materials

strong
a large stress is needed to break it
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stiff
small strains for large stresses, high YM
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elastic
returns to original form when stressor is removed
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plastic
permanent deformation under large stress rather than cracking
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tough
undergoes considerable plastic deformation before it breaks
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brittle
breaks suddenly as cracks travel through, little or no plastic deformation
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hard
resists indentation on impact
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ductile
easily stretched and deformed
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tensile stress and unit
F/A unit Nm^-2
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tensile strain and unit
x/l no unit
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maximum tensile stress is...
the breaking stress
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covalent bonds and bond properties
sharing valences, strongly directional, strong bonds
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ionic bonds and bond properties
transferring electrons creating ions, uses the electrostatic attraction, leads to a high Young Modulus because it's a strong atomic bond so stiff and brittle (because electrostatic force holds the atoms close but the alike ions repel so just breaks.
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Metallic bonds, why does it make the metal a good conductor? what force does it use?
loosely bounds electrons so free to move within energy region and bond to any other positive ion as well using electrostatic attraction, making the metal ductile and malleable (hammerable), makes it a good conductor, layers can slide over one another
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electronegativity
covalent bonds not purely covalent, it's a nucleus tug of war due to electronegativity (the relative ability for the atom to attract electrons), it's determined by the distance from the nucleus and number of electrons shielding the nucleus
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what type of bond would it be if the 2 ions had opposite extreme electronegativities? what is a polar covalent bond? what atomic pair would give the highest covalency?
large difference in electronegativities leads to more ionic bonding but in between the extremes has both ionic and covalent so is polar covalent, high covalency achieved with 2 identical atoms
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what's the strongest to weakest bond order?
ionic, covalent, metallic, intermolecular
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Van der waal bonds (secondary bond)
these are weak force between molecules due to the electrostatic attraction between dipoles (when molecules become slightly more positive at one end than the other, can be permanent or temporary).
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Hydrogen bonds (secondary bond)
N,O and F are really electronegative and attract Hydrogen by the electrostatic force (primary bond) leaving a positive region at the hydrogen and a negative region at the N,O or F so causes a secondary bond (hydrogen bond) between these regions
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what are crystalline materials?
atoms arranged in a regular geometric pattern (large symmetry).
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what is a lattice
A regular geometric pattern of points
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what is a unit cell?
a section which when repeated creates a lattice (rubix cube)
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name 4 bravais lattices
simple cubic, body-centred cubic, face-centred cubic and hexagonal close-packed lattice
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what is a hexagonal bravais lattice?
close-packed atoms with hexagonal and triangular layers in an abab order
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face centred cubic
a 3 plane hexagonal close-packed lattice in abcabc arrangement. metals can have close-packed arrangements because their bonds aren't directional.
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what is an allotrope?
same element, different material due to the atom arrangement such as carbon (can make diamond, graphite or buckerminsterfullence.
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what is an amorphous material?
atoms arranged in an irregular pattern.
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what is a polymer?
a polymer is a long chain or molecules, they can be considered semi-crystalline and can be more amorphous or more crystalline depending on how it was made/ treated.
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what are the 2 types of polymers?
thermosetting (contains covalent bonds and doesn't soften with heat) and thermoplastic (softens with heat)
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ceramics
uses covalently bonded oxides leading to a strong, brittle material resistant to high temperatures.
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composites,example: fibreglass
Materials combined to exploit their different properties for use in combination e.g fibreglass (plastic and glass) for hockey
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laminates, example: bullet-proof glass
Bullet-proof glass: layers of glass and polycarbon, a hard layer for strength and shatters to absorb energy, soft layer for elasticity and prevents complete shattering, together they slow down the bullet.
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point defects (to do with atoms)
vacancy, impurity (substitutional/ interstitial), self-interstitial
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linear defects (to do with planes)
edge dislocation, screw dislocation
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planar defects (to do with the lattices)
stacking fault (two different types of lattices next to each other) and grain boundary
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how to prevent dislocations
introducing point defects or grain boundaries so they don't slip as far, also work hardening
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how do you alter grain size?
cooling rate, fast= small grains, slow= large grains
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Alloys
A mix of metals, uses metallic binding, substitutional uses similar sized atoms, interstitial alloys use different sized atoms
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fatigue failure
a repetitive or fluctuating stress strikes a metal causing it to fail at a stress much lower than it's ultimate tensile stress. There is no plastic deformation as warning. The bigger the stressor, the smaller the number of cycles required for failure
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what is the endurance limit?
the stress level where anything below it can't lead to failure, no matter how much repetition of the stress it is (fatigue failure)
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how does fatigue failure work?
The stress causes a pile up of dislocations called slip bands which slip in and out of the surface making it uneven and susceptible to cracks. cracks can be detected using ultrasound and dye penetration. Cracks can be minimised by avoiding corners.
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what is creep?
When a large stress is applied to a material and it slowly stretches further than it's given strain, even when below the yield stress (point up to which it's elastic) which worsens as temperature increases.
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why does creep happen (atomically)
atoms diffuse through the lattice or along grain boundaries resulting in elongation of grains in the direction the stress was applied, this can be reduced using single crystals or larger grains.
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Other cards in this set

Card 2

Front

small strains for large stresses, high YM

Back

stiff

Card 3

Front

returns to original form when stressor is removed

Back

Preview of the back of card 3

Card 4

Front

permanent deformation under large stress rather than cracking

Back

Preview of the back of card 4

Card 5

Front

undergoes considerable plastic deformation before it breaks

Back

Preview of the back of card 5
View more cards

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