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


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.

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