Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

High Temperature Materials > Inför tentan > Flashcards

Inför tentan Flashcards

1
Q

Describe cobalt-based alloys

A

1) no coherent matix
dispersion strengthening NOT precipitation
2) Solid solution strengthened with
molybdenum, tungsten and tantalum
3) Can be welded!
4) Cast alloys → strengthened by carbides (addition of carbon)
5) GREAT corrosion resistance
6) Addition of cr → oxidation resistance
7) Alloyed with chrome, nickel and tungsten
W/C: influences hardness, ductility & resistance to abrasive wear
8) Wear resistant, corrosion resistant and heat resistant

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

what is meant by gamma’ hardened alloys?

A

the coherent of gamma Austenite Ni3Al

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Name some properties of Cr?

A

Cr:
+ good oxidation resistance
+ good creep resistance
- brittle → hard to shape (CVD)

needs a higher content percentage at higher temperatures to prevent corrosion

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Name some properies of W?

A

losing material:

W –> WO2 –> WO3(g)

1) can creep due to its own weight
prevent this by adding ThO2 (dispersion strengthened)
2) good creep strengthening in other materials (Nb)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Name some properties of B?

A

increase ductility

dislocations can move over gb’s

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Name some properties of Nb?

A

better resistance to intercrystalline corrosion

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Name some properties of Ti?

A

1) can’t load much at high T
unless other elements are added
2) worsens creep strengthening effect in materials (Nb)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is VAR (Vacuum art remelting)?

A

1) remelting it once or twice (or more)
2) difference in density → particles rise to the surface
3) improving the quality of the metal
- time consuming & expensive
4) melted into water cooled copper crucible

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is ESR (Electro Slag Refining)?

A

1) molten droplets goes through the slag
2) large surface/volume area
3) melted into water cooled copper crucible

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the main difference between VAR and ESR?

A

In ESR the air is excluded from the molten metal layer by a layer of molten slag instead of vacuum

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is Electron Beam Cold Hearth Refining (EBCHR)?

A

process for melting
slow, expensive
hearth = same material to avoid contamination
limit inclusions (could create cracks)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Explain Directional Solidification (DS)

A

take away heat from one direction
lower solidification rate
larger temperature gradient

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is the Pilling- Bedworth ratio?

A

PB = Volume of oxide per metal atom/ Volume of metal per metal atom

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What values are wished upon when it comes to PB-ratio?

A

1< PB< 2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What happens if PB = 3?

A

oxide layer can fall off

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What affects the oxygen affinity?

A

a more negative free energy → more stable → high thermodynamic driving force

lower partial pressure

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

How can a metal oxide be reduced?

A

by all the metals having a more negative dG

ex Cu2O and Ni is heated → Ni is more stable and will take the oxygen from the copper oxide.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

How can one Prevent oxidation of the metal/alloy in terms of protective gas atmospheres?

A

pO2 < pO2eq → metal is stable

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is important to keep in mind when using Argon or nitrogen as a protective gas?

A

will not protect metal since oxide is stable → must PURIFY Argon

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

How can you purify argon?

A

1) 500 C with cu turnings
Use of Cu due to large capacity for oxygen (can reduce it with hydrogen later)

2) reduce it further by using Mg

Make sure to use dry gas (prevent oxidation)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Describe linear oxidation

A

Typical for metals with porous or cracked oxide films

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Describe parabolic oxidation

A

1) Typical for metals with thick coherent oxides, e.g. Cu, Fe
2) Is a diffusion process

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Describe logarithmic oxidation

A

For oxidation at elevated temperature, e.g., Fe, Cu, Al; fast oxidation at the start, the rate decreases to a very low value

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Describe cubic oxidation

A

Cubic oxidation occurs when the reaction rate falls between logarithmic and parabolic kinetics. This is characterized by an initially fast logarithmic behavior followed by the slower parabolic behavior

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

What is the oxidation equation for kinear, parabolic and cubic oxidation?

A

(dm/A)^n = k* t

n = 1 linear
n= 2 parabolic
n= 3 cubic
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

What is “spce charge effect”?

A

tunt lager oxid

ex: keram leder inte bort värme → ser inget i SEM pga skikt av elektroner

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

What does Wagner’s Theory say?

A

oxide grows near the O2 for cations (positive ions: p-type)
ex: Cu| Cu2O ( → Cu+ ) | O2
oxide grows near the metal for anions (negative ions: n-type)
ex: Zr | ZrO2 ( ← O2-| O2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

What assumptions are present in Wagners Theory?

A
  • Compact oxide layer, perfectly adherent
  • Migration of ions or electrons is rate limiting
  • Equilibrium at metal/oxide & oxide/gas interfaces
  • Only small deviations from stoichiometry (oxide)
  • Local thermodynamic equilibrium throughout the oxide scale
  • Thick scale (space charge effects neglected)
  • Negligible oxygen solubility in the metal
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

How do you know which oxide that will be closest to the metal surface?

A

When several oxides form on the Surface:
lowest oxygen to metal ratio near the metal
ex: Cu → Cu2O → CuO

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

Where will carbides form? How can you prevent the formation?

A

forms near grain boundaries → corrodes

use low carbon content to avoid this

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

What happens during primary creep?

A

creep rate decreases due to strain hardening

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

What happens during secondary creep?

A

constant creep rate

balance between strain hardening and recovery (softening)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

What happens during tertiary creep?

A

intergranular cracking and/or formation of voids and cavities

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

Name 4 creep deformation mechanisms

A
  1. Cross-slip
  2. Dislocation climb
  3. Vacancy diffusion/ Diffusion Creep
  4. Grain boundary sliding
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

Describe Cross-slip

A

1) occurs at low T

(no need for vacancies)

2) happens for screw dislocations only

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
36
Q

Describe Dislocation climb

A

1) occurs at high T

(needs vacancies)

2) happens for edge dislocations
3) high stress

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
37
Q

What is the structure of Dislocation creep dependent on?

A

the structure depends on deformation hardening and recovery

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
38
Q

Describe diffusion creep

A

through grains or along gb

low stress

diffusion due to concentration differences in vacancies

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
39
Q

What types of diffusion creep can occur?

A

Nabarro Herring creep and Coble creep

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
40
Q

Explain Nabarro Herring creep

A

at high T ~0.7 *Tm
lattice diffusion
through grains

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
41
Q

Explain coble creep

A

low T ~0.4*Tm
due to GB diffusion’
along gb’s

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
42
Q

What is meant by Grain boundary sliding?

A

GB first to melt compared to interior → slide

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
43
Q

How do you know if fracture due to creep has been exposed to high or low stress?

A

high stress: contact points between 3 grains

low: “lines”

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
44
Q

How can you prevent diffusion creep?

A

Increase grain size

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
45
Q

Why are materials with high melting temperature more resistant to creep?

A

diffusion activation energy is proportional to absolute melting temperature

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
46
Q

Why is BCC less creep resistant than FCC at high temperatures?

A

More frequently vibrating atoms –> high diffusion coefficients

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
47
Q

How can you prevent dislocation creep?

A

Solid solution strengthening

dispersion hardening

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
48
Q

How can you prevent gb sliding?

A

gb precipitation or single crystal

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
49
Q

What is similar with creep and self diffusion?

A

about the same activation energy

at low T single crystals have less self diffusion

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
50
Q

Name 4 ways of hardening/strengthening a material

A
  1. Solution hardening
  2. Precipitation hardening
  3. Disperion hardening
  4. Grain boundary strengthening
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
51
Q

Describe Solution Hardening

A

for all common metals

dissolved atoms → strains in original lattice → prevent dislocation glide

more dissolved atoms :)

differences in atomic size :)

strengthening effect at high T

higher tensile + yield strength

lower ductility and electrical conductivity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
52
Q

Describe Precipitation Hardening

A

limited to certain alloy types

% solute > solubility limit at room T

crystallographic relationship with the matrix (same orientations) → strain field

Three steps:
1.solution treatment
2.quenching
3. ageing
natural (room T)
artificial (T> room T)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
53
Q

What are the requirements for precipitation hardening?

A

Conditions:
Solubility of the alloying element in the matrix must decrease strongly with temp.

Soft matrix, hard intermetallic particles

Quenching should be possible

A coherent precipitate should be formed

Note! Precipitation hardened alloys cannot be used near the ageing temperatures

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
54
Q

Describe dispersion hardening

A

good strength at high temperatures

small particles ~10 - 300 nm

small volume fraction of particles

non-coherent

usually oxide particles

less(!) hardening effect than precipitation-hardened alloys IN ROOM T

less sensitive to over ageing & grain growth

not a big decrease in strength at high temperatures

good creep resistance :)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
55
Q

Describe Grain Boundary Strengthening

A

more gb’s → harder for dislocation to move due to orientation

Zr and B in UDIMET 500

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
56
Q

How can you prevent losing/gaining material during cyclic oxidation?

A

addition of Ce

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
57
Q

What affects thermal shock?

A

want a high thermal shock parameter

conscious of cooling rate

beware of phase transformations –> stresses due to thermal shock –> cracks

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
58
Q

How does fracture toughness in composites with a brittle matrix work?

A

fiber break → takes up energy → can stop the growth of the crack

THUS a ceramic matrix can be used
(Improve fracture toughness through phase transformation)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
59
Q

Name 4 stainless steels

A

Ferritic
Martensitic
Austenitic
Duplex (F + A)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
60
Q

What is the minimum cr-content of stainless steels?

A

~12%

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
61
Q

What is a Duplex steel?

A

Ferritic + Austenitic

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
62
Q

Name 5 cast irons

A
  1. Grey Iron
  2. White Cast Iron
  3. Malleable Iron
  4. Spheroidal Graphite Iron /Ductile Iron
  5. Compact Graphite Iron
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
63
Q

What effect does C have when added to Fe-Cr?

A

enlarges gamma phase

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
64
Q

How can you protect Cr23C6 from happening when introducing C into Fe-Cr?

A

by adding Ti or Nb → forms TiC or NbC instead with ratio 1:1

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
65
Q

Why is it bad with Cr23C6?

A

leads to intergranular corrosion

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
66
Q

Describe Ferritic steel

A

bcc → magnetic
max 0.12% C
max 30% Cr
ok SCC (stress corrosion cracking)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
67
Q

Describe martensitic steel

A

always tempered

contains C to get a curing effect ~0.15 - 0.2 % is needed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
68
Q

Describe Austenitic steel

A

Ni addition

better toughness, strength

ductility

good creep resistance and corrosion resistance at high T (compared to ferritic)

non-magnetic

18% Mn in certain types → (Ni equivalent → fcc stabilizer)

kan kallbearbetas → härdas (ex stenkross → hårdare material)
the hardening effect disappears at higher T due to recrystallization

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
69
Q

What series does austenitic steel belong to?

A

300-series

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
70
Q

What do you have to keep in mind during slow cooling of austenitic steels?

A

OBS! Precipitation of carbides on slow cooling in the range 425-870 C
Cr23C6 at gb’s → intercrystalline corrosion
can lower the %C, ex. 304L, 316L more expensive but no carbides
rapid cooling

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
71
Q

Describe Duplex Steel

A

F+ A
high strength + ductility
better corrosion resistance in chloride

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
72
Q

Why is ceramics very sensitive to defects?

A

Dislocation glide is very difficult (large Burger’s vector) which makes the material very sensitive to the presence of defects (for example, cracks)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
73
Q

How can you improve the weak bonding in silicon nitrides?

A

add glass –> works as a glue

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
74
Q

Why would you use silicate glass in aluminum oxide?

A

the more SiO2 the cheaper the material!
pure material requires long sintering process –> expensive)

not as good properties as pure

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
75
Q

How can you mprove ceramics sensitivity to defects?

A

-Improve fracture toughness through phase transformation -Development of ceramic composites (for example, SiC fibre in alumina)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
76
Q

What is a “Schaeffler diagram”?

A

shows how different amounts of elements act as Ni or Cr (equivalents)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
77
Q

What types of equivalents do we normally talk about?

A

Ni- and Cr-

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
78
Q

Describe Ni-equivalents

A

stabilizes fcc structure

behaves like Ni

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
79
Q

Describe Cr- equivalents

A

stabilizes bcc structure → magnetic

behaves like Cr

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
80
Q

Give examples of Ni-equivalents

A

Co, Cu, Mn, N, C…

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
81
Q

Give examples of Cr-equivalents

A

ex: Si, Al, Mo, V, Nb, Ti, W

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
82
Q

What’s so important with Cr and Ni?

A

give better protection against cyclic oxidation

Higher Ni and Cr contents give better strength.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
83
Q

What C percentage does Cast iron have?

A

~2-4%

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
84
Q

Describe Grey irons

A

graphite flakes

good damping of vibrations

low alloying <5%

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
85
Q

Describe White Cast Iron

A

hard

Fe3C + Pearlite

rapid cooling → carbides, no graphite

brittle

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
86
Q

Describe Malleable Iron

A

heat treated White Cast Iron

more ductile

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
87
Q

Describe Spheroidal Graphite Iron /Ductile Iron

A

with Mg
high amount of Si (kiselhalt) → graphite
low amount of Si → carbide

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
88
Q

Describe Compact Graphite Iron

A

Properties between grey and ductile iron

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
89
Q

What is Austenitic Cast Iron?

A

Ductile iron (graphite nodules in ferritic matrix) –> heat treated (austenized) –> matrix becomes austenite

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
90
Q

Describe Superalloys

A

max temp ~0.8* Tm

good mechanical properties

good corrosion resistance

Fe, Ni, Co, Fe-Ni based alloys
expensive → blend with iron
higher density than steel/iron

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
91
Q

Which elements can be used for Precipitation strengthening in Nickel Alloys?

A

Al, Ti, Ta

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
92
Q

Which elements can be used for solid solution strengthening in Nickel Alloys?

A

Mo, Ta, W, Re + Cr

93
Q

Which elements can be used for grain boundary strengthening in Nickel Alloys?

A

primary B and Zr

also C and Hf

94
Q

Which elements can be used for corrosion protection in Nickel Alloys?

A

Al, Cr

95
Q

Where are carbides formed in Nickel-alloys? And why do we want to decrease the formation?

A

at gb’s

want to keep the Cr in the bulk material to improve resistance to corrosion

96
Q

Why can carbides be positive to have in gb’s?

A

resistance to grain slipping → creep resistance!

97
Q

How can be achieve different sizes of gamma’ phase in our Nickel alloy? And why would we want that?

A

age at 2 different temperatures → mixed sizes to get protection for a big interval (they won’t overage at the same time)

98
Q

When are small/big gamma’ particles more effective in Nickel alloys?

A

big particles are more effective at a lower T

small particles are more effective at a higher T

99
Q

What is Mechanical Alloying (MA)?

A

grind elements and mix them together

increasing solubility limit compared to phase diagram ~almost x2 → better strengthening effect

OBS! increasing the temperature too much → return to equilibrium

100
Q

When does gamma’ particles start to coarsen/grow?

A

gamma’-particles grow at T>0.6*Tm (called Ostwald ripening)

101
Q

How can you slow done the coarsening of gamma’?

A

addition of Co, Mo or Mo +W, Nb to slow it down

102
Q

Is single crystal or polycrystalline material stiffest?

A

poly

103
Q

Describe Iron-based Superalloys

A

-matrix based on Fe and Ni(25%min)
Cr-additions for solid solution hardening
Precipitation hardening through ordered intermetallics
Ni3Al, Ni3Ti, Ni3Nb
GB -strengthen
B, Zr
Carbides and carbonitrides may be present

104
Q

Why is directional solidification of the direction <1 0 0> used in some thermal blades?

A

1 0 0 is the direction with lowest Young’s modulus and so the thermal stresses generated in a DS blade is reduced

105
Q

Describe Co- Alloys

A

no coherent matix
dispersion strengthening NOT precipitation
Solid solution strengthened with
molybdenum, tungsten and tantalum
Can be welded! (advantage compared to Ni-alloys)
Cast alloys → strengthened by carbides (addition of carbon)
GREAT corrosion resistance
Addition of cr → oxidation resistance
Alloyed with chrome, nickel and tungsten
W/C: influences hardness, ductility & resistance to abrasive wear
Wear resistant, corrosion resistant and heat resistant

106
Q

Describe Refractory Metals

A

heavy
can withstand high T
poor oxidation resistance

107
Q

Name 5 Refreactory metals

A 
Nb
Ta
Mo
W
Re

and their alloys

108
Q

What is the major usage of refractory metals?

A

Alloying additions in steels and nickel superalloys

109
Q

Application of Nb?

A

mostly as ferroalloys

used in high strength, low alloy steels

110
Q

Application of Ta?

A

getter for oxygen, hydrogen & nitrogen → can improve vacuum → important when processing Ti
however, Ta gets a bit brittle

111
Q

Application of Mo, W?

A

alloying elements in steels, superalloys

112
Q

Application of Re?

A

catalysts, thermocouples
creep strengthening in Nb, dispersion strengthened tungsten alloys
good creep-rupture strength
high tensile strength

113
Q

Pros and cons with Mo?

A

+ withstands high loads
- High temp: Mo → MoO3 (g) :(
coat with SiO2 :D

114
Q

Describe Ta

A

poor TS and creep strength
→ add Hf + W + C

getter for oxygen, hydrogen & nitrogen → can improve vacuum → important when processing Ti
however, Ta gets a bit brittle

115
Q

How does powder production work?

A

have to grind the material
how to do it with a ductile material? → hydrides are brittle! :)
hydride → dehydride
Nb + H2(g) = NbH2 (easier to grind into powder) → treatment i vacuum at high T → Nb+H2(g)
dG < 0 at low T, dG> 0 at high T

116
Q

How does an Arkel - de Boer process work?

A

refining of metals

with transport reactions

117
Q

What’s transport reactions?

A

used for purification, in combination with a transporting agent

t. ex. Nb(with impurities)+I2 (g) → ( heating) NbI5 (g) → (heating) Nb (pure)+I2 (g)

118
Q

Describe Ti -alloys

A

light
good strength (limited at high T)
high T properties
limited > 650 C

119
Q

How is Ti produced, describe the first step?

A

TiO2 + 2C + 2Cl2 (g) → TiCl4 (g) + 2CO(g)

120
Q

How is Ti produced, describe the second step?

A

Kroll process:

TiCl4 (g) + Mg → Ti + MgCl2

121
Q

Why is Ti-production expensive?

A

addition of Mg → expensive

122
Q

Is there another way of producing Ti?

A

can use EMR (electronically mediated reaction) → add needed electron

123
Q

What “state” is Ti created in?

A

Kroll process creates Sponge Titanium

124
Q

Describe Sponge Titanium

A

porous but brittle
high solubility for oxygen and nitrogen, can ignite :( (only positive thing, solution strengthening)
Ti- not packed closely → low conductivity → can’t lead the heat away → increased T → increased oxidation

125
Q

Describe alfa- alloys

A

solution hardening → heat treatment in -region

not high T capability (cannot be strengthened by heat treatment since it only exist 1 phase)

hcp in Troom(few slip systems)

het up → change structure → more slip systems

126
Q

What happens when you quench alfa (Ti alloy)?

A

widmanstatten structure for alfa (looks like fibers)

good fracture toughness → good against fatigue :)

127
Q

What happens when you slowly cool alfa (Ti alloy)?

A

Slow cooling
alfa-plates
good creep resistance

128
Q

How can you stabilize alfa-alloys? (Ti-alloys)

A

Aluminum -small amounts

129
Q

Describe beta- alloys (ti)

A

can have -stabilizers just to get better properties

not high T capability (cannot be strengthened by heat treatment since it only exist 1 phase)

130
Q

How can you stabilize beta-alloys?

A

V ,Nb, Mo

need big amounts → expensive

131
Q

Describe alfa-beta alloys (Ti)

A

2 phases

can optimize properties with heat treatment

132
Q

Describe near - alfa alloys (Ti)

A

small portions of stabilizers

133
Q

Name 2 elements that are neutral in Ti-alloys

A

Sn (tin), and Zr

134
Q

How can you heat treat alfa-beta alloys? Name 2 ways

A

solution treatment → quench → alfa’ (martensite)→ temper

quench → beta-ss (super saturated) → ageing

135
Q

How can you solid solution strengthen Ti-alloys?

A

solid solution strengthening: Al, Zr, Tin (Sn)

136
Q

Name the Hume Rothery’s Rule for intermetallic compounds

A

formed if the (e-/atom) ratio is :
≈ 3/2 (cubic) AgCd
or 7/4 (e-brass, hcp) AgCd3
or 21/13 (-Brass, cubic) Ag5Cd8

137
Q

What’s the exception for Hume Rothery’s Rule?

A

transitional elements are ignored
Fe, Ni, Co
ex: NiAl (Al 3 valenser, Ni + Al = 2 atoms) → 3/2 → cubic

138
Q

Describe Intermetallic Compounds

A

ordered structure

low creep, good corrosion resistance

yield strength increases with temperature (Ni3Al…)

low ductility → improved by alloying

139
Q

Name some intermetallic compounds

A

Ni3Al, NiAl, Fe3Al, FeAl, Ti3Al, TiAl

140
Q

Name 2 Nickel Aluminides

A

Ni3Al and NiAl

141
Q

Describe Ni3Al

A

yield strength increases with temperature
up to a certain T ( up to about 600 degrees C)

gamma’-phase, ordered fcc

142
Q

What is a major strengthening component in superalloys?

A

Ni3Al

143
Q

What is a downside with Ni3Al?

A

Polycrystalline matrial is brittle

144
Q

How can you increse ductility in Ni3Al?

A

small additions boron → huge elongations (ductility)
B segregates to gb’s → lowers the dislocation pile stresses → dislocations can move→ no cracking
this effect is limited to Ni-rich aluminides

145
Q

What happens in the material when the yield strength increases in Ni3Al when increasing the temperature?

A

thermally activated cross-slip of screw dislocations from a close packed plane {1 1 1} to {1 0 0} –>

dislocation glide becomes more difficult

146
Q

Are all Ni3Al crystals brittle?

A

Not single crystals, just polycrystalline material

147
Q

Describe NiAl

A

High melting point ~1640

Brittle

T> 500 C → strength decreases
due to dislocation climb and slip

good oxidation resistance

148
Q

How can you increase ductility in NiAl?

A

increase T, cannot add B

HOWEVER, T> 500 → strength decreases
due to dislocation climb and slip

149
Q

How can you improve NiAl?

A

grain refining

alloying elements for supporting dislocation glide in <111>
can use Fe, Cr, Mn

Nb, Ta → increase creep strength (precip. hardening)

150
Q

Describe Titanium aluminides

A

low density

high E-modulus

good oxidation resistance up to~900 C

151
Q

Titanium aluminides look promising, especially if..?

A

low temperature ductility can be improved

152
Q

Describe Ti3Al (alfa2)

A

most alloys include Mo and V

limited ductility at room temp

153
Q

How can you increase ductility of Ti3Al (alfa2)?

A

stabilizers, like Nb, improves ductility

Nb replaces Ti → decrease in covalent bonding → Peierls-Nabarro stress decreases (stress to move one dislocation)

154
Q

Describe TiAl (gamma)

A

ordered FCT
low density
good oxidation resistance
plasticity at high T (due to twin formation)

155
Q

What happens when you ass Cr, V, M, Si to TiAl?

A

increases ductility but lower oxidation resistance

156
Q

What happens when you add Nb, Ta, Mo, W to TiAl?

A

increases oxidation resistance

157
Q

What happens when you add Si, C, N in small amounts to TiAl?

A

increases creep resistance

158
Q

Describe Iron Aluminides

A

cheap

excellent corrosion
and oxidation resistance

low density

brittle/ low ductility at low T

poor strength at high T

alloy it to increase properties

159
Q

What’s the purpose of using Cermets?

A

Ceramic - withstands high T, Metal - acts as glue

160
Q

What are Cermets often used for?

A

as cutting tools

161
Q

Give an example of a Cermet that is good as wear resistance

A

WC + Co as the binder + other carbides

162
Q

Name a Infodringsmaterial /Refractories

A

SiC

163
Q

Describe SiC, what can it be used for?

A

SiC tål hög värme + OK ledningsförmåga
behållare i hög T ugn

Refractory

164
Q

Name an engineering ceramic

A

Reaction bonded Si3N4

Al2O3

ZrO2

SiC

165
Q

What is meant by Reaction bonded?

A

when the reaction occurs the bonds will form simultaneously

166
Q

Give an example of how reaction bonding can occur

A

hard particles → not compact → introduce NH3 (ammonia) → N2* + H2 (*more reactive than just pure N2)

Si3N4

167
Q

There is a big difference between metals as ceramics. How do you know if you can trust the values of a ceramic?

A

ook at the Weibull modulus → if > 15 trust the values

168
Q

How can you improve the fracture toughness of Si3N4?

A

can improve fracture toughness by adding SiCw(whiskers)

slows down crack propagation due to absorption of energy

169
Q

How can Si3N4 be manufactured?

A

reaction bonded, hot pressed

170
Q

how will HPSN (hot pressed silicon nitride) look?

A

glass remaining at triple points –> good adhesion between the grains.

171
Q

Why is there a glassy phase between grains in HPSN?

A

often MgO as sintering aid –> reaction with SiO2 (contaminated Si3N4)

172
Q

how will SSN (sintered silicon nitride) look?

A

big white cylinders in black matrix (Liquid phase residue)

173
Q

What is good with reaction bonded Si3N4?

A

good flexure strength (~400MPa) at high T ~1500 C

174
Q

What is SiC used for?

A

electrical resistance heat elements

175
Q

How can SiC be manufactured?

A

reaction bonding, hot pressing, sintering

176
Q

Pro with sintering?

A

sintered → more even product

177
Q

-for what T should SiC be used?

A

use below 1500C

178
Q

What is Al2O3 used for and in what T?

A

used in grinding, polishing, cutting

< 1700 C

179
Q

How can you improve the fracture toughness (4-8 times) in Al2O3?

A

can ad SiC-whiskers → improved fracture toughness by 4-8 times

180
Q

What can ZrO2 be used for? why?

A

TBC (thermal barrier coatings)

one of the lowest thermal conductivity of ceramics

181
Q

What is FSZ?

A

Fully stabilized zirconia: no phase transformation → no expansion

182
Q

What is PSZ?

A

Partially stabilized zirconia:

phase transformation → expansion

bad conductivity → temperature gradient → cracks

183
Q

How does PSZ ( fracture toughening mechanism) work?

A

crack grows → compressive stresses → difficult for the crack to grow

184
Q

Name 2 new Ceramics?

A

Ti3SiC2

TiB2

185
Q

Describe Ti3SiC2

A

new material -unique properties

good resistance to thermal shock

high strength and deformation characteristics

high electrical and thermal conductivities

good oxidation and wear resistance

Good formability and strength (better than superalloys) at high temperature

186
Q

Describe TiB2

A

new material

self-propagating reaction between Ti and B

good strength at high T

good thermal conductivity

Possibility for Hot Pressing or HIP

187
Q

What are the 2 fundamental processes of powder forming processes?

A

1) solid state diffusion

2) Liquid phase transport

188
Q

WHat is meant by liquid phase sintering?

A

add small amount melt → covers pores → quicker sintering process

189
Q

Give an example of liquid phase sintering

A

ex. W-W doesn’t bind well together → add Ni, Fe or Cu as the liquid phase

190
Q

What happens during the sintering process in powder forming processes?

A

don’t have to melt the material

porosity decreases

density increases (strength increases)

component shrinks

191
Q

Name an advantage with sintering?

A

can alloy dissimilar materials (different structure, density etc)

192
Q

How can you avoid shrinkage when sintering? (What does Höganäs do?)

A

Höganäs avoids shrinkage by using Distaloy = bond between the elements!
avoids segregation! :)

193
Q

Which 3 composites do we normally talk about?

A

1) Dispersion-hardened Composites
2) Particle Composites
3) Fiber composites

194
Q

Describe Dispersion-hardened Composites

A

small particles 10-300nm
small amounts ~max 15%
usually oxide particles

195
Q

Pros with Dispersion-hardened Composites?

A

good creep resistance

better than precipitation at high T

196
Q

example of Dispersion-hardened Composite?

A

Ni-ThO2 ( 7-10 % ThO2)

197
Q

Describe particle composites

A

large particles 80m
large amounts ~85%
easy for dislocations to go through (because of the big particles)

198
Q

Name a particle composite?

A

WC- Co

85% WC

199
Q

Describe Fiber Composites

A 
short fibers 
long fibers
light
good strength
good fatigue limit
good stiffness
good strength at high T
200
Q

what fibres are often used?

A

B, C, Ceramics (SiC)

201
Q

What matrix are often used?

A

Matrix: Metals, Intermetallics, Ceramics

202
Q

What is the downside with many MMC Metal Matrix Composites?

A

poor oxidation resistance

high density of the reinforcement

203
Q

What are 2 important parameters affecting the TS of fiber composites?

A

length and orientation

204
Q

What can be done to increase the strength of Ti MMC?

A

Addition of SiC fibers:

CVD on W or C core

205
Q

What is important with Ti MMC?

A

Reaction between carbide fibre and matrix should be avoided.

limited T =600 C

206
Q

How’s carbon fibre manufactured?

A
  1. polymeren oxideras (polymer is oxidized)
  2. uppvärmning 1500 till 2000 C (förkolning/carbonized)
  3. grafitisering vid 2500 till 3000 C (bättre ordning och egenskaper) …Graphitized..better properties
207
Q

Name one pro and one con with carbon-carbon composites

A

low oxidation resistance

can be used up to 3000 C in natural environments

208
Q

When are Ceramic Matrix Composites used?

A

for high temperature applications

209
Q

Give an example of Ceramic Matrix Composite

A

for ex. SiC + nicalon fibre. SiC matrix is deposited between the fibres

210
Q

Name a pro with ceramic matrix composites?

A

Good fracture toughness, better than for ceramics.

211
Q

Composites with an Intermetallic Compound as the Matrix. Name suitable fibers

A

SiC, Al2O3, W-alloys

212
Q

Composites with an Intermetallic Compound as the Matrix. Name suitable matrix

A

NiAl, NbAl3, MoSi2

213
Q

How can materials be protected against corrosion/oxidation?

A

Cr, Al

214
Q

difference between Cr2O3 and Al2O3?

A

Chromium Oxide offers good protection up to 1000 C, al better protection

215
Q

What happens to the ductility when Cr is lowered?

A

lower chromium……better ductility.

216
Q

How can you protect large areas from corrosion?

A

“cladding” (co-extrusion) –> produces a good mettalurgical bond between inner and outer materials

217
Q

Give examples on when Cladding can be used

A

for ex. stainless steel /plain carbon steel, 50Cr/50Ni cladded Incoloy

218
Q

How can you protect small areas from corrosion?

A

Diffusion coating

219
Q

Name 3 ways of diffusion coatings

A

Chromizing
Pack Cementation
CVD

220
Q

How is “pack cementation” performed?

A

Al-powder + halide initiator + Al2O3, heated 700 - 1100 C. (inert conditions)

221
Q

What happens if the substrate is Ni- or Co-based in Pack Cementation?

A

aluminides (25 – 75 mikrometer thick) are formed after diffusion in to the alloy.

222
Q

Cons with Pack cementation

A

We cannot choose the composition of the aluminide coat –> may not be ideal

223
Q

Describe an Overlay Coat

A

MCrAlY
where M = Ni or Co or Ni+Co
50-100 mikrometer

224
Q

What’s the advantages with Overkay coatings?

A

1) Wider selection of chemical composition possible
2) superior oxidation or corrosion resistance (depending on composition)
3) superior ductility (depending on composition)
4) improved scale adherence in thermal cycling conditions due to yttrium addition

225
Q

How can you make the Overlay coating?

A

PVD, thermal spray, plasma spray, electroplating

226
Q

Describe plasma spraying (low pressure plasma spraying/vacuum plasma spraying)

A

gun sprays molten or semi-molten pre-alloyed powder onto component surface in a chamber under low pressure of inert gas.(prevent oxidation of powder)

227
Q

How can you improve corrosion/oxidation resistance when using low pressure plasma spraying?

A

add hafnium, platinum, silicon into pre-alloyed powder

228
Q

What is used for Thermal Barrier Coating (TBC)?

A

zirconia ZrO2, very low thermal conductivity compared to Ni or Fe. Acts as a thermal barrier.

229
Q

What zirconia is used in TBC?

A

PYSZ (partially yttria stabilized zirconia)

High Temperature Materials (1 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions