Materials

Plastic deformation

(Strain) is irreversible. Plastic component of deformation will be permanent

Elastic deformation

(Strain) results when inter atomic bonds are stretched/compressed. Is fully reversible, deformation will disappear if load removed

Atomic packing factor equation

(Vol of atoms)/(vol of unit cell)

(Phase diagram) A 10% sn alloy is thermodynamically stable at 150 deg. What would you expect to happen to alloy if it were cooled very rapidly to room temp

-Conditions would deviate significantly from thermo equilibrium. - phase diagram can no longer be used to predict phase proportions and compositions - rapid cooling will stifle rearrangement of atoms by diffusion, as diffusion rates decrease exponentially with decreases in temp - most likely is tin atoms remain trapped in solid α so we have solid α supersaturated with tin

Segregation(coring)

-Diffusion rates are often to slow to prevent coring or segregation -equilibrium is not maintained between solid+liquid, except at solid/liquid interface composition of solid is not homogenous Can be remedied by heat treatment in single phase solids

Precipitation hardening

-Heat treatment that is used to increase strength of alloy, requires material have a change in solid solubility with temp -involves quenching and aging operation -Objective is to produce fine distribution of precipitates to impede dislocation movement -if precipitates very small, dislocations cut through presuming coherent -if little larger it's more difficult for dislocations to cut precipitates -if precipitates become coarse dislocations will bow and multiply as pass through lattice

Types of Fracture of metals

-Highly ductile (wanted) stretch occurs, necked down to a point -moderate ductile fracture, like small teeth -brittle feature, no plastic deformation or warning, just breaks

Stages of annealing ALL PROCESS REDUCE GIBBS FREE ENERGY

-Recovery:when material first heated, thermal energy allow dislocations to move and form boundaries of sub-grains -recrystallisation:new grains with much lower dislocation densities nucleate and grow (reduction in strength and increased ductility) -grain growth: new grains grow, larger grain consume smaller reducing grain boundary surface area and energy. (Excessive grain growth can lead to reduction in strength and toughness) PRECIPITATES THAT DO NOT DISSOLVE DURING ANNEALING CAN ACT AS BARRIES TO GRAIN GROWTH

Dendrite formation

-Require undercooling of melt -crystals nucleate as spheres bit faster growth in <100> crystal direction leads to formation of tree like structure - spacing of secondary arms can provide indication of cooling rate -associated with equiaxed grain structure

Strengthening mechanisms (6)

-Work hardening: strengthening material by use of plastic deformation -solid solution:making use of alloy to distort lattice so harder for dislocation to move and therefore strengthening -precipitation hardening: introducing second phase (precipitate) to introduce new crystal structure, stops cracks propagating -grain boundary strengthening: reduce grain size-> more boundaries-> increases strength -transformation strengthening: material forced to go under phase transition. (Fast heating then cooling, forming strong hard brittle structure) -irradiation hardening: occurs when exposed to neutrons, materials become hard(strength increased) but become brittle over time (not tool to achieve specific properties, instead is a damage mechanism that needs to be accounted for in engineering design)

Influences on grain boundaries after solidification

-cooling rate during solidification can influence grain size -fast cooling causes undercooling of the melt to increase rapidly and number of viable Nucleation sites increase. Fast cooling also promotes fine grain size while slow cooling rate makes a coarse grain structure - fine grain structure is desirable so grain refiners and inoculants are added to melt to provide large Nucleation sites

Dislocations and slip planes

-dislocation glide occurs on closest packed planes in closest packed directions. -these define slip systems -3D shape change requires glide in several slip systems

Formation of grain boundaries

-grains result from random Nucleation from liquid -grain structure in solids can be modified by deformation and recrystallisation -interface between adjacent crystals or grain is GRAIN BOUNDARY

Interstitial diffusion and substitutional (interstitial faster than substitutional)

-interstitial:A diffusant will diffuse inbetween lattice structure of another crystalline. -substitutional: atoms can only move by substituting places with another atom(contingent upon available point vacancies)

Dislocation

-is a defect in the atomic packing sequence

Deformation

-occurs by yielding -occurs in crystal slip bands

Woods concept for initiation of fatigue cracks

-small defects in surface leads to localised stress concentrations -slip occurs on favourably orientated crystal planes upon loading -slip occurs in opp direction upon unloading -notch develops as consequence of those slips and crack starts to grow driven by stress concentrations at notch

Cu-Ni phase diagram

3 regions: liquid only, liquid and solid α, solid α only Lines coincide when you have purse substance (single melting point) Above line is 100% liquid Below is 100% solid

Crystals preceded growth directions

<100> crystal direction grow faster than other direction results in implications for the way the crystals grow from melt

At room temp and at atmospheric pressure nitrogen and hydrogen fluoride both exist as diatomic gases. Which, if any, if these molecules would you expect to be polar and why?

A polar bond arises in covalently bonded molecules when one of the atoms within the molecule has a greater attraction for electrons than another. In such cases electrons not shared equally. In nitrogen there are two of same atoms which have equal attraction for electrons so unlikely to be polar while hydrogen and fluorine have very different attractions to electrons so likely to be polar

Ductility

Ability of a material to deform under tensile stress Shows obvious deformation before fracture and will absorb more energy

Toughness

Ability of material to absorb energy before fracturing Energy under stress strain curve is measure of toughness. Material with high toughness has better resistance to sudden fracture. Trade off between strength and toughness

Strength

Ability of material to resist stress

Alloying additions to metals can lead to significant increases in strength, why is this so?

Alloying elements lead to distortion of crystal lattice making it more difficult to move dislocations through regions of lattice which is distorted

Solid solution strengthened aluminium alloys

Alloying with magnesium can significantly enhance strength. High yield stresses can be achieved when solid strength solution is combine with work hardening

Texture and anisotropy (opp of isotropy(uniformity))

As first order approx it's assumed metals isotopic as material has equiaxed grain structure. In many situations grain size and crystal orientation are not randomly distributed (texture) and therefore can exhibit anisotropy

Effect of carbon context

As we cool steel slowly we have a trade off between strength and toughness. As carbon content increases, the pearlite content increases, the strength increases and ductility decreases

Vacancies and diffusion

Atoms in solids don't stay in same place, thermal energy causes them to move site to site resulting in a concentration gradient encouraging atoms to move down gradient to achieve lower state - diffusion occurs by either vacancy diffusion or interstitial diffusion methods. -rate depends on number of vacancies, the structure etc

Transformation strengthening

Austenite-> ferrite+pearlite phase transformations requires carbon diffusion -Martensite can form without diffusion if large enough driving force, can also form by quenching

Brittle

Brittle materials fail without any warning

Significance of fatigue failure

Can occur even when material never experiences stress as high as yield stress Have potential to occur unexpectedly therefore of crit concern Fatigue failures lead to appearance of striations ( ridges/linear marks) on fracture surface

Phase transformations

Changes in phases that exist in solid metal or alloy usually occur by diffusion

Influence of bond type on mechanical properties

Covalent- strong and brittle Ionic- hard and brittle, since significant shear deformation cannot occur without breaking bonds

Fracture of metals

Ductile fracture is less serious than brittle as can be detected in advance due to observable plastic deformation Necking occurs in tensile test then microvoids nucleate and coalesce towards centre of specimen, then there is crack propagation along the shear plane and the sample fractures

Elastic modulus (Youngs)

E=(changeσ)/(changeε)

Close packed directions

Each atom in closed packed plane has 6 nearest neighbours 3 closed packed directions in each closed pack plane

Types of dislocations

Edge dislocation (extra half a plane of atoms) Screw dislocation(shear stress)

Fatigue

Fatigue failure can occur when a material is subjected to cyclic loading The peak stress experienced during cyclic loading is below tensile strength and yield stress of material Tend to fail where there's lots of loading and unloading

Influence of residual stress in fatigue

Fatigue life of component tends to decrease with increases in peak stress and amplitude Tensile residual stresses at surface leads to reduction in fatigue life as lead to increase in peak tensile stress at surface Compressive residual stresses at surface to increase fatigue life so some industries introduce compressive residual dtresses

Work hardening equation

For single crystals flow stress is given by: τ=το+κρ^1/2 τ is flow (shear stress) ρ is dislocation density κ is constant το is value of τ when ρ is extrapolated to 0 Work hardening behaviour can be described by stain hardening exponent σ=Κ(εp)^n σ is stress K is strength index εp is the plastic strain n is strain hardening exponent

Nucleation

Formation of stable solid particles of certain size

Primary bonds

From strongest to weakest Covalent Ionic Metallic

Gibbs free energy equation

G(p,T)=U+ρV-TS (S=entropy) (U+ρV=enthalpy) (U=internal energy) G=ΣniGi (ni is number moles in ith phase amd Gi is gibbs free energy of ith stage per mole)

Influence of grain boundaries on diffusion rates

Grain boundaries are defects with high concentration of vacancies -diffusion can occur more rapidly at grain boundaries and diffusion increases as grain size decreases BCC HAS LOWER ATOMIC PACKING FACTOR SO DASTER DIFFUSION

Martensite

Hard, brittle (due o carbon trapped in solution), fine grain size (due to high Nucleation rate) - it is metastable intermediate position between austenite and ferrite. (Bcc lattice)

Martensite

Hard, brittle, fine grain size -high hardness +brittleness due to carbon trapped in solution -small grain size due to high Nucleation rate at the large undercooling of transformation

Hardness, strength and toughness

Hardness is materials resistance to plastic deformation Toughness is how much deformation a material can take (absorb) before fracture Strength is max amount of stress object can take before deforming

Precipitation hardening

Heat treatment technique to increase strength of alloy. Objective is to produce fine distribution of precipitates to impede dislocation motion

Ultimate tensile strength (UTS)

Highest stress recorded in tensile test

Rapid cooling

If a material is cooled quickly there may not be sufficient time for diffusion ( composition within phase will not be uniform) -rapid cooling leads to inhomogeneity within material and can prevent phase changes taking place

Interstitial sites (Alloying)

If atoms smaller than host element these allying atoms can occupy sites inbetween larger host atoms. Increase in temp leads to increase in mean inter atomic spacing

In tensile test the load at point of fracture was 60kN. Why is it that failure load often does not correspond to peak load that is recorded during tension test?

In tension test strain rate is constant, as properties are dependant on strain rate. Meaning testing machine will apply whatever load necessary to keep const strain rate. During plastic deformation material undergoes work hardening, but cross section area is decreasing with increasing strain, at UTS work hardening can't keep up resulting in reduction in load, any further plastic deformation will be localised and necking will occur. Localisation of strain leads to fracture load being lower than peak load for test

Influence of thermal expansion on mech sys

Increase in temp leads to reduction in cohesive force within crystal lattice and reduction in strength and modulus of material. Temp gradient leads to generation of internal stresses

Effect of temp on elastic modulus

Inter atomic spacing increases as temp increases, resulting in weakening of interatomic bonds leading to reduction in elastic modulus

Significance of creep

Issue in industries where materials need to operate at high temp

Liquid solid and solvus

Liquid- Max temp at which solid crystals can co-exist with liquids in thermo equilibrium Solids- locus of temp below which material is solid Solvus- line which separates a homogenous solid solution from a region in which two or more phases coexist

Critical particle size

Minimum particle size for which any further growth will lead to reduction in Gibbs free energy -also minimum particle size that will be stable and for which further growth of crystal can occur spontaneously

Strength and toughness

Most strengthening mechanisms achieve increase in strength at expense of ductility and toughness -grain boundary strengthening leads to reduction in ductility but achieves an increase in toughness(in steels) -reductions in grain size lead to reduction in ductile to brittle transition temp in steels

Deformation of grains

Movement of dislocation allows one plane of atoms to move over another -glide of dislocation let's crystal reform -stress to move dislocation is small so allowed metal to yield

Heat treatment of Steals (normalising)

Normalising is process where steel is heated to form austenite and then slow cooled. Makes material softer but does not produce uniform material properties like annealing. Treatment refines grain size and improves uniformity of microstructures

Solidification

Nucleation and growth processes

Heterogenous solidification

Nucleation on existing surfaces which are already in contact with the liquid -undercooling required is often high -can be triggered by pre-existing fragments of crystals or solid films of oxides or Nucleation on solid surfaces -presence of existing interfaces reduces energy barrier to solidification -grain refiners added to melt to intentionally provide Nucleation sites and reduce grain size in alloy

Simple cube setup

One atom in each corner No close packed planes 3 close packed directions (parallel to edges of unit cell)

Fcc setup

One atom in each corner And one at centre of each face and in middle -Four closed packed planes -In each closed packed plane there are 3 close packed directions(parallel to diagonals of each face of unit cell) -12 possible close-packed slip systems, slip occurs more easily along close packed directions so Fcc have good ductility and toughness

Bcc setup

One atom in each corner and one in middle of cube -No close packed planes -4 close packed directions(parallel to diagonals) -dislocation motion can occur in Bcc once threshold temp (DBTT) exceeded but at low temp number of possible slip directions limited. Hence low toughness

Why are closed packed planes significant?

Plastic deformation is process which defects glide through crystals under shear stresses Easy for dislocation to move along close packed directions If metal had crystal structure with many close packed planes then metal will be ductile

Necking

Plastic instability. Mode of tensile deformation where strain is localised. Onset coincides with point at which UTS reached. Loads usually decreases in tensile test once necking reached as cross sectional area in necked region decreases.

Proof stress

Point where line intersects stress-strain plot

Tempering (Martensite)

Process in which steel is reheated in controlled manner -allows fine precipitation of iron carbide -hardness and brittleness are decreased by tempering as carbon leaves Martensite to form carbides

Annealing

Process in which work hardening Is reversed through heating material

Lever rule

R/(R+S)

Creep

Refers to time dependant deformation of material under static load

Natural ageing Artificial ageing

Room temp Elevated temp Over ageing reduces strength

Cubic structure types

Simple cube- rare Body centred cubic(Bcc) Face centred cubic (Fcc)

What processing conditions are necessary to achieve thermo equilibrium in metals and allows

Slow heating and cooling Any changes in proportions of each stage need to occur by diffusion which takes time

Covalent bonds

Solids that are covalently bonded have very high melting temps and strengths

Solid solution strengthening

Solute atoms that differ in size from neighbouring atoms distort lattice making it more difficult for dislocations to pass. Extra work must be done to move dislocations past distortions Interstitial alloying elements reside in core of dislocations, these sites are favoured as lead to less distortion these element pin dislocations, as high stresses required for dislocations to break free from interstitial atoms

Solid solution strengthening

Solute atoms that differ in size to neighbouring atoms distort lattice making it difficult for dislocations to pass

Precipitation hardening process

Solution Treatment: dissolve existing precipitates to form single phase solid solution. Heated to predetermined temp Quenching: rapidly cooled to room temp Or quenched to freeze solute atoms. Diffusion rate is slow and microstructure is metastable Precipitation: finely dispersed precipitates due to high Nucleation rates. Hardness and strength develop with time and temp

Grain boundary strengthening

Strength of metal depends on ability to restrict motion of dislocations. At a grain boundary dislocations are blocked, they pile up creating a driving force which pushes dislocations towards nearby grains. The bigger the grain the bigger the pileup the bigger the force. Large grains, along with a small load and force cause dislocation to move to next grain. By reducing grain size there's less pile up requiring larger stress to move dislocations therefore smaller grain require more load then large grains to move dislocations. The smaller grain metal had higher yield strength, therefore this strengthening mechanism doesn't compromise ductility

Work hardening/strain hardening/cold working

Strengthening of material through plastic deformation (eg. Metal working process) Can lead to increase in strength but can also make material too brittle

Why do we need phase diagrams

Tell us what phases are thermodynamic ally stable for given material. And tells us how to preserve these microstructures and properties

Effect of yield stress for fatigue cracks

Tendency for fatigue crack initiation depends on tendency to slip during cyclic loading Increasing yield stress generally leads to increase in fatigue life as more difficult to initiate slip. Shot pending can be used to increase yield stress and fatigue resistance

Burgers vector

The Burgers vector of a dislocation, is a crystal vector that quantifies the difference between the distorted lattice around the dislocation and the perfect lattice -for an edge dislocation the glide occurs in direction parallel to Burgers vector -for screw dislocation the glide occurs in direction perpendicular to burgers vector

Yield stress in metals

The stress at which dislocations start moving within the lattice

The theoretical strength of a metal is generally orders of mag higher than strength observed experimentally. Why?

Theoretical strength of metal is based in strength that would be required to simultaneous break all bonds in crystal plane that is orientated transversely to loading direction - real metals contain defects such as dislocations -dislocations can slip within grains at stresses much lower than theoretical strength of a metal, as not necessary to break all bonds on crystal plane simultaneously for this slip to occur

Theoretical versus real strength of metals.

Theoretical strength of metals is not realised because real crystals contain defect such as dislocations -stresses required to cause dislocation glide, are orders of magnitude lower than stress required to cause entire planes of atoms to slide past one anotjer

Term "stable" (phase diagrams)

Thermo stability is achieved in metals when GIBBS FREE ENERGY is at a minimum

Quenching

To make material harder. Metal is heated to specific temp and then rapidly cooled (drawback is metal becomes brittle) Therefore this treatment is followed by tempering. Preheating quenched material to temp below crit range and then cooling to obtain desired properties. Temp chosen for tempering impacts hardness of workpiece, higher the temp the lower the hardness Also low temperature leads to finer carbide distributions, higher strength and reduced toughness

Undercooling/supercooling

To over energy barrier from homogeneous Nucleation liquid must be cooled to temp below equilibrium freezing temp

True stress equation True strain equation Engineering stress equation Engineering strain equation

True stress=F/A True strain=ln(L/Lo) Engineering stress=F/Ao Engineering strain=changeL/Lo

Phase diagrams

Unary- one component, t and p vary Binary- two comp, const p, t and composition vary Ternary- three components const p, t and composition vary

Homogenous nucleation

When Nucleation occurs spontaneously, within a liquid, without any preferred Nucleation sites being present. -Requires creation of new interface between solid nucleus and liquid phase -Energy barrier associated with this new interface. -Spherical nucleus energy required is 4πr^2Y

Non equilibrium microstructures in steels (how Martensite is formed)

When austenite is cooled below eutectoid temp the driving force for transformation increases with undercooling but diffusion rates decrease with temp. -if cooling rate exceeds crit value the temp is low enough to prevent diffusion before transformation takes place -eventually driving force for change in structure becomes so great lattice undergoes shear transformation forming martensite

Yielding of metals

Yielding(plastic deformation) occurs by gliding of planes over eachother, it is driven by shear stresses

Carbon table terminology

γ=austenite (FCC) α= ferrite (BCC) Fe3C= ceminite α+Fe3C= pearlite (alt layers of ferrite and ceminite)

Engineering strain

ε=(L1-Lo)/Lo

Stress

σ=F/A

Yielding of metals

τ=(σ1-σ2)/2 σ1 is up and down, σ2 is left and right -in unaxial tensile test max shear stress occurs on plane orientated at 45 degree to loading direction