Metal & Alloys Flashcards
Which is the only alloy (in BDS2) to undergo a setting reaction.
Amalgam
What does a HIGH YOUNG’S (ELASTIC) MODULUS look like on a stress strain diagram and what does this indicate?
Looks like a very steep gradient.
This indicates the material is very RIGID.
So large stress => small strain (small deformation)
What is ductility?
The amount of plastic deformation prior to fracture (the extent a material can be shaped/ manipulated)
What are grains in metal?
The formation of crystals (from the nuclei of crystallisation) as the metal cools, that grow until they impinge on other crystals - called GRAIN BOUNDARY. Impurities and defects concentrate at grain boundaries.
Would you rather have small grains or large grains in a metal alloy and why?
SMALL GRAINS
Because it is will have a high elastic limit stress value – so deformation requires a large stress to be applied.
&
it results in an increased fracture strength and surface hardness (better mechanical properties)
BUT LESS DUCTILE
What is quenching?
Rapid cooling. Heat just above mpt. before cooling, use small quantity of metal.
Causes CORING.
Quenching achieves small grains which is desirable
What is SLIP and what role do grain boundaries play in it?
Slip is when a dislocation (imperfection) is propagated along the lattice when a force is applied to a metal/ alloy.
Presence of grain boundaries prevents dislocation so a metal/ alloy with small grains means dislocations are impeded.
Results in increase in elastic limit, fracture strength and surface hardness.
Effects of cold working (changing shape of metal below mpt/ recrystallisation temperature)
Higher - elastic limit, fracture stress, hardness
Lower - ductility, impact strength, lower corrosion resistance.
What is the purpose of annealing?
Heating a metal or alloy causes greater thermal vibrations allows migration of atoms - eliminates instability or distortion. DOES NOT CHANGE GRAIN STRUCTURE, used after cold work.
What is a substitutional solid solution?
Where atoms of one metal replace (or substitute for) the other metal in the crystal lattice/grain. Can be random (metal atoms must be similar in size, valency etc.), or ordered (regular lattice arrangement with same conditions size, valency etc.)
NOTE - solid solution formed when 2 metals are soluble
What is interstitial solid solution?
2 atoms to be markedly different in size - smaller atoms between larger atoms.
NOTE - solid solution formed when 2 metals are soluble
What is the difference between a pure metal and alloy crystallising temperature?
metal - crystallises at one temperature
alloy - crystallises over temperature range (Tl - crystallisation begins, Ts - crystallisation ends)
How is coring caused?
Rapid cooling of alloys
Coring = where the composition of grains vary throughout.
What is a negative consequence of CORING?
Reduces the corrosion resistance of the alloy = OVERALL UNDESIRABLE. Coring caused by the separation of the LIQUIDUS and SOLIDUS (the greater the separation the more coring)
Why are alloys more fracture resistant (stronger) than metals?
Alloys require greater stress to move dislocations than metal (greater resistance to dislocations improve mechanical properties) BECAUSE there are two types of metal in the grain.
What is a solid solution alloy?
each grain has both types of metal present
both types of metals are soluble in one another
What is a eutectic alloy?
Both metals are insoluble in one another so they form separate grains.
Dental application - soldering alloy parts together.
What are the ideal properties of a partial denture alloy?
Rigid (YM) Strong (Elastic limit stress) Hard Ductile Precise casting (limited shrinkage)
What does cold working do?
Pushes dislocations to the grain boundaries
How do you change the rigidness (YM) of a material without changing the structure of it?
Change the thickness
Thick = high YM (base) Thin = low YM (clasps - to flex)
What characteristic does Nickel give in a CoCr PDA?
Ductility
What does chromium add in a CoCr PDA that cobalt does not?
Chromium enhances CORROSION RESISTANCE.
Both increase strength, hardness, rigidity
What is CoCr hardness?
370 hardness units - much harder than type IV gold.
Components of CoCr
Co (35-65%) 54% Cr (25-30%) 25% Ni (0-30%) 15% Mo (5-6%) 5% C (0.2–0.4%) 0.4%
State some drawbacks of CoCr
Shrinks during casting
Is not as ductile as Type 4 Gold
More difficult to polish than Type 4 Gold
State some reasons why CoCr is a good PDA
Low density - more lightweight for patient
Very rigid (high Young’s modulus)
Very hard
Fairly high abrasion resistance
