GFD’s – When are dislocations necessary?

Geometrically necessary dislocations are a concept used to explain the difference of behaviour to materials seen when testing small volumes by indentation. Especially the higher hardness values found when testing materials by nano-indention.

I’m not sure that dislocations can ever be necessary – just sometimes their existence becomes very likely – surely geometrically favoured dislocation (GFD) would be a better name than geometrically necessary dislocation (GND). Maybe that is too philosophical but I already saw reference to geometically necessary twinning (GNT) and am preparing myself for geometically necessary grains boundaries (GNGB), grains (GNG), phase transformations (GNPT).

A few things annoy be about the name, firstly I don’t like acronyms, secondly these dislocations aren’t necessary – their existence would only be favourable not necessary, thirdly they aren’t favoured by the geometry, the same geometry could be achieved with a different arrangement of atoms. If the material was subsequently annealed and the surface was constrained the dislocation density would be able to decrease.
Huajian Gao and Yonggang Huang discussed GND in the context of size-dependent plasticity in Scripta materialia, 48 (2003) 113-118 and you can find more useful references from their article no doubt.


A periodic array of dislocations is ‘necessary’ to generate a lattice curvature (a) and (b), H. Gao and Y. Huang 2003.


Geometrically necessary dislocations under an indenter. H. Gao and Y. Huang 2003

Advertisements

2 Responses

  1. Hi, I am a graduate student in materials science from China. I searched GND and saw this entry. There is a little mistake : it is “Scripta materialia”, not “Scripta materailia”.
    I am very impressed by your blog, so I feed it. Thank you for your sharing.

  2. Thanks/welcome.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: