External Flashcards
Why does FCC do not have a DBTT?
- edge and screw dislocations relatively athermal
- number of active slip systems homogeneous
- more than 5 independent slip systems
- allow to accommodate arbitrary plastic strain
Why does BCC experience a DBTT?
- screw dislocation motion is not athermal
- to allow plastic deformation the material under stress must have enough energy to thermaly activate screw dislocation movement
- if energy is missing then no plastic deformation and fracture happens.
What needs to happen to accommodate plastic deformation in BCC and FCC?
DISLOCATION MOBILITY
Are dislocations mobile at room temperature for BCC? What about FCC?
Mobile in FCC but not in BCC
Is the stress required to move dislocations temperature dependent in FCC? BCC?
Not temperature dependent in FCC but yes in BCC
In what scenarios does FCC experience brittle failiure?
final stage of fatigue failure
wHAT DOES THE PRESENCE OF DBTT imply in terms of defromation modes?
It implies there are insufficient (ductile) deformation modes at low temperatures to support plastic deformation
Why does fracture occur?
to release energy/load that is not passible to unload in plastic deformation due to the lack of deformation modes at low temperatures.
What is the root cause of the DBTT in the BCC materials?
The critical temperature for screw dislocation mobility is not present (the thermal contribution to the rearrangement of the BCC screw dislocation core structure)
Can BCC support plastic deformation?
Yes if at high enough temperatures for dislocations to be mobile
Why does FCC remain ductile at room temperature in terms of dislocations and slip systems?
- Dislocations are mobile
- closest-packed planes for each slip system avaliable.
Why does BCC does not remain ductile at room temperature in terms of dislocations and slip systems?
- dislocations are not mobile (thermally activated)
- many slip systems but dislocation happen only as a line of atom jumps from one potential energy valley to another
In FCC dislocation movememnt what atom moves?
The corner atom moves to center of the face
In BCC dislocation what atom move?
The entire line of atoms need to jump from one potential energy valley to another
What is require to move dislocations in BCC?
High temperature
What does it mean to be at temperatures below the DBTT in terms of fracture toughness and stress?
That below this point the fracture toughness is exceeded before the stress is sufficient to induce dislocation motion resulting in brittle fracture
Why is nickel not used in BCC fast reactor alloys?
Because it aids in the DBTT shift along with copper impeding dislocation movement to the already thermally restricted dislocations.
How many slip systems are needed for a material to be ductile?
5 independent slip systems
How many slip systems does HPC have?
one plane and three direction = 3 * 1 = 3
How many slip systems does BCC have?
48
If BCC has 48 slip systems, then why is it so brittle?
Because the slip systems interfere or usually obstruct each other. or because at low temperatures not enough of them are thermally activated.
What is one main component difference between ht9 and 316ss for rpv?
The lack of nickel and the addition of BCC stabilizers
What type of structure is ht9?
martensitic ss
What are some of the benefits of martensitic ht9?
- higher thermal conductivity
- low thermal expansion coefficient
- high temperature strength
- very low void swelling rate
What is the main difference between martensitic ht9 and austenitic ss?
the very low void swelling rate under irradiation of ht9 comapre to austenitic ss
What are the biggest issues with HT9?
- low temperature irradiation embrittlement especially after neutron irradiation
- high temperature creep
- rupture strength
What does the high temperature strength in martensitic steel rely on?
stability of heat treatment microstructures and secondary phases under neutron irradiations at operating temperature
Why is the high temperature strength property of ht9 change during irradiatino?
Due to irradiation enhanced diffusion and segreation which are important factors for the formation and stability of new phases in martensitic alloys that can cause degradation in this property
Why does the thermal conductivity decrease with temperature?
Because even though number of free electrons increases, the lattice vibrations also increase consequently obstructing the flow of free electrons through the medium.
Why does martensitic alloys form other more stable phases under irradiation
Due to irradiation enhanced diffusion and segregation
What is the dpa equation
dpa = timefluxdisplacement cross section
What is the displacement cross section equation
displacement cross section = scattering cross section * number of atomic displacements
Where does the hourglass shape come from in fuel pellet clad interaction?
Due to the combine effects of swelling in the fuel and cladding compression due to differential pressures at BOL.
What are luders band?
also known as slip bands or stretcher-strain marks, are localized bands of plastic deformation in metals experiencing tensile stresses, common to low-carbon steels and certain Al-Mg alloys
What elements are FCC stabilizers?
NiCoMn
What elements are BCC stabilizers?
CrAlTiSMoV
What is a dislocation
is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms
What is the burgers vector?
the distance and direction of movement it causes to atoms
Is cementite formed in martensite? Why?
The material is cooled at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite
Why are dislocations created in the austenite to martensite tranformation
Austenite is gamma-phase iron (γ-Fe), a solid solution of iron and alloying elements. As a result of the quenching, the face-centered cubic austenite transforms to a highly strained body-centered tetragonal form called martensite that is supersaturated with carbon. The shear deformations that result produce a large number of dislocations, which is a primary strengthening mechanism of steels.
What happens if we quench eutectoid steel austenite to form martensite?
There will be a significant percentage of retained austenite.
For non eutectoid steels with carbon content 0-0.6% what is the martensitic structure?
For this carbon content the martensite has the appearance of lath along with plate martensite
What happens as the percentage of retained austenite increases in martensitic steels?
The strength decreases.
What happens if the cooling rate is slower than the critical cooling rate for martensite formation?
Some pearlite will form at the grain boundaries until the martensitic start temperature is reached
At what carbon content do we start getting retained austenite in martensitic formations?
Starting approximately at the eutectoid carbon concentration (0.6%)
What is tempering
the destruction of martensitic structure by application of heat.
How do we stop the degradation of tempering in martensitic alloys in high temperature applications?
Addition of elements that interfere with cementite nucleation including tungsten.
What is a ttt curve?
a TIME TEMPERATURE transformation curve. It explains the formation of martensite, pearlite and bainite formation.
Why is zirconium alloys used as cladding instead of stainless steel?
- Corrosion resistance along with good mechanical properties
- very low thermal neutron cross section.
- 30% more thermal conductivity that SS alloys
- 1/3 linear expansion coefficient that of SS meaning dimension stability at elevated temperatures
What two components caused the explosion in fukishima
hydrogen and air
Where did hydrogen come from in the fukishima accident?
Through the oxidation of zirconium by steam.
Explain oxidation of zirconium by steam in LOCA?
- Zr rapidly reacts with steam at high temperatures
- Oxidation of Zr results in release of hydrogen gas
Zr + 2H20 -> ZrO2 + 2H2
What happen to the hydrogen produced in the LOCA in fukishima?
Hydrogen gas was vented into the reactor maintenance halls and resulting explosive mixture of hydrogen with air oxygen detonated.
What is the melting point of sodium?
around 98C
What is the boiling point of sodium?
around 880
What is the operating temperature of the liquid metal cooled reactor?
around 500C
What is the operating temperature for a MSR?
860C
What is the operating temperature for a high temperature gas cooled reactor
950C
What is the melting point of FLiBe?
460
What is the boiling point of MSR?
1430
What is the composition of HT-9
12%Cr and 1%Mo + Fe W Ni V C
What are some advantages of HT-9?
- good creep rupture strength
- oxidation resistance
- high thermal conductivity
- low thermal expansion coefficient
- good high temperature strength
- low void swelling rate under irradiation
What are disadvatanges of HT-9?
- low temperature irradiation embrittlement
- high temperature creep property
- rupture strength
Why does HT-9 present low temp irradiation embrittlement?
Because of the BCC structure, ferritic/martensitic steels undergo a transition from ductile to brittle fracture with decreasing temperature.
Is irradiation embrittlement severe during all conditions as long as there is irradiation?
No, it is temperature dependent.
Does HT-9 have good high temperatur strength?
Yes
What are some issues relating high temperature strength of HT-9?
That this strength relies on the ferritic/martensitic stability of heat treatment microstructures and secondary phases.
As sson as irradiation begins irradiation-enhanced diffusion and segregation are important factors that steer the alloy to stable phases meaning degradation of high temperature performance.
Since HT-9 looses its high temperature stregnth due to irradiation enhaned diffusion what can be done?
oxide dispersion strengthened high-Cr steels
What are oxide dispersion strengthened alloys?
consist of a metal matrix with small oxide particles dispersed within it
What is the inverse kirkendal effect?
Flow of point defect-solute complexes to sinks at different diffusion rates.
What are the main topics to talk about when comparing materials?
- Radiation induced segregation and precipitation
- void swelling
In terms of radiation induced segregation, what is the level of segregation with temperature?
- low temp irradiation = low vacancy mobility = high recombination rate
- high temp irradiation = high recombination rate due to enhanced back diffusion of solutes
- intermediate temps defect recombination rates are minimum
What is the result of RIS in austenitic steels?
To the Cr depletion and Ni enrichment causing enhanced corrosion and embrittlement
Why do dislocations preferentially attract interstitials?
Because of the stronger elastic interaction. Relatively larger strain field surrounding a SIA than a vacancy.
Why does sweeling happen in terms of point defects and dislocations.
SIA preferentially absorbed at dislocations meaning excess vacancies become supersaturated and void nucleation and growth take place
At what temperatures do swelling occure?
0.3 - 0.5Tm
What is the swelling rate for austenitic steels?
1%/dpa
Is the swelling rate of austenitic steels dependent on irradiation temperature? how about ht-9?
No, and No
Is HT-9 less or more swelling resistant relative to austenite?
More swelling resistant
What is the swelling rate of ht9
0.1%/dpa
What is the proposed mechanism for swelling resistance of BCC
- Solute trapping due to weak interactions between Cr and vacancies
- character of dislocation loop structure
- lower dislocation bias related to the BCC structure
- extensive subgrain and lath boundaries in temperated martensitic microstructure
- high density of second phase precipitates
