Metals- Dislocation Reactions, Cross Slip and Climb

Question 1

Where is compression and tension relative to a slip plane?

Answer 1

Extra half plane of atoms above the slip plane pits region in compression. The region below the slip plane goes into tension

Question 2

How do the stress contributions from dislocations combine?

Answer 2

The resulting stress at any point is the sum of the stress contributions from each dislocation.

Question 3

When are dislocations attracted or repelled from one another?

Answer 3

Two dislocations of the same sign on the same slip plane will repel one another.
Two dislocations of opposite sign on the same slip plane will attract one another and in some cases annihilate.
Dislocations of the same sign moving on different slip planes may be attracted to one another to form arrays

Question 4

How does dislocation recombination work?

Answer 4

They combine into a single dislocation. Two dislocations with b1 and b2 mean the new dislocation has b3=b1+b2. New dislocation line vector l3 is intersection of the slip planes of the original two dislocations. Slip plane for new dislocation is one containing both its new b3 and l3

Question 5

When can two dislocations combine?

Answer 5

When the energy of the new dislocation is lower than the energy of the summation of the energies of the two dislocations combining. Otherwise they will not combine

Question 6

Frank’s rule for dislocation recombination

Answer 6

For two dislocations to combine into a single dislocation the following must be true:
b3^2 is less than b1^2+b2^2
This is because the energy of a dislocation is 1/2 Gb^2 but G is constant for a given material

Question 7

The written form of the Burgers vector

Answer 7

b=a/2 [uvw]

Question 8

What is the Lomer Lock?

Answer 8

A type of sessile dislocation, formed from the combination of two dislocations, which cannot move and will block the movement of further dislocations on both slip planes (of the original dislocations). This happens when this new dislocation has a slip plane that is not valid for the material’s crystal structure (determined from needing to contain b3 and l3)

Question 9

What happens when one dislocation passes though another?

Answer 9

It creates a jog or a kink with the direction and magnitude of the Burgers vector of the dislocation that cut it

Question 10

Kinks

Answer 10

Dislocation steps on the same slip plane. Like a 2D rectangle sticking out

Question 11

Jogs

Answer 11

Dislocations steps on a different slip plane. Like a single 3D step out of the original slip plane

Question 12

Why do kinks and jogs provide strengthening?

Answer 12

Increased energy required to create them.
Difficulty in moving them around.
Production of locked segments

Question 13

What is cross slip?

Answer 13

The process through which a screw dislocation may glide on a different plane to pass obstacles

Question 14

Why is cross slip only possible for screw dislocations?

Answer 14

Because their Burgers vector and line vectors are parallel. This means they can glide on any plane that contains both b and l.

Question 15

What is double cross slip?

Answer 15

When the dislocation returns to its original glide plane (or at least a parallel one after briefly moving along another one)

Question 16

What is climb?

Answer 16

Where edge dislocations can move onto other slip planes to avoid obstacles or to arrange themselves into energetically favourable configurations. Requires the incorporation of either vacancies or atoms to the dislocation core

Question 17

Positive or negative climb

Answer 17

Positive is where vacancies diffuse to the dislocation core and move it upwards. Negative climb is where atoms diffuse to the core and move it downwards

Question 18

Effect of temperature on climb and cross slip

Answer 18

Higher temperatures, or thermal activation, will make both mechanisms easier. Most evident in climb where high temperatures mean increased rates of diffusion