Lecture 1 and 2 Flashcards
Materials made of
atoms bonded together
stress and strain related by
mechanical properties
altering the properties of a material can be done by
altering the method of processing the raw material
Present day dependant on what type of materials
none renewable
binary alloy
two atomic species together
ternary alloy
two different atomic species together
Material science atm focuses on
nano materials - altering materials at the atomic scale
over how many engineering materials
100,000
Issues with new materials
have to alter manufacturing processes to use - economics
environmental issues
is it legally allowed
what percentage of inovations comes from materials
70%
hard materials are
quite brittle - small crack likely to break but keep an edge
choose a material based on
properties
performance
processing
structure
microstructurally insensitive
arise from bonding energy arrangements and the packing of atoms density elasticity/YMS melting point thermal conductivity
microstructurally sensitive
imperfections within crystal structure
hardness
ductility
hardness and how is it related to yield strength
how difficult is it is to scratch a material related to yield strength as permanently deforming the material
density is
mass over volume kg/m3
approximate density of water
1000
Metals density of most metals
8000
requirements to float on water
density should be lower than water
properties change with
temperature therefore need to specify temperature at which the mechanical property is measured
a ductile fracture
will require a lot of energy - will have a higher impact fracture toughness
brittle fracture
clean brake one crack - ductile many cracks form before break
Different processing methods will
change the material properties of a material
why is it important to process a material correctly
to make sure the product at the ends properties at the ones we desire - are not poor
all materials (solids liquid gasses) are made of
atoms
when breaking a material what are you actually breaking
chemical bonds between the atoms to form two separate surfaces
material properties determined by
how atoms are arranged strength and nature of bonds between them
outside shell of an atom called
valence shell
atoms want to have
full outer shell - shell goes 2 8 8 8
outer electrons responsible for
chemical bonding - can be shared or transferred - how it does this determines mechanical properties
metallic bond
valence electrons shared between all atoms - inner shell kept, generates sea of electrons that can move freely
good thermal and electrical properties
easy to rearrange material - ductile and malleable
generally materials
covalent bond
sharing of electrons - carbon has 4 valence electrons hydrogen has 1 therefore 4 hydrogen to one carbon,
charge interaction generates strong and directional bonds - not easy to rearrange
therefore stiff and hard material
diamond
ionic bond
complete transfer of electrons between atoms to form positive and negative ions
atoms give up or take in valence electrons
chemical reaction donating atoms or taking in to fill or get rid of outer shell
method of measuring atomic densities
RDF - radial distribution function
how does rdf work
choose an atom
draw round set of concentric atoms
count number of atoms inside ring divide by average density
move ring outwards
repeat
seeing how close its nearest neighboring atom is
RDF of gas
no regular arrangement
no long range order
none near it
as you go outwards rises to density of gas
RDF of liquid
no regular arrangement vibrate and move about each other
sticky- viscosity
no nearest neighbour nearby then large spike
short range order
but no long range order
RDF of solid
atoms and molecules tightly packed usually regular pattern
can vibrate around fixed position
have a number of atoms at fixed distance - spike
then none until get to next nearest neighbour -spike
atoms on regular crystal array have to go so far to see next nearest atom
vibration of atom leads to a slight cone shape
crystalline solid
RDF of solid in glassy state
same as liquid - when you process material dont have time to get to crystal state freezes in liquid state
rapidly remove energy - cool quickly
RDFs of materials
not fixed to described RDFs, these RDFs tell you about the structure of the material no whether it is solid liquid or gas
how do solids liquids and gasses act in a container
gas fills container - lots of space between atoms easy to compress
liquid fills bottom of container - difficult to compress
solids retain their shape no easy to compress
solid to liquid
melting reverse freeze
liquid to gas
vaporise reverse condense
solid to gas
sublimation
gas to solid
deposition
which state has highest energy
gas
pressure temperature diagram
can vary pressure to change state (with constant temperature)
given mass of liquid has
fixed volume - varies slightly with temperature and pressure - similar to solid
given mass of gas has
variable volume very dependent on temperature and pressure
solids strong interactions between atoms mean
it can support stresses - has moduli and a given strength
liquids atomic molecules in what type of state
atomic molecules in high energy state as move past each other sticky - visocscity
method to think about bonds
like springs - although different in detail force energy curves are similar for all types of bonds
lennord jones potential equation, which part is repulsive which is attractive when does it apply
order of 12 term = repulsive
order of 6 term = attrative
when thinking about bonds as springs - ionic case for simplicity electrons repel each other ions attract each other
describes potential energy of two atoms seperated by distance r
what are the material constants in the lennord jones equation
epsilon and sigma
where does atom want to sit on energy distance curve
lowest point the well lowest energy state distance known as ro
diagram of lennord jones curve
potential energy vs distance potential energy well epsilon sigma ro
energy required to break bond lennord jones curve
energy to push out of well distance from ro to xaxis
lennord jones curve it atom is to the right of well
atom attracted falls into well
lennord jones curve if atoms left of well
atoms really close together
repel each other
epsilon lennord jones
depth of well in joules in J minium point on lennord jones curve
sigma lennord jones
distance away from atom (m) where lennord jones curve crosses x axis potential energy = 0
what happens when ro distance apart
atoms happy
lowest energy
what happens when you compress spring - ie push atoms closer together
spring wants to force atom back
atoms repel each other due to electronic charge repelling
high energy state - requires a lot of energy to do difficult
repulsive term dominates
what happens when you expand the spring ie pull atoms apart
spring wants to bring atoms back together
atoms attracted to each other
attractive term dominates
eventually pull so far apart break bond
How do you convert energy to temperature
boltzman constant kb E= KbT
Temperature energy can change the
bond energy - make well shallower as atoms vibrate up side of well
if temperature energy increases
mean inter atomic distance increases as atoms vibrating
bond stiffness decreases - easier to melt material
low melting point metals
have a shallower well - their epsilon value is lower
Relating bond energy to melting point
high bond energy = high melting point
highest melting point
covalent and metallic bonds
ionic slightly lower but have higher bond energy
secondary type of bonds
van der waals and hydrogen - not the exchange of atoms much lower bond energy and melting point
Extensive properties
linearly dependent on the amount of substance. Examples: mass, volume
Intensive Properties
independent of the amount of substance. Examples:
temperature, pressure, density
