materials 2

What is an essential requirement for high temperature materials?

Good oxidation resistance in dry environments

What does positive oxidation energy mean?

Metal is stable

What does negative oxidation energy mean?

Metal will oxidise

How do oxide layers control the rate of oxidation?

Oxide film acts as a barrier to separate oxygen and iron atoms

What happen to oxidation rate as temperature increases?

Increases exponentially (follows Arrhenius's law)

How can KL be negative in linear oxidation?

If the oxide evaporates away from the metal surface as soon as it's formed, so the material loses weight

What does a low diffusion coefficient represent?

Better protective layers

examples of metals exhibiting a negative oxidation rate

tungsten, vanadium, molybdenum

how much does it cost to replace corroding parts in the UK per year?

£4 billion

What are the properties of an oxide film?

Low diffusivity, high resistivity, non-porous, adhere

How can a crevice function as a corrosion site?

wide enough to allow liquid penetration, sufficiently narrow to maintain a stagnant zone

Vol. oxides << Vol. material

microcracking

Vol. oxides >> Vol. material

spalling

what type of oxidation does spalling and microcracking lead to?

linear

what type of oxidation occurs when PB ratio is 1 or 2?

parabolic

properties of the oxide layer for parabolic oxidation

adherent, non-porous, protective film

how to avoid crevice corrosion

welding instead of riveted or bolted joints, nonadsorbing gaskets, removing accumulated deposits frequently, avoid stagnant areas in designs

what is pitting?

surface roughness or non-uniform chemical composition distribution results in holes or pits

why is pitting so destructive?

difficult to detect, penetrates the metal so failures occur suddenly

how can you minimise pitting?

polish, adding chemicals to homogenize the composition distribution

why does Galvanic corrosion occur on the more reactive metal?

higher tendency to form a cation

when is Galvanic corrosion more severe?

when the corroding metal has a much smaller area

how can you prevent Galvanic corrosion?

choose metals with similar reactivity, large anode area, insulate metals from each other, connect a third anodic metal to the other two

galvanising protection definition

Anodic protective coating that corrodes very slowly

sacrificial protection

Anodic metal connected to cathode so cathode doesn't corrode 

intergranular corrosion

occurs along grain boundaries and leads to macroscopic fractures

how can you prevent intergranular corrosion?

heat steel to a high temp to dissolve carbide, lower carbon content, alloying the steel

why does alloying the steel prevent intergranular corrosion?

carbon reacts with alloying element instead

what happens to the dislocation due to the volume around the dislocation having higher energy than the crystal?

dislocation is as short as possible and straight

Dislocation yield strength/ critically resolved stress definition 

The shear stress required to move a dislocation through a slip plane

how do coatings prevent corrosion?

Prevents the air from reacting with metal

how does plating prevent corrosion?

A metal is covered in a thin layer of metal with better corrosive resistance

how do diffusion coatings prevent corrosion?

Lower the diffusion coefficient of the interface between air/vapour and metal

how does cladding prevent corrosion?

Covering a metal with another metal. The secondary metal has better corrosion resistance

how do oxidising agents prevent corrosion?

removing oxygen from water, formation of protective deposits on metal surface

twinning definition

Part of lattice displacement such that it forms a mirror image of the undeformed lattice

why are polycrystalline crystals stronger than single crystals?

a single grain cannot deform until the adjacent grains are capable of slip

what are the conditions for twinning

low temperatures, high loading rates

solid solution hardening

Impurities in a crystal increase resistance to dislocation motion

when are substitutional solid solutions formed?

when foreign atoms replace parent atoms

Interstitial solid solution hardening

Alloying atoms partially cancel the dislocation stress field

how does precipitation hardening increase hardness?

Precipitates act as dislocation movement barriers and increase resistance of dislocation motion

how does grain boundary strengthening work?

Grain boundaries act as obstacles to dislocation movement

how does grain size affect metals?

finer grain size makes metals stronger, tougher and harder

why do grain boundaries resist dislocations?

grains are at different orientations, atomic disorder at grain boundary

what is recovery and recrystallisation used for?

To obtain a high grain structure

yield definition

the point where plastic deformation begins to occur

how does elastic deformation occur?

an increase (or decrease) in atomic separation

What stress is yielding controlled by?

shear

why does neutron diffraction only measure elastic strain?

Only measures atom separation

what is a flow curve?

A true stress-true plastic strain curve

what information does a flow curve provide?

the instantaneous value of stress required to continue plastically deforming the material

what does N=1 represent in the Rambert-Osgood model?

Linear elastic material

what does N=3 represent in the Rambert-Osgood model?

strongly strain hardening material

what does N>20 represent in the Rambert-Osgood model?

Non-strain hardening, tend to behave like elastic- perfectly plastic materials

Where does necking start on a stress-strain graph?

ultimate tensile stress

poisson’s contraction definition

test sample elongates as cross sectional area decreases

how does a neck form?

Heterogeneities (defects) act as stress concentration points causing local fluctuations in stresses and strains

When does necking initiate?

at the point of maximum load

how does plastic strain cause damage?

causes voids to nucleate, grow and coalesce

what is the strain hardening coefficient equal to?

true strain at which necking occurs

why are FCC/HCP not sensitive to strain?

Close packed planes

why are FCC/HCP not sensitive to temperature?

Long range obstacles cannot be overcome by thermal energy

when will yield stress increase?

low temperature under impact conditions

how much does yield stress increase by for structural steels at -200C?

by a factor of ~2

how much does yield stress increase by for aluminium alloys and austenitic stainless steels at -200C?

10% to 20%

what are Al-Mg alloys and 18-8 austenitic stainless steels used for?

low temperature vessels and plant for liquid nitrogen

why are Al-Mg alloys and 18-8 austenitic stainless steels used for low temperature vessels and plant for liquid nitrogen?

they are ductile at low temperature

when do C-Mn structural steels show brittle behaviour?

at ambient or sub-ambient temperatures, particularly under impact loading

why does strain ageing occur?

diffusion of C and Ni pinning dislocations

what does equivalent stress control?

yielding

what does principle stress control?

brittle fracture

what does hydrostatic stress cause?

volumetric expansion but no yielding

what happens if the point is outside the tresca locus?

yielding occurs

what happens if the point is inside the tresca locus?

no yielding

what happens if the point is on the tresca locus?

yielding has just occurred

when does yielding occur in von mises criterion?

when shear strain energy attains a critical value

Isotropic hardening definition

Yield surface remains the same shape but expands with increasing stress

Kinematic hardening definition

Yield locus translates in the direction of the stress causing the hardening

What happens in forward plastic deformation?

Dislocations interact with obstacles, back stress is generated

What happens in reverse deformation?

Bauchinger effect

What is the bauchinger effect?

Dislocations repel obstacles, stress field forms that helps move the dislocation in reverse strain direction

How can hydrostatic stress cause a material to move?

Elastic expansion

Deviatoric stress definition

Difference between principle and mean stresses

What does deviatoric stress cause?

Yielding

Stress triaxiality definition

There are stresses acting in three directions

Lowest value for stress triaxiality

1/3 (uniaxial tension)

Stress triaxiality for hydrostatic tension

Infinity