial solid insolubility alloy
Once initial solid solution is formed, on further cooling (specific conditions) - precipitation of a second phase occurs
= Same metals mixed in different quantities with different unit cells
Resulting alloy is STRONGER (harder, more brittle) because the 2nd phase pins the movement of the dislocations
intermetallic compounds
compounds with specific and well defined stoichiometry/composition
MERCURY AMALGAMS
You cannot really change the rigidity of a metal, but you can change the
strength
We use metals in dentistry because metals can be both very ____ and ____
strong
tough
metals used inthe mouth must be biocompatible, aka…
resistant to corrorsion in aggressive environment of mouth
NO release of metallic ions
Limits range of metals that can be used
Which metallic alloys are used in dentistry?
Noble alloys - nonreactive
Stainless/passivated alloys - initially reactive then stop
mercury
What are dental examples of noble alloys?
alloys based in:
gold
platinum
palladium
Dental examples of stainless/passivated alloys
stainless steel
chromium based
titanium based
List the applications of alloys in dentistry
amalgam inlays, crowns, bridges, dentures bonding to porcelain wires, brackets, bands implants instruments
