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Materials Basics > External > Flashcards

External Flashcards

1
Q

Why does FCC do not have a DBTT?

A
  • edge and screw dislocations relatively athermal
  • number of active slip systems homogeneous
  • more than 5 independent slip systems
  • allow to accommodate arbitrary plastic strain
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2
Q

Why does BCC experience a DBTT?

A
  • 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.
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3
Q

What needs to happen to accommodate plastic deformation in BCC and FCC?

A

DISLOCATION MOBILITY

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4
Q

Are dislocations mobile at room temperature for BCC? What about FCC?

A

Mobile in FCC but not in BCC

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5
Q

Is the stress required to move dislocations temperature dependent in FCC? BCC?

A

Not temperature dependent in FCC but yes in BCC

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6
Q

In what scenarios does FCC experience brittle failiure?

A

final stage of fatigue failure

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7
Q

wHAT DOES THE PRESENCE OF DBTT imply in terms of defromation modes?

A

It implies there are insufficient (ductile) deformation modes at low temperatures to support plastic deformation

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8
Q

Why does fracture occur?

A

to release energy/load that is not passible to unload in plastic deformation due to the lack of deformation modes at low temperatures.

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9
Q

What is the root cause of the DBTT in the BCC materials?

A

The critical temperature for screw dislocation mobility is not present (the thermal contribution to the rearrangement of the BCC screw dislocation core structure)

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10
Q

Can BCC support plastic deformation?

A

Yes if at high enough temperatures for dislocations to be mobile

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11
Q

Why does FCC remain ductile at room temperature in terms of dislocations and slip systems?

A
  • Dislocations are mobile

- closest-packed planes for each slip system avaliable.

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12
Q

Why does BCC does not remain ductile at room temperature in terms of dislocations and slip systems?

A
  • 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

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13
Q

In FCC dislocation movememnt what atom moves?

A

The corner atom moves to center of the face

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14
Q

In BCC dislocation what atom move?

A

The entire line of atoms need to jump from one potential energy valley to another

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15
Q

What is require to move dislocations in BCC?

A

High temperature

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16
Q

What does it mean to be at temperatures below the DBTT in terms of fracture toughness and stress?

A

That below this point the fracture toughness is exceeded before the stress is sufficient to induce dislocation motion resulting in brittle fracture

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17
Q

Why is nickel not used in BCC fast reactor alloys?

A

Because it aids in the DBTT shift along with copper impeding dislocation movement to the already thermally restricted dislocations.

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18
Q

How many slip systems are needed for a material to be ductile?

A

5 independent slip systems

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19
Q

How many slip systems does HPC have?

A

one plane and three direction = 3 * 1 = 3

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20
Q

How many slip systems does BCC have?

A

48

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21
Q

If BCC has 48 slip systems, then why is it so brittle?

A

Because the slip systems interfere or usually obstruct each other. or because at low temperatures not enough of them are thermally activated.

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22
Q

What is one main component difference between ht9 and 316ss for rpv?

A

The lack of nickel and the addition of BCC stabilizers

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23
Q

What type of structure is ht9?

A

martensitic ss

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24
Q

What are some of the benefits of martensitic ht9?

A
  • higher thermal conductivity
  • low thermal expansion coefficient
  • high temperature strength
  • very low void swelling rate
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25
Q

What is the main difference between martensitic ht9 and austenitic ss?

A

the very low void swelling rate under irradiation of ht9 comapre to austenitic ss

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26
Q

What are the biggest issues with HT9?

A
  • low temperature irradiation embrittlement especially after neutron irradiation
  • high temperature creep
  • rupture strength
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27
Q

What does the high temperature strength in martensitic steel rely on?

A

stability of heat treatment microstructures and secondary phases under neutron irradiations at operating temperature

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28
Q

Why is the high temperature strength property of ht9 change during irradiatino?

A

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

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29
Q

Why does the thermal conductivity decrease with temperature?

A

Because even though number of free electrons increases, the lattice vibrations also increase consequently obstructing the flow of free electrons through the medium.

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30
Q

Why does martensitic alloys form other more stable phases under irradiation

A

Due to irradiation enhanced diffusion and segregation

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31
Q

What is the dpa equation

A

dpa = timefluxdisplacement cross section

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32
Q

What is the displacement cross section equation

A

displacement cross section = scattering cross section * number of atomic displacements

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33
Q

Where does the hourglass shape come from in fuel pellet clad interaction?

A

Due to the combine effects of swelling in the fuel and cladding compression due to differential pressures at BOL.

34
Q

What are luders band?

A

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

35
Q

What elements are FCC stabilizers?

A

NiCoMn

36
Q

What elements are BCC stabilizers?

A

CrAlTiSMoV

37
Q

What is a dislocation

A

is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms

38
Q

What is the burgers vector?

A

the distance and direction of movement it causes to atoms

39
Q

Is cementite formed in martensite? Why?

A

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

40
Q

Why are dislocations created in the austenite to martensite tranformation

A

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.

41
Q

What happens if we quench eutectoid steel austenite to form martensite?

A

There will be a significant percentage of retained austenite.

42
Q

For non eutectoid steels with carbon content 0-0.6% what is the martensitic structure?

A

For this carbon content the martensite has the appearance of lath along with plate martensite

43
Q

What happens as the percentage of retained austenite increases in martensitic steels?

A

The strength decreases.

44
Q

What happens if the cooling rate is slower than the critical cooling rate for martensite formation?

A

Some pearlite will form at the grain boundaries until the martensitic start temperature is reached

45
Q

At what carbon content do we start getting retained austenite in martensitic formations?

A

Starting approximately at the eutectoid carbon concentration (0.6%)

46
Q

What is tempering

A

the destruction of martensitic structure by application of heat.

47
Q

How do we stop the degradation of tempering in martensitic alloys in high temperature applications?

A

Addition of elements that interfere with cementite nucleation including tungsten.

48
Q

What is a ttt curve?

A

a TIME TEMPERATURE transformation curve. It explains the formation of martensite, pearlite and bainite formation.

49
Q

Why is zirconium alloys used as cladding instead of stainless steel?

A
  • 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
50
Q

What two components caused the explosion in fukishima

A

hydrogen and air

51
Q

Where did hydrogen come from in the fukishima accident?

A

Through the oxidation of zirconium by steam.

52
Q

Explain oxidation of zirconium by steam in LOCA?

A
  • Zr rapidly reacts with steam at high temperatures
  • Oxidation of Zr results in release of hydrogen gas

Zr + 2H20 -> ZrO2 + 2H2

53
Q

What happen to the hydrogen produced in the LOCA in fukishima?

A

Hydrogen gas was vented into the reactor maintenance halls and resulting explosive mixture of hydrogen with air oxygen detonated.

54
Q

What is the melting point of sodium?

A

around 98C

55
Q

What is the boiling point of sodium?

A

around 880

56
Q

What is the operating temperature of the liquid metal cooled reactor?

A

around 500C

57
Q

What is the operating temperature for a MSR?

A

860C

58
Q

What is the operating temperature for a high temperature gas cooled reactor

A

950C

59
Q

What is the melting point of FLiBe?

A

460

60
Q

What is the boiling point of MSR?

A

1430

61
Q

What is the composition of HT-9

A

12%Cr and 1%Mo + Fe W Ni V C

62
Q

What are some advantages of HT-9?

A
  • good creep rupture strength
  • oxidation resistance
  • high thermal conductivity
  • low thermal expansion coefficient
  • good high temperature strength
  • low void swelling rate under irradiation
63
Q

What are disadvatanges of HT-9?

A
  • low temperature irradiation embrittlement
  • high temperature creep property
  • rupture strength
64
Q

Why does HT-9 present low temp irradiation embrittlement?

A

Because of the BCC structure, ferritic/martensitic steels undergo a transition from ductile to brittle fracture with decreasing temperature.

65
Q

Is irradiation embrittlement severe during all conditions as long as there is irradiation?

A

No, it is temperature dependent.

66
Q

Does HT-9 have good high temperatur strength?

A

Yes

67
Q

What are some issues relating high temperature strength of HT-9?

A

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.

68
Q

Since HT-9 looses its high temperature stregnth due to irradiation enhaned diffusion what can be done?

A

oxide dispersion strengthened high-Cr steels

69
Q

What are oxide dispersion strengthened alloys?

A

consist of a metal matrix with small oxide particles dispersed within it

70
Q

What is the inverse kirkendal effect?

A

Flow of point defect-solute complexes to sinks at different diffusion rates.

71
Q

What are the main topics to talk about when comparing materials?

A
  • Radiation induced segregation and precipitation

- void swelling

72
Q

In terms of radiation induced segregation, what is the level of segregation with temperature?

A
  • 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
73
Q

What is the result of RIS in austenitic steels?

A

To the Cr depletion and Ni enrichment causing enhanced corrosion and embrittlement

74
Q

Why do dislocations preferentially attract interstitials?

A

Because of the stronger elastic interaction. Relatively larger strain field surrounding a SIA than a vacancy.

75
Q

Why does sweeling happen in terms of point defects and dislocations.

A

SIA preferentially absorbed at dislocations meaning excess vacancies become supersaturated and void nucleation and growth take place

76
Q

At what temperatures do swelling occure?

A

0.3 - 0.5Tm

77
Q

What is the swelling rate for austenitic steels?

A

1%/dpa

78
Q

Is the swelling rate of austenitic steels dependent on irradiation temperature? how about ht-9?

A

No, and No

79
Q

Is HT-9 less or more swelling resistant relative to austenite?

A

More swelling resistant

80
Q

What is the swelling rate of ht9

A

0.1%/dpa

81
Q

What is the proposed mechanism for swelling resistance of BCC

A
  • 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

Materials Basics (11 decks)

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