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