Superalloys - Disclocation Theory Flashcards
What is anomalous yield behaviour?
When a material gets stronger at higer temperatures. Superalloys possess this property.
What are dislocations?
Line defects that govern plastic deformation. They are heavily influenced by crystal stucture.
What are the most favourable slip planes for an FCC crystal structure?
The closed packed planes. [1,1,1] in the <1,1,0> direction.
What are partial dislocations?
Sometimes in the gamma phase it is energetically preferable to make two smaller dislocations rather than one large one.
What can partial dislocations generate?
Stacking faults as the partial dislocations arn’t lattice vectors.
Can dislocations in the gamma phase transition into gamma prime phase?
Yes due to the coherence however they must be ‘perfect’ and not partial to transition.
What is a superpartial dislocation?
As the gamma prime has a different structure to the gamma phase, when a perfect dislocation transitions it becomes a partial one. And becase the gamma prime is a superlattice the dislocations are called superpartial dislocations.
Do the superpartial dislocations create a stacking fault in gamma prime?
They do, but as the crystal structure is different, they also disrupt the chemical ordering of the structure. This has an energy penalty (increases strength) and is called an Anti-Phase boundary.
How can the gamma prime return to normal?
Another superpartial dislocation retuns the structure to normality.
How is cross slip enabled?
Temperature must be high enough to activate slip planes.
What is cross slip?
The ability for a dislocation to switch slip planes
What is a Kear-Wilsdorf lock?
As a dislocation attempts cross slip the gamma prime phase leaves behind a large anti-phase boundary which is very energetically unfavourable and so locks the disclocation in place.
What is the result of Kear-Wilsdorf locks?
It means that strength improves and explains the anomalous yield behaviour. These locks are ‘unlocked’ when new slip planes are activated at higher temperatures.
