Metals and Alloys Flashcards
give 4 properties of metals
- hard
- closely packed regular crystalline structures
- opaque
- conductors of heat and electricity
which metal is an exception to these properties?
mercury
what is a binary alloy?
an alloy composed of two metals
what is a tertiary alloy?
an alloy composed of 3 metals
define an alloy system
all possible combinations of an alloy
what occurs in the metals in an alloy when in molten state?
they display mutual solubility
what 2 things occur when cooling from a molten metal?
- impurities enable metal atoms to deposit upon them
- then nucleate crystal growth
what are crystals in metal called?
grains
when does grain growth stop?
when all the melt metal is consumed
define a grain boundary
when the grains grow and push on neighbours creating a source of tension
what are the two types of nucleation of metal crystals (grains)?
homogeneous and heterogeneous
what is homogenous nucleation? what are the 2 requirements of it?
nucleation of metal grains at a single site.
requires
- 4 metal atoms to come together
- specialised equipment to control crystal orientation
what is heterogeneous nucleation?
where grain nucleation occurs at many sites.
- majority of dental applications
- happens more randomly
does grain size influence the physical properties?
yes
how is grain size controlled?
by the manufacturer
what are the 3 ways that the manufacturers can influence grain size?
- rapid solidification
- provision of ‘extra’ nucleation sites
- both of the above (promotes small grain size)
what other 2 factors affect grain size?
- the temperature of the mould
2. the shape of the mould
if the mould is similar temperature to the melt then… 2 things
- slow cooling
- large grain size
if the mould is a different temperature to the melt then… 2 things
- fast cooling
- small grain size
how are grain boundaries visualised?
a light reflecting microscope
how can the quality of grain boundaries be assured?
by using the correct etching chemical that shows up areas under high stress
4 ways that metals and alloys can be shaped?
- hammering
- rolling
- drawing through a wire
- pressing
define ductility
the maximum degree of EXTENSION in response to an applied TENSILE force
define malleable
the maximum degree of COMPRESSION in response to an applied COMPRESSION force
what is casting?
molten to investment mould
what is amalgamation?
alloy and mercury mixing in a hardening reaction that brings about crystallisation
why are elemental metals really used practically?
as there are severe limitations of their properties that are inferior to that of an alloy
what are the 4 possible solubilities of molten metals that occurs on cooling?
- remain soluble in each other (SOLID SOLUTION)
- are completely insoluble in each other
- are partially soluble in each other
- from inter metallic compounds if the metals have affinity for each other (e.g Ag3Sn)
what are the 3 possible solid solutions?
- ordered solid solution
- random solid solution
- interstitial solid solution
whats the benefit of solid solutions?
- the hardening effect due to atoms of differing radii form a mechanical resistance to dislocation
when 2 metals are totally insoluble in each other way can occur and is it common?
it is rare.
risk of electrolytic corrosion due to 2 dissimilar metals
what are the 4 main properties of an alloy where inter metallic compounds have been formed on cooling?
- hard
- brittle
- low ductility
- few crystal imperfections means less potential fro slip movement
what are the two axis on a cooling curve diagram?
temperature vs time
what does Tm mean?
melting point
when does crystallisation occur?
at Tm
what does a plateau in the cooling curve indicate?
a constant temperature during crystallisation
what is a phase diagram?
a series of cooling curves for alloys of different compositions
what does a large separation between liquidous or solidous indicate..
a greater coring. (more of a cored structure)
what does an eutectic alloy behave like?
a pure metal
give a fact about the melting point of an eutectic alloy
it is lower than that of the component metals
how are eutectic alloys applied?
by a solder
