程序代写案例-MATS43202/64502

Student Name______________________ University ID_________________ MATS43202/64502
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2020 SEMESTER 2 FORMATIVE ASSESSMENT
br>UNIVERSITY OF MANCHESTER
MSc, MEng DEGREES, POSTGRADUATE DIPLOMA
SUPERALLOYS & HIGH PERFORMANCE MATERIALS

Student Name______________________________ University ID_________________

Please check to confirm that you have:
- answered ALL THREE questions in SECTION A: ☐ YES ☐ NO
- answered ONE question in SECTION B: ☐ Q4 ☐ Q5 ☐ Q6


Submit your Assignment as a SINGLE PDF FILE via Blackboard before 16:00 on 28 May,
2020.

Student Name______________________ University ID_________________ MATS43202/64502
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SECTION A
Answer ALL questions from this section
1.
(a) The thermal expansion of -Titanium is highly anisotropic at a single crystal level
with the thermal expansion along the axis being half of the thermal expansion
perpendicular to the axis.
(i) Considering such anisotropy in thermal expansion, what type of residual
stresses are observed when cooling CP-Ti with a random texture from 700C
to room temperature.
Answer:
- At high temperature the grain will be stress free.
- The axis perpendicular to the axis is the
axis.
- On average, and because the texture is random, during cooling the bulk material will shrink
according to an average value of the two thermal expansion values.
- This will mean that the
axis of a single crystal is constrained by the bulk material and
cannot shrink as much as it would like to according to the thermal expansion, whereas the
axis is constrained and will be forced to shrink more than it would like to.
- Therefore, the residual stresses in the material will stress the crystals in compression along
the axis and in tension along the
axis.

(ii) Considering the elastic anisotropy of the crystal structure of -Titanium, how
might the stresses discussed in (i) might affect the stress state at a grain level
when the material is loaded to 70% of its yield point.
Answer:
Note, there was an error in this exam question, the above sentence should state that this is
tensile loading.

- Remember, the axis is elastically (and plastically) stiffer than the
axis.
- All grains undergo the same level of elastic strain during mechanical loading.
- But, grains with the -axis aligned with the loading direction are stiffer than the grains
orientated with the axis perpendicular to the loading direction.
- Hence, the grains with the -axis aligned with the loading direction will be stressed the
highest.
- The thermal stresses described in (i) will add to this effect, as in principle the tensile stresses
in those grains are even higher during cooling.

(b) Wrought two-phase Titanium alloys, such as Ti-6Al-4V, are often solution heat
treated in the high two-phase region. Describe the microstructure evolution when
the material is cooled from such temperature with 1C/min, 100C/min and
8000C/min.
Student Name______________________ University ID_________________ MATS43202/64502
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Answer:
- At the solution temperature there is large proportion of  phase and some primary  grains.
- 1C/min: As the material is cooled down slowly, there is sufficient time for  stabilisers (Al)
left in  phase to diffuse to the primary  grains and consequently they grow until  phase is
only found in the triple points between the primary  grains.
- 100C/min: as the cooling rate is now too fast to simply grow primary , the only way for 
stabilisers to leave the  phase is by nucleation and then epitaxial growth of secondary 
lamellae within the  phase. Consequently, primary  only grows slightly during cooling. The
result is a bi-modal (duplex) microstructure.
- 8000C/min: such fast cooling rates will result in no primary  growth and the  phase will
transform martensitically. The result is a bi-modal (duplex) microstructure. In contrast to the
microstructure generated at 100C/min, here the lath structure formed in the former  phase is
very fine due to the diffusionless martensitic phase transformation.
- A sketch might be helpful but not essential.
2. Titanium Aluminides have long been considered a material that could replace to some
extend Titanium alloys and Nickel-base Superalloys in aeroengines.
(a) Discuss advantages and disadvantages of Titanium Aluminides compared to
Nickel-base Superalloys and Titanium alloys.
Answer:
- Significantly lighter than Ti-alloys and dramatically lighter than Nickel-base Superalloys
- Burn-resistant (compared to Ti-alloys)
- TiAl have temperature capability exceeding Ti-alloys and comparable with some Nickel-base
Superalloys
- TiAl intermetallics have been developed for more than 50 years now and their application is
still extremely limited because they are very brittle at room temperature.
(b) What are the stoichiometries of 2 and  Titanium Aluminides?
Answer:
Ti3Al and TiAl

(c) Discuss the high temperature oxidation resistance of Titanium alloys together with
2 and  Titanium Aluminides.
Answer:
- The high temperature oxidation resistance increases with increasing Aluminium content,
hence  is more oxidation resistant than 2, which is more resistant than Ti-alloys.
- Oxygen solubility also decreases dramatically with increasing Al content.
- The increased high temperature oxidation resistance is explained by having increasing
amount of Al2O3 in the oxide with increasing Aluminium content in the material.

Student Name______________________ University ID_________________ MATS43202/64502
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(d) Discuss deformation by prismatic and basal slip in 2 Titanium Aluminides and
the consequences for the ductility of the material.
Answer:
- 1/3 <11-20> slip preferred option
- Slip along basal slip creates disorder
- There is easy (creates no wrong nearest neighbour) and hard prismatic slip (creates two
wrong nearest neighbours)
- As a result of this, very few slip systems are available and the material is brittle at room
temperature.
3. Low density, high hardness, thermal stability and shock resistance make silicon nitride
based ceramics attractive for a wide range of applications, including components of
diesel engines, bearings, jet thrusters and cutting tools.
(a) Among alpha, betta and gamma Si3N4 phases, which one requires high pressure to
be synthesised and which synthesis method should be used in this case?
Answer:
- Gamma Si3N4, which attains a cubic spinel structure, requires above ~13 GPa for the
synthesis
- Such high pressure can be achieved only using a multi-anvil press.

(b) The micrograph below shows a microstructure of polycrystalline Si3N4 hot-
pressed at 1650 oC in presence of MgO addition:

Figure. A microstructure of polycrystalline Si3N4 hot-pressed at 1650
oC in
presence of MgO addition
Student Name______________________ University ID_________________ MATS43202/64502
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(i) Identify the main phase constituents of the ceramic microstructure.
Answer:
Dark elongated grains – β-Si3N4.
The bright continuous grain boundary phase – amorphous or partly crystallized Mg silicate
and/or a silicate-rich oxynitride phase.
(ii) Discuss the microstructure formation during hot pressing and the role of MgO
addition in it.
Answer:
Hot-pressing starts with α-Si3N4 powder and MgO as a densification aid.
Under pressures of >14 MPa and temperatures of 1650° to 1750°C, some of α-Si3N4 particles
reacts with the MgO additive producing a thin layer of SiO2. Further reaction produces a
liquid silicate in which the remaining α-Si3N4 is dissolved.
Next, the elongated grains β-Si3N4 re-precipitate from the silicate. On completion of the α 
β transformation, the elongated β grains are surrounded with residual silicate oxynitride grain
boundary phase.
(c) Discuss the effects of the microstructural and phase transformations during hot
pressing on the mechanical behaviour of silicon nitride:
(i) Explain how and why strength and hardness are affected.
Answer:
The resulting β-Si3N4 is softer than the initial α phase.
However the liquid phase densification reduces porosity and the silicate oxynitride phase
increases grain boundary strength. So both hardness and strength will increase.
(ii) Which toughening mechanisms are expected to prevail in such structure?
Answer:
Since the β-Si3N4 grains are elongated, with typical size of 0.5 by 4 um and aspect ratio of
R=4-5, they would deflect cracks in the process zone and act as fibre reinforcement in the
wake/bridging zone.
(iii) Which microstructural constituent limits the high-temperature strength of the
material?
Answer:
Hot-pressed silicon nitride ceramics exhibit room temperature flexural strengths of 690 MPa
or higher and retain these strengths to at least 1000° to 1200°C.
Further increase in operating temperature is limited by the reduced strength and chemical
stability of the grain boundary silicate phase.
(d) Identify TWO main factors that limit a wider use of silicon nitride ceramics.
Answer:
High manufacturing and processing costs
Difficult to form into and machine complex shapes
Student Name______________________ University ID_________________ MATS43202/64502
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SECTION B
Answer ONE question from this section
4.
(a) Precipitation hardened nickel-base superalloys provide high temperature
capabilities typically required in gas-turbine engines. Nickel-base superalloys are
composed of a large number of alloying elements with each of them serving a
particular purpose.
(i) Name two alloying elements that promote  or  formation
Answer:
possible answers: Al, Ti, Ta

(ii) Name one alloying element that promotes stable oxide scales.
Answer:
possible answers: Al, Cr

(b) Describe particle-strengthening mechanisms operating in  strengthened nickel-
base superalloys.
Answer:
- Needs to first mention that  is an ordered phase that is coherent with the  matrix.
- Deformation happens by  shearing of weakly or strongly coupled dislocation or Orowan
looping and each mechanism has to be clearly described.
- Shearing takes place on the {111} in the <110> direction.
- Requires mentioning of AFB and how it affects weakly and strongly coupled dislocations

(c) Draw a schematic that describes the role of precipitation size on strengthening
mechanisms in a precipitation-hardened nickel-base superalloy.
Answer:
Student Name______________________ University ID_________________ MATS43202/64502
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- Labelling each axis correctly gives
- Correct shape of curves
- Labelling of each section correctly
(d) After solution heat treatment followed by a cooling rate to room temperature of
about 10C/min,  strengthened nickel-base superalloys typically exhibit a bi-
modal intragranular  size distribution. Explain the bi-modal  formation and its
consequences regarding  chemistry.
Answer:
- Due to the relatively slow cooling rate and the high propensity of  to form during early
stages of cooling,  starts forming as soon as the material has been cooled below the -
solvus.
- The lowest energy sites for precipitation of  are at the grain boundaries of the  (FCC)
matrix, which form primary  precipitates.
- As cooling proceeds, the initial  continues to grow by diffusion of  stabilising elements to
the existing  particles.
- With decreasing temperature the solubility limit of the  stabilising elements decreases. As
diffusion rates decrease with cooling at some stage it becomes energetically more favourable
for the  stabilising elements to form new secondary  particles rather than trying to reach
existing primary  particles. Hence a second family of  forms.
- As  forms during cooling, i.e. over a range of temperatures, the chemistry of  is size
dependent.

5. Zirconium alloys are widely used for fuel cladding and fuel spacer grids in almost all
water-cooled reactors.
(a) Corrosion of zirconium alloys in an aqueous environment is principally related to
the oxidation of the zirconium by the oxygen in the coolant.
(i) Explain the growth direction of the oxide layer on the Zr metal surface.
Answer:
- Because diffusion of Zr ions trough ZrO2 is slower than oxygen ions the diffusion rate is
inwards.
Student Name______________________ University ID_________________ MATS43202/64502
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(ii) Describe schematically the corrosion kinetics observed during autoclave
testing of Zr alloys.
Answer:


1 mark for x-axis properly labelled, 1 mark for y-axis properly labelled, Pre-transition, 1st
transition, post-transition, breakaway corrosion

(iii) Discuss proposed mechanisms leading to transition from pre- to post-
transition corrosion kinetics in Zr alloys.
Answer:
- The cyclic corrosion kinetics typically observed are accompanied by oxide layers appearing
and composed of successive layer of columnar and equiaxed grains (of few m thickness).
- Columnar grains have superior performance compared to equiaxed grains. Columnar grains
slow corrosion rates up to pre-transition, whereas equiaxed grains nucleate post-transition.
- At transition, destabilisation of the oxide layer could be caused by:
- Cracking of the oxide as a result of the accumulation of compressive stresses in the oxide
from imperfect accommodation of the volume expansion attendant upon oxide formation.
- Cracking of the oxide as a result of the transformation of initially tetragonal ZrO2 to the
monoclinic modification which results in volume expansion. This transformation can be
caused by reduction in the compressive stress, or large tetragonal grain size (stabilised by
higher Sn content).
- Cracking of the oxide as a result of the oxidation of intermetallic precipitates, initially
incorporated in metallic form, resulting in volume expansion.
- The porosity formed in the oxide reaches a percolation condition, leading to easy access of
the coolant to the underplaying metal.
(b) An important side effect of the Zr corrosion process is that the metal picks up
hydrogen. Absorption of hydrogen is a major contributor to degradation of
zirconium alloys during service in nuclear reactor systems. In this context, discuss
hydride formation and why hydrides mechanically degrade the clad.
Answer:
Student Name______________________ University ID_________________ MATS43202/64502
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- Once the hydrogen level has exceeded the terminal solid solubility, hydrides start forming in
Zr alloys. Since the solubility level of hydrogen in Zr at typical operation temperatures is only
around 100 ppm, this will happen even when the oxide has not grown very substantially.
- Hydrides precipitate on basal plane of hcp lattice. The formation of hydride platelets is
associated with a volume expansion (so most parts of the hydrides are in compression, but can
form tensile stress concentrations at the ends, which lead to material weakness).
- Hydrides are considered to be brittle and therefore reduce material ductility and fracture
toughness.
- Hydrogen can diffuse to cold spots causing clusters of hydride lenses and hydrides can
diffuse to regions of tensile stress, causing delayed hydride cracking (DHC).
- Hydrides are particularly detrimental if they are arranged radially as they then present an
easy pass for a through thickness crack to grow.
6. Deposits of calcium-magnesium alumino-silicates (CMAS) are detrimental to the
performance of turbine blades in jet engines.
(a) By answering the following questions, discuss how the thermal barrier coatings
protect gas turbine blades in general:
(i) Provide a schematic diagram explaining the architecture of a typical thermal
barrier coating applied to gas turbine blades. Show characteristic length scales
of the main coating constituents.
(ii) Show typical temperature distribution across the coating.
(iii) Discuss functions of and critical materials requirements to the main coating
constituents.
Answer:

Correct answers should contain diagram showing 3 layers; correct length scales;
temperature profile with correct slopes; functions of the three layers; materials requirements
to each layer.
(b) Explain the thermomechanical mechanism of environmental attack on the thermal
barrier coating due to CMAS deposits.
Student Name______________________ University ID_________________ MATS43202/64502
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Answer:
(a sketch might be helpful but not essential)
In terms of thermomechanical damage, the role of CMAS consists primarily in stiffening of
the top part of t’- YSZ coat leading to shallow or deep delamination.
Further infiltration into the porous structure of YSZ thermal barrier layer may cause chemical
attack to and damage of the TGO layer
Finally, the damage of TGO layer may cause outward diffusion of elements from the
bondcoat and aggravation of intrinsic forms of degradation (e.g. rumpling and formation of
cavities)
(c) Identify and discuss the main approaches that are currently pursued to mitigate the
CMAS-mediated degradation of thermal barrier coatings.
Answer:
The mainstream approaches are based on manipulating the chemical reaction between the
TBC and the melt to accomplish two goals simultaneously: (1) Immobilize the melt by
capturing its main constituents into crystalline phases, e.g. silicates (CaAl2Si2O8) or RE
zirconates (Gd2Zr2O7).
(2) Generate sufficient volume of precipitated products to fill the pore spaces and block
access of any residual melt to the remaining TBC.
A separate approach consists in decreasing CMAS wettability by adjusting the topology of the
coating grains in the top layer, e.g. by creating cauliflower structures, which would prevent
penetrating the molten material in to the porous structure of TBC
END OF ASSESSMENT

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