Edge, Screw, and mixed dislocations

[scott_alexsk]

Does anyone have an idea as to the relative abundance of each of these types? Why will one type occur over the other in a given situation?

Thanks,
-Scott

 

[Vanechka]

An excellent question. In reality mixed type prevails (if you go as far as to consider 3D dislocation lines in a realistic material with its imperfect structure including point and volume defects, most dislocations have both edge and screw parts) since both main types are rather constrictive in their definitions, and its for example easy to "introduce" an edge part to a screw dislocation. I got to check this out from somewhere but remember when one material physics Prof. was "pushed" with the same question at some point very reluctantly he was "willing" to give edge dislocations a fraction of 10-20% in a typical metallic crystal (without giving any specifics - he probably just wanted to end the unease at that point ). I've no idea how accurate that estimate is - it does make sense "a somewhat" - but the problem as I see it is the answer is specific (like lattice dependent for one) and in reality the classification is quite difficult since our edge and screw models are only components of the real ones with all their jogs and complicated arrays. I'll see what Cottrell for one has to say.

 

[Gokul43201]

Does anyone have an idea as to the relative abundance of each of these types? Why will one type occur over the other in a given situation?

Typically, edge dislocations are stabilized (possibly, they have a very low mobility) at high temperatures, and screw dislocations at low temperatures.

 

[scott_alexsk]

Thanks guys!

 

[Astronuc]

I just picked up a nice little book -

Elementary Dislocation Theory by Johannes and Julia Weertman.

I just started it and it seems decent.


In my experience, I've seen mostly mixed and that includes dislocation loops and tangles. However most of my experience is with hcp Zr and Zr-alloys, and refractory alloys, and most of that is irradiated.

I believe dislocations in alloys and poly-crystalline metals are mostly mixed.

 

[Vanechka]

It seems like the successes of discrete & continuum dislocation dynamics are resulting in a new rise in everything 'dislocations' related. Or whether it was ever really "down" ... . Year ago picked up a very good example of this: Computer Simulation of Dislocations, by Bulatov & Cai, which have enjoyed immensely. Johannes Weertman also has an excellent book "Dislocation Based Fracture Mechanics", which contains pretty much everything relevant need to know of the particular field (Griffith-Inglis crack, Zener-Stroh-Koehler crack, Yoffe crack, Bilby-Cottrell-Swinden-Dugdale crack, lots about dislocation mechanics, shielding and antishielding, and plenty of elastic-plastic theory). Julia and Johannes must have some pretty "interesting" table conversations .