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
2
Q
what is meant by gamma’ hardened alloys?
A
the coherent of gamma Austenite Ni3Al
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
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)
5
Q
Name some properties of B?
A
increase ductility
dislocations can move over gb’s
6
Q
Name some properties of Nb?
A
better resistance to intercrystalline corrosion
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)
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
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
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
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)
12
Q
Explain Directional Solidification (DS)
A
take away heat from one direction
lower solidification rate
larger temperature gradient
13
Q
What is the Pilling- Bedworth ratio?
A
PB = Volume of oxide per metal atom/ Volume of metal per metal atom
14
Q
What values are wished upon when it comes to PB-ratio?
A
1< PB< 2
15
Q
What happens if PB = 3?
A
oxide layer can fall off
16
Q
What affects the oxygen affinity?
A
a more negative free energy → more stable → high thermodynamic driving force
lower partial pressure
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.
18
Q
How can one Prevent oxidation of the metal/alloy in terms of protective gas atmospheres?
A
pO2 < pO2eq → metal is stable
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
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)
21
Q
Describe linear oxidation
A
Typical for metals with porous or cracked oxide films
22
Q
Describe parabolic oxidation
A
1) Typical for metals with thick coherent oxides, e.g. Cu, Fe
2) Is a diffusion process
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
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
25
What is the oxidation equation for kinear, parabolic and cubic oxidation?
(dm/A)^n = k* t
```
n = 1 linear
n= 2 parabolic
n= 3 cubic
```
26
What is "spce charge effect"?
tunt lager oxid
| ex: keram leder inte bort värme → ser inget i SEM pga skikt av elektroner
27
What does Wagner’s Theory say?
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
28
What assumptions are present in Wagners Theory?
* 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
29
How do you know which oxide that will be closest to the metal surface?
When several oxides form on the Surface:
lowest oxygen to metal ratio near the metal
ex: Cu → Cu2O → CuO
30
Where will carbides form? How can you prevent the formation?
forms near grain boundaries → corrodes
| use low carbon content to avoid this
31
What happens during primary creep?
creep rate decreases due to strain hardening
32
What happens during secondary creep?
constant creep rate
balance between strain hardening and recovery (softening)
33
What happens during tertiary creep?
intergranular cracking and/or formation of voids and cavities
34
Name 4 creep deformation mechanisms
1. Cross-slip
2. Dislocation climb
3. Vacancy diffusion/ Diffusion Creep
4. Grain boundary sliding
35
Describe Cross-slip
1) occurs at low T
(no need for vacancies)
2) happens for screw dislocations only
36
Describe Dislocation climb
1) occurs at high T
(needs vacancies)
2) happens for edge dislocations
3) high stress
37
What is the structure of Dislocation creep dependent on?
the structure depends on deformation hardening and recovery
38
Describe diffusion creep
through grains or along gb
low stress
diffusion due to concentration differences in vacancies
39
What types of diffusion creep can occur?
Nabarro Herring creep and Coble creep
40
Explain Nabarro Herring creep
at high T ~0.7 *Tm
lattice diffusion
through grains
41
Explain coble creep
low T ~0.4*Tm
due to GB diffusion’
along gb’s
42
What is meant by Grain boundary sliding?
GB first to melt compared to interior → slide
43
How do you know if fracture due to creep has been exposed to high or low stress?
high stress: contact points between 3 grains
low: "lines"
44
How can you prevent diffusion creep?
Increase grain size
45
Why are materials with high melting temperature more resistant to creep?
diffusion activation energy is proportional to absolute melting temperature
46
Why is BCC less creep resistant than FCC at high temperatures?
More frequently vibrating atoms --> high diffusion coefficients
47
How can you prevent dislocation creep?
Solid solution strengthening
dispersion hardening
48
How can you prevent gb sliding?
gb precipitation or single crystal
49
What is similar with creep and self diffusion?
about the same activation energy
at low T single crystals have less self diffusion
50
Name 4 ways of hardening/strengthening a material
1. Solution hardening
2. Precipitation hardening
3. Disperion hardening
4. Grain boundary strengthening
51
Describe Solution Hardening
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
52
Describe Precipitation Hardening
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)
```
53
What are the requirements for precipitation hardening?
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
54
Describe dispersion hardening
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 :)
55
Describe Grain Boundary Strengthening
more gb’s → harder for dislocation to move due to orientation
Zr and B in UDIMET 500
56
How can you prevent losing/gaining material during cyclic oxidation?
addition of Ce
57
What affects thermal shock?
want a high thermal shock parameter
conscious of cooling rate
beware of phase transformations --> stresses due to thermal shock --> cracks
58
How does fracture toughness in composites with a brittle matrix work?
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)
59
Name 4 stainless steels
Ferritic
Martensitic
Austenitic
Duplex (F + A)
60
What is the minimum cr-content of stainless steels?
~12%
61
What is a Duplex steel?
Ferritic + Austenitic
62
Name 5 cast irons
1. Grey Iron
2. White Cast Iron
3. Malleable Iron
4. Spheroidal Graphite Iron /Ductile Iron
5. Compact Graphite Iron
63
What effect does C have when added to Fe-Cr?
enlarges gamma phase
64
How can you protect Cr23C6 from happening when introducing C into Fe-Cr?
by adding Ti or Nb → forms TiC or NbC instead with ratio 1:1
65
Why is it bad with Cr23C6?
leads to intergranular corrosion
66
Describe Ferritic steel
bcc → magnetic
max 0.12% C
max 30% Cr
ok SCC (stress corrosion cracking)
67
Describe martensitic steel
always tempered
| contains C to get a curing effect ~0.15 - 0.2 % is needed
68
Describe Austenitic steel
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
69
What series does austenitic steel belong to?
300-series
70
What do you have to keep in mind during slow cooling of austenitic steels?
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
71
Describe Duplex Steel
F+ A
high strength + ductility
better corrosion resistance in chloride
72
Why is ceramics very sensitive to defects?
Dislocation glide is very difficult (large Burger’s vector) which makes the material very sensitive to the presence of defects (for example, cracks)
73
How can you improve the weak bonding in silicon nitrides?
add glass --> works as a glue
74
Why would you use silicate glass in aluminum oxide?
the more SiO2 the cheaper the material!
pure material requires long sintering process --> expensive)
not as good properties as pure
75
How can you mprove ceramics sensitivity to defects?
-Improve fracture toughness through phase transformation -Development of ceramic composites (for example, SiC fibre in alumina)
76
What is a "Schaeffler diagram"?
shows how different amounts of elements act as Ni or Cr (equivalents)
77
What types of equivalents do we normally talk about?
Ni- and Cr-
78
Describe Ni-equivalents
stabilizes fcc structure
| behaves like Ni
79
Describe Cr- equivalents
stabilizes bcc structure → magnetic
| behaves like Cr
80
Give examples of Ni-equivalents
Co, Cu, Mn, N, C…
81
Give examples of Cr-equivalents
ex: Si, Al, Mo, V, Nb, Ti, W
82
What's so important with Cr and Ni?
give better protection against cyclic oxidation
Higher Ni and Cr contents give better strength.
83
What C percentage does Cast iron have?
~2-4%
84
Describe Grey irons
graphite flakes
good damping of vibrations
low alloying <5%
85
Describe White Cast Iron
hard
Fe3C + Pearlite
rapid cooling → carbides, no graphite
brittle
86
Describe Malleable Iron
heat treated White Cast Iron
more ductile
87
Describe Spheroidal Graphite Iron /Ductile Iron
with Mg
high amount of Si (kiselhalt) → graphite
low amount of Si → carbide
88
Describe Compact Graphite Iron
Properties between grey and ductile iron
89
What is Austenitic Cast Iron?
Ductile iron (graphite nodules in ferritic matrix) --> heat treated (austenized) --> matrix becomes austenite
90
Describe Superalloys
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
91
Which elements can be used for Precipitation strengthening in Nickel Alloys?
Al, Ti, Ta
92
Which elements can be used for solid solution strengthening in Nickel Alloys?
Mo, Ta, W, Re + Cr
93
Which elements can be used for grain boundary strengthening in Nickel Alloys?
primary B and Zr
also C and Hf
94
Which elements can be used for corrosion protection in Nickel Alloys?
Al, Cr
95
Where are carbides formed in Nickel-alloys? And why do we want to decrease the formation?
at gb's
want to keep the Cr in the bulk material to improve resistance to corrosion
96
Why can carbides be positive to have in gb's?
resistance to grain slipping → creep resistance!
97
How can be achieve different sizes of gamma' phase in our Nickel alloy? And why would we want that?
age at 2 different temperatures → mixed sizes to get protection for a big interval (they won’t overage at the same time)
98
When are small/big gamma' particles more effective in Nickel alloys?
big particles are more effective at a lower T
small particles are more effective at a higher T
99
What is Mechanical Alloying (MA)?
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
When does gamma' particles start to coarsen/grow?
gamma'-particles grow at T>0.6*Tm (called Ostwald ripening)
101
How can you slow done the coarsening of gamma'?
addition of Co, Mo or Mo +W, Nb to slow it down
102
Is single crystal or polycrystalline material stiffest?
poly
103
Describe Iron-based Superalloys
-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
Why is directional solidification of the direction <1 0 0> used in some thermal blades?
1 0 0 is the direction with lowest Young's modulus and so the thermal stresses generated in a DS blade is reduced
105
Describe Co- Alloys
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
Describe Refractory Metals
heavy
can withstand high T
poor oxidation resistance
107
Name 5 Refreactory metals
```
Nb
Ta
Mo
W
Re
```
and their alloys
108
What is the major usage of refractory metals?
Alloying additions in steels and nickel superalloys
109
Application of Nb?
mostly as ferroalloys
| used in high strength, low alloy steels
110
Application of Ta?
getter for oxygen, hydrogen & nitrogen → can improve vacuum → important when processing Ti
however, Ta gets a bit brittle
111
Application of Mo, W?
alloying elements in steels, superalloys
112
Application of Re?
catalysts, thermocouples
creep strengthening in Nb, dispersion strengthened tungsten alloys
good creep-rupture strength
high tensile strength
113
Pros and cons with Mo?
+ withstands high loads
- High temp: Mo → MoO3 (g) :(
coat with SiO2 :D
114
Describe Ta
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
How does powder production work?
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
How does an Arkel - de Boer process work?
refining of metals
with transport reactions
117
What's transport reactions?
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
Describe Ti -alloys
light
good strength (limited at high T)
high T properties
limited > 650 C
119
How is Ti produced, describe the first step?
TiO2 + 2C + 2Cl2 (g) → TiCl4 (g) + 2CO(g)
120
How is Ti produced, describe the second step?
Kroll process:
TiCl4 (g) + Mg → Ti + MgCl2
121
Why is Ti-production expensive?
addition of Mg → expensive
122
Is there another way of producing Ti?
can use EMR (electronically mediated reaction) → add needed electron
123
What "state" is Ti created in?
Kroll process creates Sponge Titanium
124
Describe Sponge Titanium
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
Describe alfa- alloys
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
What happens when you quench alfa (Ti alloy)?
widmanstatten structure for alfa (looks like fibers)
| good fracture toughness → good against fatigue :)
127
What happens when you slowly cool alfa (Ti alloy)?
Slow cooling
alfa-plates
good creep resistance
128
How can you stabilize alfa-alloys? (Ti-alloys)
Aluminum -small amounts
129
Describe beta- alloys (ti)
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
How can you stabilize beta-alloys?
V ,Nb, Mo
| need big amounts → expensive
131
Describe alfa-beta alloys (Ti)
2 phases
| can optimize properties with heat treatment
132
Describe near - alfa alloys (Ti)
small portions of stabilizers
133
Name 2 elements that are neutral in Ti-alloys
Sn (tin), and Zr
134
How can you heat treat alfa-beta alloys? Name 2 ways
solution treatment → quench → alfa' (martensite)→ temper
quench → beta-ss (super saturated) → ageing
135
How can you solid solution strengthen Ti-alloys?
solid solution strengthening: Al, Zr, Tin (Sn)
136
Name the Hume Rothery's Rule for intermetallic compounds
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
What's the exception for Hume Rothery's Rule?
transitional elements are ignored
Fe, Ni, Co
ex: NiAl (Al 3 valenser, Ni + Al = 2 atoms) → 3/2 → cubic
138
Describe Intermetallic Compounds
ordered structure
low creep, good corrosion resistance
yield strength increases with temperature (Ni3Al…)
low ductility → improved by alloying
139
Name some intermetallic compounds
Ni3Al, NiAl, Fe3Al, FeAl, Ti3Al, TiAl
140
Name 2 Nickel Aluminides
Ni3Al and NiAl
141
Describe Ni3Al
yield strength increases with temperature
up to a certain T ( up to about 600 degrees C)
gamma'-phase, ordered fcc
142
What is a major strengthening component in superalloys?
Ni3Al
143
What is a downside with Ni3Al?
Polycrystalline matrial is brittle
144
How can you increse ductility in Ni3Al?
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
What happens in the material when the yield strength increases in Ni3Al when increasing the temperature?
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
Are all Ni3Al crystals brittle?
Not single crystals, just polycrystalline material
147
Describe NiAl
High melting point ~1640
Brittle
T> 500 C → strength decreases
due to dislocation climb and slip
good oxidation resistance
148
How can you increase ductility in NiAl?
increase T, cannot add B
HOWEVER, T> 500 → strength decreases
due to dislocation climb and slip
149
How can you improve NiAl?
grain refining
alloying elements for supporting dislocation glide in <111>
can use Fe, Cr, Mn
Nb, Ta → increase creep strength (precip. hardening)
150
Describe Titanium aluminides
low density
high E-modulus
good oxidation resistance up to~900 C
151
Titanium aluminides look promising, especially if..?
low temperature ductility can be improved
152
Describe Ti3Al (alfa2)
most alloys include Mo and V
limited ductility at room temp
153
How can you increase ductility of Ti3Al (alfa2)?
stabilizers, like Nb, improves ductility
| Nb replaces Ti → decrease in covalent bonding → Peierls-Nabarro stress decreases (stress to move one dislocation)
154
Describe TiAl (gamma)
ordered FCT
low density
good oxidation resistance
plasticity at high T (due to twin formation)
155
What happens when you ass Cr, V, M, Si to TiAl?
increases ductility but lower oxidation resistance
156
What happens when you add Nb, Ta, Mo, W to TiAl?
increases oxidation resistance
157
What happens when you add Si, C, N in small amounts to TiAl?
increases creep resistance
158
Describe Iron Aluminides
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
What's the purpose of using Cermets?
Ceramic - withstands high T, Metal - acts as glue
160
What are Cermets often used for?
as cutting tools
161
Give an example of a Cermet that is good as wear resistance
WC + Co as the binder + other carbides
162
Name a Infodringsmaterial /Refractories
SiC
163
Describe SiC, what can it be used for?
SiC tål hög värme + OK ledningsförmåga
behållare i hög T ugn
Refractory
164
Name an engineering ceramic
Reaction bonded Si3N4
Al2O3
ZrO2
SiC
165
What is meant by Reaction bonded?
when the reaction occurs the bonds will form simultaneously
166
Give an example of how reaction bonding can occur
hard particles → not compact → introduce NH3 (ammonia) → N2* + H2 (*more reactive than just pure N2)
Si3N4
167
There is a big difference between metals as ceramics. How do you know if you can trust the values of a ceramic?
ook at the Weibull modulus → if > 15 trust the values
168
How can you improve the fracture toughness of Si3N4?
can improve fracture toughness by adding SiCw(whiskers)
slows down crack propagation due to absorption of energy
169
How can Si3N4 be manufactured?
reaction bonded, hot pressed
170
how will HPSN (hot pressed silicon nitride) look?
glass remaining at triple points --> good adhesion between the grains.
171
Why is there a glassy phase between grains in HPSN?
often MgO as sintering aid --> reaction with SiO2 (contaminated Si3N4)
172
how will SSN (sintered silicon nitride) look?
big white cylinders in black matrix (Liquid phase residue)
173
What is good with reaction bonded Si3N4?
good flexure strength (~400MPa) at high T ~1500 C
174
What is SiC used for?
electrical resistance heat elements
175
How can SiC be manufactured?
reaction bonding, hot pressing, sintering
176
Pro with sintering?
sintered → more even product
177
-for what T should SiC be used?
use below 1500C
178
What is Al2O3 used for and in what T?
used in grinding, polishing, cutting
| < 1700 C
179
How can you improve the fracture toughness (4-8 times) in Al2O3?
can ad SiC-whiskers → improved fracture toughness by 4-8 times
180
What can ZrO2 be used for? why?
TBC (thermal barrier coatings)
one of the lowest thermal conductivity of ceramics
181
What is FSZ?
Fully stabilized zirconia: no phase transformation → no expansion
182
What is PSZ?
Partially stabilized zirconia:
phase transformation → expansion
bad conductivity → temperature gradient → cracks
183
How does PSZ ( fracture toughening mechanism) work?
crack grows → compressive stresses → difficult for the crack to grow
184
Name 2 new Ceramics?
Ti3SiC2
| TiB2
185
Describe Ti3SiC2
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
Describe TiB2
new material
self-propagating reaction between Ti and B
good strength at high T
good thermal conductivity
Possibility for Hot Pressing or HIP
187
What are the 2 fundamental processes of powder forming processes?
1) solid state diffusion
| 2) Liquid phase transport
188
WHat is meant by liquid phase sintering?
add small amount melt → covers pores → quicker sintering process
189
Give an example of liquid phase sintering
ex. W-W doesn’t bind well together → add Ni, Fe or Cu as the liquid phase
190
What happens during the sintering process in powder forming processes?
don’t have to melt the material
porosity decreases
density increases (strength increases)
component shrinks
191
Name an advantage with sintering?
can alloy dissimilar materials (different structure, density etc)
192
How can you avoid shrinkage when sintering? (What does Höganäs do?)
Höganäs avoids shrinkage by using Distaloy = bond between the elements!
avoids segregation! :)
193
Which 3 composites do we normally talk about?
1) Dispersion-hardened Composites
2) Particle Composites
3) Fiber composites
194
Describe Dispersion-hardened Composites
small particles 10-300nm
small amounts ~max 15%
usually oxide particles
195
Pros with Dispersion-hardened Composites?
good creep resistance
| better than precipitation at high T
196
example of Dispersion-hardened Composite?
Ni-ThO2 ( 7-10 % ThO2)
197
Describe particle composites
large particles 80m
large amounts ~85%
easy for dislocations to go through (because of the big particles)
198
Name a particle composite?
WC- Co
| 85% WC
199
Describe Fiber Composites
```
short fibers
long fibers
light
good strength
good fatigue limit
good stiffness
good strength at high T
```
200
what fibres are often used?
B, C, Ceramics (SiC)
201
What matrix are often used?
Matrix: Metals, Intermetallics, Ceramics
202
What is the downside with many MMC Metal Matrix Composites?
poor oxidation resistance
high density of the reinforcement
203
What are 2 important parameters affecting the TS of fiber composites?
length and orientation
204
What can be done to increase the strength of Ti MMC?
Addition of SiC fibers:
CVD on W or C core
205
What is important with Ti MMC?
Reaction between carbide fibre and matrix should be avoided.
limited T =600 C
206
How's carbon fibre manufactured?
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
Name one pro and one con with carbon-carbon composites
low oxidation resistance
can be used up to 3000 C in natural environments
208
When are Ceramic Matrix Composites used?
for high temperature applications
209
Give an example of Ceramic Matrix Composite
for ex. SiC + nicalon fibre. SiC matrix is deposited between the fibres
210
Name a pro with ceramic matrix composites?
Good fracture toughness, better than for ceramics.
211
Composites with an Intermetallic Compound as the Matrix. Name suitable fibers
SiC, Al2O3, W-alloys
212
Composites with an Intermetallic Compound as the Matrix. Name suitable matrix
NiAl, NbAl3, MoSi2
213
How can materials be protected against corrosion/oxidation?
Cr, Al
214
difference between Cr2O3 and Al2O3?
Chromium Oxide offers good protection up to 1000 C, al better protection
215
What happens to the ductility when Cr is lowered?
lower chromium......better ductility.
216
How can you protect large areas from corrosion?
"cladding" (co-extrusion) --> produces a good mettalurgical bond between inner and outer materials
217
Give examples on when Cladding can be used
for ex. stainless steel /plain carbon steel, 50Cr/50Ni cladded Incoloy
218
How can you protect small areas from corrosion?
Diffusion coating
219
Name 3 ways of diffusion coatings
Chromizing
Pack Cementation
CVD
220
How is "pack cementation" performed?
Al-powder + halide initiator + Al2O3, heated 700 - 1100 C. (inert conditions)
221
What happens if the substrate is Ni- or Co-based in Pack Cementation?
aluminides (25 – 75 mikrometer thick) are formed after diffusion in to the alloy.
222
Cons with Pack cementation
We cannot choose the composition of the aluminide coat --> may not be ideal
223
Describe an Overlay Coat
MCrAlY
where M = Ni or Co or Ni+Co
50-100 mikrometer
224
What's the advantages with Overkay coatings?
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
How can you make the Overlay coating?
PVD, thermal spray, plasma spray, electroplating
226
Describe plasma spraying (low pressure plasma spraying/vacuum plasma spraying)
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
How can you improve corrosion/oxidation resistance when using low pressure plasma spraying?
add hafnium, platinum, silicon into pre-alloyed powder
228
What is used for Thermal Barrier Coating (TBC)?
zirconia ZrO2, very low thermal conductivity compared to Ni or Fe. Acts as a thermal barrier.
229
What zirconia is used in TBC?
PYSZ (partially yttria stabilized zirconia)
