metals and alloys 2 Flashcards
alloy
a combination or mixture of 2 or more metals, or metals with a metalloid (Si, C)
3 advantages of alloys
Mechanical (EL, UTS, hardness) than metals
Corrosion resistance (potentially)
lower melting point than individual metal
4 dental examples of alloy uses
STEEL - burs, instruments
AMALGAM - filling material
GOLD ALLOY - inlays, crowns, bridges, partial dentures, wires
NICKEL CHROMIUM - crowns, bridges, wires
etc……
phase
Physically distinct homogenous structure (can have more than one component)
solution
Homogenous mixture at an atomic scale
Lattice arrangement of 2 or more metals
- Metals coexist in lattice structure called a solution
how many phases are there in a metal formed of one metal atom only
1 phase
1 atom homogenous
how many phases are there in a metal made of individual grains of 2 phases situated in a lattice network
2 phases
don’t exist in same grain - so 2 phases
number of individual grains of different metals
how many phases are there in a metal made of 2 metals in a homogenous mix
1 phase
2 metal atoms coexisting in same lattice structure on atomic scale
- Homogenous
- Single phase
Solid solution
- Several grains of 2 different metal atoms
- Grains of varying shape and size
what is the physical property of metals when molten
soluble usually
what are the 3 types of solid solution that can occur on crystallisation of molten alloy
insoluble (no common lattice; 2 phases)
form inter-metallic compound with specific chemical formulation
be soluble and form a solid solution
properties of solid solution
form a common lattice, co-exist in solid-solution
when an alloy is in a molten state
all metal components are soluble in one another
what is substitutional solid solution
atoms of one metal replace the other metal in the crystal lattice/grain.
RANDOM:- metal atoms similar in:- SIZE, VALENCY, CRYSTAL STRUCTURE (e.g. fcc)
eg AuAg, AuCu
ORDERED:- metal atoms in regular lattice arrangement, conditions as above
Regular fashioned arrangement, form ordered solid solution
what is random substitutional solid solution
atoms of one metal replace the other metal in the crystal lattice/grain.
metal atoms similar in:- SIZE, VALENCY, CRYSTAL STRUCTURE (e.g. fcc)
eg AuAg, AuCu
what is ordered substitutional solid solution
atoms of one metal replace the other metal in the crystal lattice/grain.
metal atoms in regular lattice arrangement, conditions as above
- Regular fashioned arrangement, form ordered solid solution
what are the 2 types of substitutional solid solution
random
ordered
what is interstitial solid solution
atoms markedly different in size
smaller atoms located in spaces in lattice/grain structure of larger atom (cannot predict where smaller atoms are)
e.g Fe-C stainless steel BDS3
what can vary between the cooling curves of different compositions of alloys
Each composition have to measure the cooling curve
- TL and TS
cooling curve of metal
crystallises at one temperature
cooling curve of alloy
crystallises over TEMPERATURE RANGE
state of soluble solid solution
solid solution formed (homogeneous mixture of metals in each grain)
state of insoluble solid solution
grains of individual metals formed
phase diagram
Can plot on phase diagram the varying Tl and Ts
Temperature Vs alloy composition
Top curve TL values is liquidous
Lower curve TS values is solidus
top curve on phase diagram is
liquidous
lower curve on phase diagram is
solidus
what state is the alloy if the temperature is above the lilquidous line
the 2 metals will be molten
what happens to the alloy as its cooled to the liquidous line
Some will crystallise to grains
Part liquid part solid
what happens to the alloy as its cooled to the solidus line
All solid
Crystallisation is complete - completely solid
LIQUIDUS
line representing the temperatures which different alloy compositions begin to crystallise
SOLIDUS
line representing the temperatures which different alloy compositions have completely crystallised
slow cooling of alloy
allows metal atoms to diffuse through lattice
ENSURES grain composition is homogeneous
BUT this results in LARGE GRAINS
- Undesirable
Do not want to cool slowly
what does rapid cooling of alloy create
coring
only way to determine structure/compositions of grains of final grain is from drawing tie lines to solidus
rapid cooling cooling of alloy make grains of
Different concentrations/ compositions of grains
Not homogenous (cored structure)
- Different % within each grain
- more likely to corrode
- Not desirable
rapid cooling of molten alloy leads to
prevents atoms diffusing through lattice
causes CORING
as composition varies throughout grain.
NOTE: initial grain composition IS NOT the same as the molten alloy.
what are conditions needed for coring
fast cooling of liquid state;
Liquidus and Solidus must be separated (far apart on Phase Diagram) and determines extent of coring
- eg Au-Pt
- if close together then not a large amount of coring
what is the main disadvantage of coring/rapid alloy cooling
reduce corrosion resistance
fast cooling of alloy advantages and disadvantage
generates MANY SMALL grains which impede dislocation movement, improving its MECHANICAL PROPERTIES
BUT causes coring, which is undesirable
what is homogenising anneal
once solid cored alloy formed REHEAT to allow atoms to diffuse and so cause grain composition to become homogeneous
- get rid of cored structure
NOTE: keep below recrystallisation temperature, otherwise grains altered
- Allows grains to move around and eliminate cored structure
why must the temperature be kept below recrystallisation temperature for homogenising annealing
higher will alter grains structure
- Allows grains to move around and eliminate cored structure
how are defects eliminate in metal lattices
Metal lattice is perfect with planes as all atoms the same size
- defect slides along the lattice plane when a force applied
- Until reach grain boundary
Defect “rolls” over the atoms in the lattice plane.
- Little energy/force is needed for defect to move along slip plane
solid solution atoms sizes
Alloys forming a SOLID SOLUTION and consisting of metals of different atomic size have a distorted grain structure
Not perfect as metal lattice
- Big, small, big, small
benefit of solid solution impeding dislocation movement
improves mechanical properties (EL, UTS, hardness, more fracture resistant)
more likely to prevent defects from moving in the structure
how does solid solution impede dislocation movement
Defect does not “roll” over the lattice plane.
Instead it falls into the larger space existing between large & small atom.
More energy/force is needed for the defect to overcome the different-sized atoms, and move along lattice to the grain boundary.
- requires greater stress to move dislocations in a solid solution
- making alloys inherently more fracture resistant (i.e. stronger) than metals.
what is benefit of order hardening
Alloys forming an ORDERED SOLID SOLUTION (atoms distributed at specific lattice sites) have a distorted grain structure (eg Au-Cu)
which IMPEDES resits dislocation movement and so improves mechanical properties (EL, UTS, hardness)
benefited by improved mechanical properties due to fact atoms are in ordered fashion
ordered solid solution
atoms distributed at specific lattice sites have a distorted grain structure (eg Au-Cu)
which IMPEDES resits dislocation movement and so improves mechanical properties (EL, UTS, hardness)
eutectic alloys properties
metals are soluble in liquid state
metals INSOLUBLE in solid state (so 2 PHASES) not soluble in each other
- i.e. each metal forms physically distinct grains
- CONTRAST this with Au-Pt alloy
not used much in dentistry
what is the unique quality of the melting point of binary eutectic alloys
Less than point A (mp of A) and point E (mp of B)
Less than individual metals
eutectic alloy compositor
where Liquidus and Solidus coincide
(i.e. where crystallisation process occurs at a single temperature)
where grains of individual metals formed simultaneously
lowest melting point - at eutectic composition: used for solder dental technicians
hard but brittle relatively easy to fracture
poor corrosion resistance
physical properties of eutectic alloys
lowest melting point - at eutectic composition: used for solder dental technicians
hard but brittle relatively easy to fracture
poor corrosion resistance
non-eutectic alloy compositon
excess metal crystallises first
then liquid reaches eutectic composition
and BOTH metals crystallise (forming separate grains)
solid solution forming alloy
alloy of 2 metals coexist in same lattic structure
eutectic alloy
2 metals exist in separate grain
solubility limit line
indicates that a range of compositions of Ag and Cu (corresponding to the horizontal section of the solidus (H1 to H2) ) ARE NOT POSSIBLE
- cool down rapidly get grains of both no matter starting composition
Hence molten alloy of composition Z
- DOES NOT cool rapidly to produce a 50:50 grain comprising Ag and Cu;
- instead grains of alpha and beta are formed
only extremes exists
what happens to partially soluble alloys on annealing
in partially soluble alloys on annealing, a supersaturated alloy will undergo PRECIPITATION HARDENING
One of the atoms (silver or cooper) pushed to grain boundary and makes the alloy stronger and harder
- Enhances the mechanical properties
precipitation hardening
annealing partially soluble alloys
One of the atoms (silver or cooper) pushed to grain boundary and makes the alloy stronger and harder
Enhances the mechanical properties
e.g. Type IV gold Has Ag and Cu present Benefit when anneal the alloy - Cool to room temp - Get rid of grain structure
positive and negative of alloys
Have better mechanical properties than metals (e.g. fracture strength, rigidity, elastic limit surface hardness) due to :
- solution, order & precipitation hardening
But Cored structure must be removed by annealing
