Chapter 6 - Collision and Defects Flashcards
what is radiation damage?
localized disruption of the crystal lattice of a solid (defect production) by high energy radiation passing through it
what are radiation effects?
consequence of radiation damage of the mechanical and or phsyical properties of the solid
What is the energy range of the electrons bound to the nucleus?
keV to eV
What are the three main energetic particle types?
- Neutral elementary particles
- Charged elementary particles
- high energy atoms or ions
Energetic charged particles interactions are characterized by what quantity?
scattering cross sections
how are interactions between charged particles and electrons in a solid medium?
characterized by the stopping power, or energy loss per unit path length.
At what energyes is electronic stopping dominat?
high energies
What happens to the electrons when a charged particle interacts with them?
they are excited and removed from the nucleus leading to ionization of the medium along ion track. These electrons are thermalized and re=captured by nuclei
What does electros being thermalized mean?
ionization energy is degraded into heat
at what energies are nuclear interactions between charged particles and nuclei in a solid medium?
low energies
what two possible pairs do nulcear interactions (scattering) invovle?
- bare nuclei at high particle energy
- nuclei and bound electrons at low energies
What do nuclear interactions produce in solid medium?
permanent atomic displacements within the solid
What do atomic displacements cause?
physical and mechanical property changes - radiation damage and radioation effects
In hard sphere collisions what values of energy transferred are more probable?
all of thema re equally probable
What is the average energy transfered to the struck atom by a projectile?
Tmax/2
What is the average energy of an incoming neutron?
2 MeV
What does energy imparted to struck atoms by a projectile cause?
displacements from the lattice sites in the crystal of the solid (frenkely pairs) -> radiation damage
For a 1MeV incoming neutron to iron m=56 the max possible transferred energy is ? What is the average?
0.07 MeV= 70 keV = 70000eV and the average is 0.035 MeV = 35 keV = 35000 eV
What is the common name of the first sturck atom?
primary knockon atom (PKA)
What is the displacement energy?
it is the energy needed to displace a lattice atom from its site in the crystal leaving a vacant lattice site
What is the average displacement energy?
25 eV
What can the PKA cause?
it stirkes another lattice atom since M=m (atom displaced collides with same type of atoms)
What is the enrgy transfered from the PKA to the secondary knockon atom?
1/2 PKA energy (17.5 keV)
What is the average energy of the recoils of the nth generation?
one half of that of the previous generations
Until when does the knockon atoms generation continues as a chain reaction
until the energy of the nth knockon atom is less than the displacement energy (25 eV)
When should electronic energy loss (bethe-bohr) be considered? What does this mean for displcaments/?
When the energy transfered to the struck atom is higher than the energy of the projectile particle. This means no atomic displacements.
When should electronic energy loss (bethe-bohr) be not considered? What does this mean for displcaments/?
When the energy transfered to the struck atom is LESS than the energy of the projectile particle. Displacements happen producing vacancies and SIA (nculear stopping).
What big particles affect fuel uo2?
fission fragments
what particles affect cladding and sturcture?
neutrons
What particles affect structures other than neutrons?
photons (damage by compton scattering
What does the displacement energy depend on?
On the crystal structure of atoms in a solid and the PKA direction.
What does the displacement energy represents?
The average energy needed for displacements overa ll possible directions.
is the pka energy small or large comapred to the displacement energy? what does this mean for number of displacements or V-I pairs?
very large meaning that a neutron generates many tens and even hundreds of VI pairs
What two parts make an atom?
the nucleus and the electrons bound to the nucleus
What particle does the neutron interact with?
with the atomic nuclei notthe electrons
Do gamma rays interact with electrons?
Yes, pair production, compton scattering, and photo-electric effect
What is pair production?
is the process that results in the conversion of a photon into an electron–positron pair. Since photon has no rest mass, we can say that this process converts energy into mass according to Einstein’s mass energy relation E=mc2.
gamma ray -> neutron -> electron and positron ejected
What is compton scattering?
is the scattering of a photon by a charged particle, usually an electron. If it results in a decrease in energy (increase in wavelength) of the photon (which may be an X-ray or gamma ray photon), it is called the Compton effect.
photon -> electron -> recoil electron and scattered photon (less energy)
What is the photoelectric effect?
phenomenon in which electrically charged particles are released from or within a material when it absorbs electromagnetic radiation (photon). The effect is often defined as the ejection of electrons from a metal plate when light falls on it
name the two main charged elementary particles?
protons and electrons
Name exaples of high energy atoms or ions
fission fragments, accelerator produced heavy ions, recoil atoms from primary collisions.
What interactions are characterized by the stopping power?
interactions between charged particles and electrons
What is the stopping power?
the energy loss per unit path length, is the retarding force acting on charged particles, typically alpha and beta particles, due to interaction with matter, resulting in loss of particle energy. Its application is important in areas such as radiation protection, ion implantation and nuclear medicine.
What is electronic stopping?
refers to the slowing down of a projectile ion due to the inelastic collisions between bound electrons in the medium and the ion moving through it.
What is inelastic collision?
The term inelastic is used to signify that energy is lost during the process (the collisions may result both in excitations of bound electrons of the medium, and in excitations of the electron cloud of the ion as well)
At what energy is electronic stopping dominant?
high energies
Does electronic stopping excite the medium?
Yes, the electrons in themedium are excited (energized) and removed from the nucleus leading to ionization of the medium along ion-track
What happens to excited electrons that left the nucleus due to electronic stopping?
Most are thermalized and recaptured by nuclei. The energy is lost in the form of heat.
At what energy do nuclear interactions predominate with?
low energies
What do nuclear interactions produce in terms of the atoms in the solid? What is the end product?
permanent atomic displacements which cause physicals and mechanical property changes (radiation damage and radiation effects).
What are the main 4 mechanisms of creep
The general creep mechanism are (i) dislocation slip; (ii) climb; (iii) grain-boundary sliding; and (iv) diffusion flow caused by vacancies.
What is the von mises stress?
The von Mises stress is used to predict yielding of materials under complex loading from the results of uniaxial tensile.
What is DBTT?
The temperature at which there is a pronounced decrease in a material’s ability to absorb force without fracturing. At this point, a material transitions from ductile to brittle. Also known as DBTT.
What is the DBTT dependent on? What is the DBTT for steel?
The ductile to brittle transition temperature is strongly dependant on the composition of the metal. Steel is the most commonly used metal that shows this behaviour.
For some steels the transition temperature can be around 0°C, and in winter the temperature in some parts of the world can be below this. As a result, some steel structures are very likely to fail in winter.
What is the displacement per atom?
is the number of times that an atom is displaced for a given fluence/flux. In other words it is the number of displacements in a given volume by a given flux.
Can neutrons, alphas, x rays and electrons all damage metals?
Yes, as long as the avaliable energy is above the displacement threshold energy
Can neutrons, alphas, x rays and electrons all damage metals?
Yes, as long as the avaliable energy is above the displacement threshold energy
What is the reaction rate a function of?
R = atom_density * scattering cross section * flux
What is the energy of the incoming projectile?
E = 1/2 * MV^2
What is the maximum energy transfered in head on collision?
T_max = [ (4mM)/(m+M)^2 ] * E
What is the average displacement energy?
25 eV
What is the maximum transfer energy of a 1 mev neutron to iron? The average? The energy transfereed to the second lattice atom strucked by PKA? The final number of displacements?
0.07 MeV, 0.035 MeV, 0.0175 MeV, 700 displacements
What is the displacement energy?
The energy needed to displace a lattice atom from its crystal site leaving behind a vacant lattice site.
What is the energy transferred from a PKA to the enxt atom? Why?
It is half of the PKA energy since M=m (same atom collision).
What is the average energy of the recoils of the nth generation? When does it stop?
one half of that of the previous generation until the recoil energy is less than the displacement energy threshold.
Is the creation of the PKA elastic or inelastic collision?
elastic collision
When should electronic energy loss be considered?
When the PKA’s energy T is higher than Ec (the minimum ion energy for transferring enough energy to an electron to remove it from the atom)
What is the displacement energy a function of?
the crystal structure of atoms in a solid, more specifically the direction (angle) at which the projectile strucks the stationary atom. (depends on the PKA direction)
What does the displacement energy really represent?
The energy needed to produce a frenkel pair
What is the average value of the displacement energy?
25 to 50 eV
When does a PKA cascade ceases?
When the average knockon energy is 2Ed = T/2^{NF}
What is the final number of displaced atoms given a PKA with an energy T
v = 2^{Nf} = T/2Ed
What needs to happen for energy transfer from moving atom to electron?
a head on collision of atom and electron must transfer a minimum energy equivalent to the binding energy of electrons in medium (I)
What is the energy of an incoming particle to generate only one pka and why?
Ed < T < 2Ed because two times threshold energy means after creation it will still have the energy to displace one more PKA
What does Ec stand for in terms of threshold energy?
minimum ion energy for transferring enough energy to an electron to remove it from the atom:
What is the displacement rate
the number of displacements per unit volumen second
DR = RR * v
What is the kinchin pease model?
A simple model for calculating the atomic displacements
What are the assumptions of the kinchin pease model?
- cascade is created by a sequence of two body elastic collision between atoms
- the energy Ed consumed in displacing an atom is neglected in the energy balance
- energy loss by electronic stopping is treated by a cutoff energy
- energy transfer cross section is given by a hard-sphere model
- the arrengement of atoms in a solid is random and effects due to the crystal strucutre are neglected
What is the minimum energy for removing electrons Ec in iron?
56 keV
What is the displacement cross section a function of?
the scattering (inelastic and elastic) cross section, the displacement energy threshold, the energy of the incoming neutron, the alpha factor.
What is the displacement cross section for a fast neutron at 1 MeV in Iron?
1245 barns (fission)
What is the dpa a function of?
dpa = flux * displacement cross section * time
Why is the displacement cross section formula not applicable at high neutron energies?
because inelastic scattering begins and anisotropic scattering dominantes. Transmutations occur.
Where do vacancies coalese into?
- small clusters making depleted zones in the form of stacking fault tetrahedra for fcc metals and cavities in bcc.
- large clusters (voids or cavities)
Where do interstitials coalese into?
dislocation loops
What is responsible for mechanical hardenign and embrittlement and or physical property changes in displacement context?
point defect clusters of vacancies and interstitials along with radiation induced/enhanced diffusion, precipitation and segregation
what is segregation?
segregation is the enrichment of atoms, ions, or molecules at a microscopic region in a materials system
What are the five recovery stages of SIA and vacancnies?
- collapse of close frenkel pairs, recombination and uncorrelated recombination
- interstitial cluster grow leading to identifiable small interstitial loops
- vacancies migrate and adulate at interstitial clusters
- the vacancy clusters sruviving stage 3 grow in size and the vacancy clusters dissociate thermally.
What is the displacement spike?
• Because the cross section for atomic collisions becomes large at the low energy end of the ion track, displacements are created in a compact region called a displacement spike
Why is the energy transfer cross section for ions much larger than for neutrons?
Because ions deal with an atomic cross section (10^-17 cm2) while neutrons is a nuclear cross section (10^-24)
In a thin layer, does dpa distribution is uniform or varies?
For ions it varies with penetration depth while for neutrons it is uniform.
What is radiation damage?
microscopic defects produced in materials due to irradiation
What is radiation effects?
the changes in physical, mechanical, and chemical properties resulting from accumulated radiation damage
What are the three main radiation sources categories that can cause damage?
- neutral elementary particles
- charged elementary particles
3 high energy atoms or ions
Interactions with energetic charged particles in the material can be divided into two categories:
interactions with electrons, and interactions with nuclei.
Where do electron interactions dominate in terms of energy?
High energies
Where do nuclear interactions dominate in terms of energy?
low energies
How is ionization characterized?
by the stopping power, or energy loss per unit path length of the radiation.
What ar the most common effects of electron interactions?
Most of the energetic electrons are thermalized and re-captured, which leads to the energy being dissipated as heat.
What is the dipslacmenet energy?
The displacement energy of lattice atoms in a given material is the binding energy that keeps the atom in its lattice site:
What is the typical energy range for displacement energy?
10 to 60 eVW
What is displacement?
scattering events between them provide the atom with enough energy to get knocked out of its lattice site.
What is the primary knoc on atom
The atom that was knocked off of its lattice site
What is the maximum energy transfer from an incident projectile to a target? What is the average energy transfered?
Tmax = [(4mM)/(m+M)^2] * E
The average is just half of the maximum theoretical transfer
What is the maximum energy transfered by a 1 MeV neutron to an Fe 56 atom?
35 KeV
What is the displacement energy of iron
25 eV
Can a PKA displace more atoms?
Yes, if it has enough energy.
Can a neutron displace more atoms than the PKA?
Yes, it can create several more PKA since it has enough energy
Why are there no displaced atoms at very high energies? What are these energies?
Because at these energies the particle may interact with electrons first prior to displacing atoms.
The electron ionization energy is in the order of KeV
What is the kinchin pease model?
A simplified model describing the interactions between a PKA and a material
What are the assumptions of the kinchin pease model in terms of energy loss?
That all energy loss by the particle is lost by ionization and no displacements are produced above Energy Ec. Similarly, below Ec all energy loss is by dipslacement collisions.
What are the assumptions of the kinchin pease model?
- Cascade is created due to sequence of two-body elastic collisions which assume a hard sphwere model
- no quantum effects therefore no displacements until T > Ed
3 No energy passsed to the lattice during collision phase - no atomic displacements above Ec and no ionization below Ed
- effect of crystal structure is neglected and istherefore random
- no annhihilation of defects assumed.
What are depleted zones?
When an energetic particle displaces atoms from their lattice sites, the immediate result is vacant lattice sites which lead to zones devoid of atoms
How are voids formed? What is the effect on the overal material?
From displacments forming a big enough depelted zone.
The effect is swelling
How can voids be stabilizied?
If elements like B, Ni, and Fe are present, (n,alpha) reactions lead to the prdoction of Helium whih can stabilize the voids by forming gas filled caviites or bubbles.
Can voids be removed by annealing?
Depends, if voids are from depelted zones yes but if stabilized by He bubbles, then no.
Name some radiation defects induced by high energy neutrons?
- vacancies and SIA
- iimpurity atoms produced by transmutaiton
- thermal spikes due to impacted nuclei going into high energy states
- displacement spikes (regions with high density of displacements, vacancies, SIA, produced by PKAs and secondary knock on atoms)
- voids
- bubbles/cavities (void stabilized by gases)
How many of the vacancy-sia defects survive? Why/
1% or less due to recombination of interstitials with adjacent vacancies.
What is the dose in materials?
Measurement of radiation damage can be tabulated in a non-material specific manner using the unit dpa = displacements per atom.
What is the dpa? Explain.
displacements per atom
A dose of 5 dpa indicates that theoretically, every atom in the piece of material has been displaced 5 times.
What is the displacement rate damage a functio of ?
Rd = N * xs_disp * flux
What is dpa a functio of (calculated)?
dpa = displacement damage rate * time / number density
What is the displacement cross section a functio of?
xs_dpa = int( xs_n * v(t))dT
elastic cross section
sum of the number of atomic displacements (v(T)) produced by PKA with energies T from Ed to Tmax.
What is one major flaw tih KP model?
That even before reaching the cut off energy a major fraction of the energy will be lost into electronic excitation and ionization.
What are some general observation son radiation effects?
- defect concertation increase with fluence
- nuclear transmutation occurs
- chemical reactivity changes
- diffusion increases
- new phases,
- impurities are produced
Do interstitial atoms exists in isolation?
No
What is the effect of interstitials?
Distortions on the lattice and aggregation due to the high binding energy
What is the binding energy o f interstitials
1 eV
Why are solutes impurity traps of vacancies?
Because binding vacancies and impurity atoms results in alower overall free energy of the solid.
The binding energy of a vacancy and a solute can be equal to that of the interstitials (0.2 to 1 eV)
are interstitials clusters stable? vacancy clusters?
Interstitial clusters are stable and posses higher mobility than vacancy clusters which are often unstable.
Where are interstitial clusters mostly created? What is the fate of interstitial clusters?
clusters form at the casade and the migrate away from the cascade region and can be absrobed at various sinks such as dislocations and grain boundaries.
What are dislocation loops?
are a form of extended defect which eventually results from clustering of both vacancies and interstitials. Dislocation loops resulting from vacancy and interstitial condensations are created from clusters of the respective defects, and either shrink or grow depending on the flux of defects reaching the embryo. Once they have reached a critical size, the loops become stable and grow until they unfaulty by interactions with other loops or with the network dislocations.
What comes fierst? voids or dislocation loops? Why?
voids because the void is the stable form for small clusters of vacancies. However, as the number of vacancies in the cluster grows, the loop becomes the energetically favorable configuration.
Why do you often see a large number of voids and not dislocation loops as expected by thermodynamics?
the collapse of a void embryo into a vacancy loop is NOT favored by the presence of inert gas, such as He, in the void, and this is why voids can survive and grow.
Why do we observe a higher number of vacancies than interstitials?
dislocations preferentially absorb interstitials because they interact more strongly with interstitials than vacancies due to the larger lattice distortion caused by interstitials. This leaves the lattice depleted of interstitials and rich in vacancies. Therefore, voids swelling does occur.
In what four scenarios is void swelling more significant?
- intermediate temperatures (500C)
- if less cold woring is present
- in materials of larger grain size
- in materials with fewer precipitates
Why is void swelling more significant in intermediate temperatures?
because at low temperatures defects are not mobile enough and at too high of temperatures, the vacancy concentration reaches a value closer to the thermal equilibrium value, so there is no longer this driving force that would lead to void growth.
Why is void swelling more significant if less cold working is present?
Cold working increases the dislocation density of a material, which restricts the nucleation and growth of voids.
Why is void swelling more significant in materials of larger grain size?
Smaller grain size means more grain boundaries, which act as sinks for point defects, so they do not go on to form clusters and extended defects as readily
Why is void swelling more significant in materials with fewer precipitates
Precipitates act as recombination sites for vacancies and interstitials, thus reducing swelling.
Why does hardness and strenghth increease in irradiation?
Radiation harening is caused by increased number of defects: including point defects, impurity atoms, small defect clusters, dislocation loops, dislocation lines, cavities (voids/bubbles), and precipitates
When does radiation hardening start in terms of melting temperature and dpa?
Hardening starts to appear at 𝑇<0.4𝑇𝑚 (where in situ recovery effect is less), and at >0.1 dpa.
What does irradiation hardening do to elongation?
Elongation percentage is reduced (more brittle)
What is helium embrittlement? Where is helium produced?
Helium gas is produced through transmutation of the component elements.
The gas is insoluble in metals and thus tends to precipitate into bubbles. This can produce sever embrittlement and intergranular cracking
What is the difference between helium and defect produced embrittlement?
that helium embrittlement cannot be healed by annealing.
What happens with fracture toughness as fluence increases?
it decreases
What is the reason creep appears at lower temperatures that thermal creep would usually kick in?
Materials under stress can experience creep under irradiation from energetic particles at much lower temperatures than where thermal creep usually requires to kick in normally due to the generation of vacancies enhancing the thermal creep rate
What are the 6 main mechanical property effects of irradiation?
- radiation hardening
- radiation embrittlement
- Helium embrittlement
- fracture toughness decreases
- irradiation enhanced creep
6 low cycle fatigue life decrease and high cycle fatigue life increase
What are the 4 main physicals properties changed due to irradiation?
- Density decreases (radiation swelling)
- Electrical resistivity increases (conductivity decreases)
- magnetic susceptibility decreases
- thermal conductivity decreases due to the increase defect concentration.
What are the 2 main chemical properties changed due to irradiation?
- Enhanced corrosion
2. Irradiation assisted stress corrosion cracking
Why is corrosion enhnaced in irradiated environments?
One reason is the radiolytic dissociation of environment including water. The presence of more radicals accelerates corrosion effects.
What are the 5 main causes of IASCC?
- irradiation=induced depletion of chromium at grain boundaries
- radiation hardening
- localized deformation
- selective internal oxidation
- irradiation creep
