Materials Flashcards
Spring in series/parallel
Like capacitor arrangement-
-Parallel u add meaning f/k=x meaning more force need for same extension
Vice versa
Poisons ratio
v=strain lateral/strain axial
Hardness
Measure of resistance to permanent deformation
Resistance from scratches and indentation
Strength
Amount of force needed to deform a material
Toughness
Amount of force needed to fracture
Yield strength
The load or stress at which material starts to plastically deform
Ductile material
Lower yield strength and higher elongation failure
Brittle
Higher yield strength and exhibits little or no plastic deformation at failure
Bonding and properties
Stronger the interatomic bonding higher the modulus (stiffer).
Elastic properties depend on the Orientation and interconnection of bonds
Periodic trends
- atomic radius radius reduces across and electron affinity increases ▶️ because number of proton increases hence stronger attraction from nucleus
- ionisation energy (to remove electron) goes up across▶️ and above 🔼 because when radius gets bigger electron further from nucleus away less attraction
Primary bonding
Primary bonds
•metallic bonding
•ionic
•covalent
Secondary bonds
•hydrogen bonding
•van der waal (London forces)
Primary bonds are stiff, while secondary bonds are less stiff
Metallic bonding
- Sea of delocalised electrons (good conductivity of electricity and heat)
- give up electrons easily
- bond strength proportional to melting point
- non directional bonding allows dense packing to form metallic lattices
Ionic
- metal loses electron and gives it to a non metal (bonding between metal and non metal)
- elements of very different electronegativity
- non directional bonding
- electrons in a fixed position means no movement hence bad conductivity
- bonding is strong meaning high modulus meaning stuff hence materials are hard stiff and brittle
Covalent
- two or more non metals share electrons to complete shell (for stability)
- very strong and stiff
- directional bonding (cannot easily move the bonds around the atom)
- electrons tightly bound hence bad conductivity
- polymer chains predominantly made of covalent carbon chains
- found in ceramics
Non directional bonding
Allows close packing in certain patterns and produces dense materials
Van der waals
Arise from the polarity of the dipole. (Asymmetric distribution of electron charge between two atoms)
Hydrogen bonds
Stronger than van der waal
•when hydrogen bonds with highly electronegative atoms O,F,N
Important part in polymers
Polymers properties
- very long chains of carbon atoms
- covalent bonding hence directional bonding hence they can rotate to form complex shapes
- viscoelastic behaviour
- low modulus
- low strength
- not conductive
- poor thermal resistance
- cheap and easy to make (fabrication by moulding)
- low chemical reactivity
Viscoelastic behaviour
- low temperature: elastic at small strains (deformation instantaneous and reversible) stiffer
- high temperature: viscous (deformation time dependent and not reversible) flows more because secondary bonds broken
- intermediate temperatures: viscoelastic (instantaneous elastic strain + viscous time depend strain) So elastic for rapid applied stress and viscous for slow applied stress (behaviour dependent on the rate of strain)
Thermoset vs thermoplastic
Two classes of polymers
•thermoplastics has no branching therefore chains mobile when heated hence allows polymers to form a viscous liquid at high temperature and can be reversed (can melt)
•thermosets large amounts of branching occurs via chemical reactions (more covalent). Cannot melt you can soften and cannot be reversed
Bending polymers
Modulus is different in the middle of polymers because stress distribution is not linear
Ceramics and glasses properties
- non metals and comprising of metals meaning can have ionic and covalent bonding.
- strong
- stiff
- hard
- brittle (strong bonds high modulus) no plastic deformation
- high electrical and thermal insulation (poor electron mobility)
- excellent temperature (primary bonding are strong)
Manufacturing ceramics
- brittle so not easily machined or cut
- powder processing diffusion to make (dry under pressure and heat powder liquid)
Defects in ceramics
- strength is dependent on volume (smaller the diameter higher the strength) small fibre cannot have big defects (internal defects can not be reversed)
- surface defects can be reduced by: flame polishing, acid polishing and coatings and glazes
Statistical analysis
Use weibul distribution to estimate failure of ceramics as distribution is not normal
Weibul modulus (m) is the opposite of standard deviation in terms of large m small spread narrow distribution (reliable material)
What is a composite material
- A mixture of two or more discrete materials
- materials are separate
- dispersed phase usually either particles (eg carbon) or fibres
Why use composites
- to combine the good points of two materials
- to limit the size of defects in brittle materials
- better properties than it’s constituents eg a composite made of two brittle materials can be very tough
Particulate composites
Particle reinforced polymers
-carbon black soot is vital to reinforce rubber in car tyres
Fibre composites
Carbon fibres- expensive but have better stiffness and durability
Aramid fibres (Kevlar)- impact resistance used in combination with carbon or glass
In the fibre direction stronger (longitudinal) dominated by fibre strength
In the transverse direction weak and complex dominated by matrix strength not uniform stress hence predicting failure very difficult
Disadvantages of composites
- strength and failure difficult to predict
- ingredients can be expensive
- quality control can be difficult
- time consuming and often hard to make
Crystals
When in solid form atoms pack together in a regular structure to minimise energy loss. (Many ceramics and polymers form crystalline structures under normal solidification conditions)
-materials that do not are termed non-crystalline or amorphous
Unit cell
Crystal structures defined by their unit cell (the smaller repeated unit)
BCC, FCC, HCP
- Face centred cube (FCC)-copper gold aluminium (one atom at each corner and one in the centre of each side of cube) closely packed with abcabc
- Hexagonal close packed (HCP)-close packed stacked with AB AB AB forms hexagonal unit (cobalt zinc titanium)
- body centred cubic (BCC)-one atom centre of cube and one at each corner of cube (not closely packed) tungsten chromium
Diffusion why it’s important
•changing composition for example putting extra carbon or nitrogen into the surface of a steel component to increase its hardness and strength. (Works in vice versa if too much hydrogen diffusion makes material brittle)
Diffusion
- atoms vibrate and with their energy and amplitude increase with temperature
- may jump into a different position (random)
- results to movement of the material at high temp
- diffusion allows atoms to move through solid at high temp
- controlled by concentration and strain in the crystal structure
*elevated temps needed to make atoms diffuse in metals
Case hardening
- carbon diffuser into the surface of steel to make it harder and more resistant to wear
- driven by concentration
Sintering and creep
Both are driven by crystal strain and permanent deformation occurs
Mechanisms of diffusion
- Interstitial substitution: interstitial atoms are small and can diffuse easily through the gaps between larger atoms. (Fast as there are more channel of space for atoms to move) eg carbon in iron
- substitutional diffusion: cannot easily fit through the gaps between atoms and diffusion occurs mainly by vacancy diffusion eg copper in nickel
Vacancy diffusion
Where there is a missing atoms, a neighbouring atom can into the space (mixing of metals by the process is relatively slow)
Fick’s first law
J=-D (dC/dx)
Diffusion flux is the rate atoms move through a unit cross sectional area.
Diffusion flux= diffusion coefficient* change in concentration/change in position in the material
Diffusion high concentration to low concentration
Steady state diffusion
Diffusion flux does not change with time.
Rate if diffusion is proportional to the concentration gradient dC/dx
(Gas diffusion through a thin metal plate with a constant gas concentration on each side.
Non steady state diffusion
- if the concentration in any point varies with time we have non steady state diffusion
- atoms diffuse into the the surface immediately but the time they take to penetrate deeper increase exponentially (difficult to diffuse atoms along long distances through metal)
Fick’s second law
dC/dt=D d^2C/dx^2
The diffusion coefficient varies with temperature (increases as temp increases gradient increasing graph)
Solubility
- No limit to solubility in a substitutional diffusion mechanism
- there Is a limit to solubility in interstitial diffusion as there is a limited amount of gaps (interstitial areas)
Phase
Material that has uniform physical and chemical characteristics
Equilibria and metastability
- phase equilibria: phase changes takes time and eventually will reach an equilibrium state (eg if u cool tea to sub zero really quickly from it solubility limit it will still be liquid but if u do slowly u will see the solid sugar grains)
- Metastable structures: when equilibrium is not reached but for practical purposes nothing changes so it’s not a near perfect equilibrium state
Lever rule
Ws=R/R+S
Weight fraction
Eutectic transformation (v shape)
When going from all liquid to all solid. When goes from single phase to two phase
Solidification happens rapidly therefore diffusion cannot happen occur over long distances
Eutectoid for solid to liquid
Hypo- or hyper eutectic
Hypo eutectic is a composition less than the eutectic composition.
Hyper eutectic is vice versa
Structure of iron
Phase diagram of pure iron and carbon
- pure iron forms a bcc structure called ferrite or alpha below 912 degrees c.
- above 912 degrees c this changes to fcc structure called austenite or gamma.
- above 1394 degrees c another bcc structure called ferrite
- very little carbon is soluble in ferrite compared to austenite
Equilibrium phase changes
- on heating above the eutectoid austenite is formed and dissolved up to 2.14% C.
- on cooling the austenite transforms to ferrite which rejects all the carbon
- a mixture of ferrite and cementite is formed at room temperature
Steel - carbon phase diagram
More used in engineering than iron
Low to high => ferrite+ pearlite then ferrite then austenite
Pure iron carbon phase diagram
Ferrite then austenite then ferrite
Pearlite
Mixture of ferrite and cementite.
You get this when you cool austenite
Cementite is an Iron carbide
Cooling the mixture
Cooling the mixture of phases formed depends on the composition of C
- less than 0.8% C ferrite and pearlite (hypo eutectoid)
- eutectoid mixture at 0.8% pearlite throughout
- more than 0.8% C pearlite and cementite (hyper eutectoid)
Heat treatment
We use heat heat treatment to change properties of material while keeping the composing of C the same. Making more ductile while still being high in hardness
TTT diagram time- temp transformation
Mechanical properties
- the presence of carbon strengthens the steel by solid solution hardening.
- the formation of pearlite produces a very small grain size strong
- the cementite phase is very hard and brittle
- thus the yield stress and tensile strength increases with C content
When something is hard it’s brittle. When quite ductile quite soft
Time effects
- the transformation from austenite to ferrite require considerable amount of diffusion and atomic rearrangement (diffusion in solid slow) cooling
- this takes some time as carbon atoms must move some distance
- if this cooling is rapid the rate of diffusion may become too slow to allow the transformation to finish
Bainite
Finer than pearlite as give more time to diffuse.
Different properties to pearlite
Martensite
Cool down austenite really quickly do not get normal transformation because does not cross any lines
You get instead martensite is bcc (only happens low carbon %) very hard and brittle
Annealing (Heat treatment process)
If we anneal the material we can improve it’s ductility, toughness (resistance to fracture) and machinability. Relieve residual stress
To anneal you heat up slowly and hold then cool down quickly
Cheap as not high very high temp
Normalising (heat treatment process)
Similar to full annealing but cooled in still air results in finer grain size and pearlitic structure.
Higher temp than annealing
Tempering (heat treatment)
Done after quench hardening (to toughen up the brittle parts)
Heats up the martensite to get tempered martensite consists of ferrite and cementite with no actual martensite present
Quenching
Rapid cooling to room temperate transforms austenite to martensite
Hardenability
Is the property that determines how easily a metal may be hardened and how the resulting hardness profile varies through its depth.
(Not same as hardness)
Jominy test
- how the cooling rate influences the hardening constant, allows comparison between materials
Results
•quenched end cooler rapidly, maximum hardness, up to 100%
•cooling rate decreases with distance from quenched end therefore hardness
•slow cookers allows carbon diffusion and formation of pearlite as well as martensite and bainite.
•high hardenable steel will have high hardness relatively long distance from quenched end
Case hardening
A hard and wear resistance case while still tough and strong in the middle
Methods (strongest in hardness to weakest)
-Nitriding - diffuses nitrongen into the surface forming nitrides that inhibit dislocation movement.
- carburising- diffuses carbon into the surface of a steel part
- Induction hardening- harden high carbon steel by local heating
- volume hardening
