Materials and Manufacturing Flashcards
1
Q
What is valency?
A
- the ability of the atom to enter into chemical
combination with other elements - losing and gaining electrons
2
Q
where are electropositive elements located on the periodic table?
A
Left
have few electrons in outer shell,can remove easilylow ionisation energy
3
Q
where are electronegative elements located on the periodic table?
A
Right
prefer to gain extra electrons to fill the shell
4
Q
what are in the primary class of bonding
A
metallic, ionic, covalent
5
Q
what are in the secondary class of bonding
A
van der waals, hydrogen bonding
6
Q
ionic bonding
A
transfer e from one atom to another creating ions
held together by electrostatic attraction
7
Q
properties of elements ionically bonded
A
◦ high melting and boiling points
◦ high hardness
◦ high Young’s modulus
◦ high degree of brittleness
8
Q
covalent bonding
A
- atoms sharing electrons between them to form molecules or solids
- e- held in region between two atom nuclei
- bonds are directional
9
Q
What are the types of bonds found in covalent molecules, and how do they affect the properties of materials?
A
◦ bonds within molecules are covalent - strong
◦ bonds between molecules may be secondary, eg. van der waals - weak (determine many of the overall
material properties)
10
Q
properties of elements covalently bonded
A
◦ high Young’s modulus, often brittle
◦ high melting and boiling point
11
Q
metallic bonding
A
- Sharing of the valence electrons between all the atoms
- electrons are considered to be completely
disconnected from the atoms and free to move anywhere, leads to electrical conductivity - Electrostatic attraction between electron cloud and metal ions binds the metal
together
12
Q
properties of elements metallically bonded
A
- relatively strong
- Good tensile and compressive strength
- Electrical conductivity
- High ductility – how much a material can be plastically deformed
13
Q
what is secondary bonding
A
- They are weak interactions that can play a significant part in determining
material properties (eg. H2O and polymers) - There is no transfer or sharing of electrons
14
Q
van der waals forces
A
- WEAK - is caused by a distortion between the e- and +ve nuclei (creates an induced dipole)
- overall the attraction forces outweight he repulsion forces, so there is a small force developed that holds two atoms together
15
Q
hydrogen bonding
A
- Weak forces, but stronger than other van der Waals forces
- Hydrogen bonding takes place between molecules that have H-F, H-O or H-N
bonds within them. e.g. water H2O, ammonia NH3 ( creates δ+ on H) - Electrostatic attraction
between the small positive and negative charges hold
the molecules together. - Hydrogen bonding accounts for the melting point of water
16
Q
what are the two small scale crystal structure?
A
- crystalline (regular)
- amorphous (irregular)
17
Q
What are teh 4 large scale structures of solid material?
A
- single crystal
- amorphous
- semi-crystalline
- polycrystalline
18
Q
what is a unit cell?
A
smallest segment of a structure that can be repeated in three dimensions
◦ usually contains only a few atoms
◦ defined by 6 lattice paramters (three edges (a, b, c) and the angles between them (α, β, γ))
19
Q
what is a polymorphic material?
A
can be many types of crystal structures
(can depend on pressure, tenp)
20
Q
what is allotropic
A
elements that adopt different crystal structures at different
temperatures and pressures
21
Q
types of defects in material structure
A
- vacancies
- grain boundaries
- dislocations
22
Q
what are grain boundaries
A
- At grain boundaries the crystal planes **don’t match **and there is a transition region a few atomic layers wide
- grain boundaries have higher energy than the bulk of the material due to the irregular bond lengths and coordination numbers of the atoms
23
Q
what are vacancies
A
- missing atom in a structure (hole in crystal structure)
- no. of vancancies depends on temp (high temp = more vancancies)
- vacancies allow atoms to easily diffuse through a solid lattice
24
Q
what are dislocations
A
defect in the planes of atoms
- screw and edge dislocations
- edge are caused by an extra incomplete plane of atoms
typical length 10m per mm3
25
elastic deformation
only** stretches the bonds** – returns to its original shape when the load is removed
26
plastic deformation
**permanently deformed** – it does not return to
its original shape
27
what is edge dislocation slip
- slip of dislocations makes material ductile
- shear stress moves the dislocation along a plane
- can continue until it reaches the crystal edge/grain boundary
28
how can dislocation slip be controlled
By controlling the number and difficulty of movement of dislocations it is possible to control the mechanical properties of materials
29
what property does dislocation slip make **metals** have
metallic bond enables dislocations to slip relatively easily past each other – **ductile**
30
what property does dislocation slip make **ionic** materials have
for slip to occur, ions with the same charge must be forced past each other - requires too much energy and material tends to break before slip occurs – **brittle**
31
what property does dislocation slip make **covalent** materials have
the bonds are strong and very directional - it is difficult for one plane of atoms to slide past another - **brittle**
32
what happens to youngs modulus (E) as temp increases
higher temp = lower E
33
what is poisson's ratio, v
in elastic region, ν is the negative ratio of the lateral to axial strain
* 𝑣 = − 𝜀𝑥/𝜀𝑧
* 𝑣 = − 𝜀y/𝜀𝑧
34
why is ceramic hard to tensile test?
- very brittle so little to no plastic deformation
- dislocation slip almost impossible without fracture
- very poor in tension, better in compression
35
what is hardness
the ability of a material to **resist localised plastic deformation** (surface indentation or scratching)
36
what are the 4 hardness testing types
* brinell test
* vickers test
* rockwell test
* knoop test
37
what is the brinell test
- tungsten carbide ball is indented into material
- diameter of indent determines hardness
38
what is brittleness?
sudden fracture with little or no plastic deformation under tensile loading
39
what is toughness?
ability of a material to absorb energy during deformation and fracture
| can be work/energy required to break a material - area under stresstrain
40
What are the 2 impact tests called
Charpy and Izod
41
how does the izod method work?
- uses a swinging pendulum to measure the amount of energy absorbed by the test piece during fracture
- Energy absorbed = (mg hi) - (mg hf)
42
what happens to when temp increases
43
what happens to yield strength when temp increases
decreases
44
what happens to tensile strength when temp increases
decreases
45
what happens to hardness when temp increases
drecreases
46
what happens to brittleness when temp increases
decreases
47
what happens to ductility when temp increases
increases
48
how does a material fracture
- crack initiation
- propagation
49
differences between ductile and brittle fracture
- In ductile materials, crack propagation proceeds with significant plastic deformation in front of the crack – get some warning before failure
- In brittle materials there is very little / no plastic deformation and the crack can spread
rapidly – very little (or no) warning before failure
50
what causes crack intiation?
microscopic flaws from]
- crystal defects – vacancies and dislocations
- manufacturing defects – porous castings, surface scratches
51
what is entropy
disorder or randomness in a system
52
what is enthalpy
measure of total heat in a system
53
what happens to entropy and enthalpy in a favourable system
entropy increases
enthalpy change is negative
54
what is gibbs free energy
ΔG=ΔH−TΔS
ΔG = change in Gibbs free energy
ΔH = change in enthalpy (heat content)
T = temperature in Kelvin
ΔS = change in entropy (degree of disorder)
| aim is for ΔG to be negative
55
what are solute and solvent
* SOLVENT is the material in greater proportion
* SOLUTE is the material in smaller proportion
56
why do solids have different phases?
- Most mixtures of molten metals show complete solubility of the atoms, however in the solid state this is much less likely
- Within the solid can get different phases that have different compositions depending on the ability of the atoms to mix
57
what happens in substitutional solid solutions?
the solute atoms replace the solvent atoms in the crystal structure - this can occur in a random manner or occasionally in a
regular array
- occurs best when the atoms are of similar size
58
what happens in interstitial solid solutions?
* Interstitial solid solution – the solute atoms fit into the interstitial sites within the crystal structure
* Can only occur for small solute atoms as the spaces are small and large atoms would distort the crystal lattice
59
# phase diagrams
what are intermediate compounds?
- These are combinations of elements (metals and / or non-metals) that produce a compound
- can have very different properties to the alloy and the individual components (often hard and brittle)
60
what is diffusion?
movement of atoms, ions, or molecules within a solid, liquid or gas
it is driven by the desire to increase the entropy
61
what is vacancy diffusion?
where atoms move into neighbouring vacancies
62
what is interstitial diffusion?
only suitable for small atoms such as C that can pass between atoms in the structure
63
use of diffusion
manufacturing processes
- solidification of alloys (casting)
- heat treatments (annealing, age hardening)
64
consequence of diffusion
gas permeation of plastics in food packaging
65
what is solidification?
crystalline material transforms from **disordered arrangement** in **liquid** state to **regular order** in **solid** state which also causes **shrinkage**
66
what is undercooling?
cooling the metal below the melting point so solidification can commence
67
describe the process of solidification?
- embryo is formed randomly (small solid region)
- embryo grows into a nucleus
- if it isn't broken more atoms attach and the nucleus grows
- it attachs to other nuclei until the liquid becomes a solid (forms a plycrystalline structure)
68
what is homogenous nucleation?
takes place in the liquid without solid impurities or surfaces present. Very rare and requires a lot of undercooling to enable solidification to start.
69
what is heterogeneous nucleation
when there are solids present e.g. container walls or particles - requires much less undercooling
- surface of container allows nuclei to to form and can conduct heat away
70
whta is the liquidus line?
temp at which freezing starts when cooling down
71
what is the solidus line?
completion of freezing when cooling down
72
what is the lever rule
fL = b / a+b
fα = a / a+b
fL + fα = 1
73
what happens when alloys are rapidly cooled
microstructures form with varying composition called cores as the atoms can't diffuse in the short time
74
what is a eutectic alloy
a mixture of metals/alloys that solidify at a lower temp/ eutectic temp that forms a super lattice
75
define work hardening
plastically deforming a metal below the critical temp
76
what does work hardening do to the properties of a metal/alloy
increases strength
decreases ductility
- extra dislocations are formed and the can't slide past each other
77
what processes work harden
- cold rolling
- forging
- drawing
- extrusion
- forming
78
what is grain size strengthening?
by reducing the size of the grains, the number of grain boundaries increases. the slip of dislocations into other grains is nearly impossible due to all the different orientations, so it reduces disolaction slip in the metal.
79
how can grain size be controlled?
using inoculants (prevents undercooling) so heterogenous nucealtion occurs in casting
80
what is solid solution strengthening?
adding large solute atoms to the metal so dislocation slip is reduced
81
how does solid solution strengthening work?
by adding larger or samller solute atoms, dislocation slip is reduced and it strengthens the metal
82
what is dispersion strengthening?
precipitation of a second phase material to hinder slip dislocations
83
what is precipitaion hardening?
heat is used to disperse the second phase throughout structure
84
how does precipitation hardening work?
1. alloy is heated so it goes into a single phase region
2. cooled rapidly by quenching (freezes the single phase in as diffusion can't occur in short time)
3. ageing - alloy is heated gently so some second phase can form in much smaller and more dispersed areas
85
what is ageing (strengthening alloy process)
- alloy is heated gently so some second phase can form in much smaller and more dispersed areas
- can sometimes age at room temp
- if particles grow too large, its not effective against dislocation slip - over ageing
86
what is annealing?
process used to increase the ductility of a metal and restore the original softness typically after cold working
87
what are the 3 stages to annealing?
1. Recovery
2. Recrystallisation
3. Grain growth
88
what is a ferrous metal alloy
contains iron
89
what is a non ferrous metal alloy
doesn't contain iron
90
what does allotropic mean?
crystal structure depends on the temp
91
