Twinning entails the intergrowth of crystal structures in which each individual is related to the other by a symmetry element external to the structure. It does not necessarily ensue from displacement.

Dear Prof. Aragon. Thanks for the reply. I was not aware of this very generalized definition for twinning. My question was specific to Deformation Twinning which is very readily seen in metals/alloys on application of stress under unavailability of slip-systems. I had a doubt becuase one can find both the terms available "twinning deformation" as well as "twinning transformation" in litterature.

My personal preference would be to preserve the strict crystallographic meaning of twinning but I do realize that usage sometimes takes precedence. In any case, we would all appreciate it if authors made their definition explicit.

When dealing with stacking defects there is always a large confusion. Quite often for instance e.g. the terms "twin" and "twin band" are used as synonyms (even if they can be quite different objects) and "multiply twinned" structures often contain other types of stacking faults. You always have a problem of stacking and of local symmetry operation differing from the global ones.

Therefore I do not fully agree with the definition of Ricardo.. it is true that in twinning you have some crystallographic relationship between different parts of a crystal, but I don't agree on the fact that you have two intergrown structures. You can find some useful definition in the mathematical crystallography group of the IUCr

http://www.crystallography.fr/mathcryst/twins.htm

Twinning and martensitic transformation lead to two different situations. in twinning you just change locally (usually just on one plane, especially in your case) the structure. In a martensitic transfomation you change an extended portion of the lattice into a different one.

Deformation twinning is the occurrence of a local change in the orientation of part of a lattice due to the action of a force (shear stress). The local symmetry on the twinning plane is different from that of the two sides (that are equal, just related e.g. by a mirror operation). A martensite transformation causes the transformation of a structure into another one: the interface between austenite and martensite separates again two regions, but in this case the regions have different, albeit related, crystallographic structures (that were the same before the transformation occurred).

Dear Dr. Leoni. Thanks for the clarification. I believe from your explanation, two differentiating grounds come out, for Twin and Martensitic Transformation, which are:

1) Extended Change v/s Local Change

2) Crystal Structure Change v/s No Crystal Structure Change.

The martensitic transformation is caracterised by its habit plan which is not deformed nor rotated. the transformation matrix is generally given as the product of a bain deformation and a rotation, but this combination keeps one line invariant (no deformation and no rotation). to keep a plan invariant we add another deformation: twinning or slip. so the twinning contributes to keep a plan invariant (habit plan) in the martensitic transformation. Pr Bhadeshia has a very good book about this topic, "Geometry of Crystals".

Twinning and Martensitic transformation is two different phenomena.

First come to twinning

Twinning - generally formed due to deformation process, twinning may also form during the recrystallization process called annealing twins.

Twinning form due to the shear displacement of atoms

Generally observed in the low slip systems.....which comes under the category of HCP unit cell system.

Deformation twinning observe in HCP material like (Ti, Mg etc...) at room temperature

However BCC which is higher number of slip system than HCP also shows deformation twinning, when deformed at low temperature......FCC shows very least tendency of showing deformation twinning. In term of the Stacking fault energy the least SFE system is highly suspectable to twinning. But when the solute concentration is increased in FCC system it shows twinning as the stacking fault energy is decrease. Ex: Brass, Bronzes, Cupro-nickel etc...

whereas Martensitic transformation "diffusion less process"....it is the metastable stage of the system. the driving force for a martensitic transformation is the chemical free energy difference between the two phases.

The similarity between the martensitic transformation and twining lies in the fact that in both the cases the parent phase are realigned as new crystal lattices.

In twins this realignment reproduces the original crystal structure, but with new orientation.

In martensitic plate, not only a new orientation produces, but also a basically different crystal structure. for example Fe-C (fcc to bct) with habit plane (225), Fe-Ni (fcc to bcc) with habit plane (9 23 33), Fe-Ni-C (fcc to bct) with habit plane (3 10 5) pure Ti (bcc to hcp) (8 9 12).

necessary condition for martensitic transformation, it need at least a single Glissle interface.

go through the some of the lecture of Proff Badeshia.....and the book Physical Metallurgy Principles by Reed Hill

http://www.youtube.com/watch?v=FffY381eMhU

http://www.youtube.com/watch?v=S-5ZMhE7Iak

The difference between transformation twinning and deformation twinning: in the latter case the twinning shear is realized by the motion of dislocations, while in the first case it is not. See for instance

https://www.researchgate.net/publication/233802377_A_continuum_model_for_initiation_and_evolution_of_deformation_twinning

Article A continuum model for initiation and evolution of deformation twinning

Though I have not worked on the subject of martensitic transformation, the following references may be useful.

1) "On the origin of mechanical twinning—Deformation or transformation?" by R. J. Wasilewski , Metallurgical Transactions A, Vol. 8, pp. 391-396 (1977).

2) "Deformation and martensitic transformation", by V. Seetharaman, Bulletin of Materials Science, Volume 6, Issue 4, pp 703-716 (September 1984).

Best wishes.

In terms of geometry and crystallography, they are almost identical. The key difference between them is that the driving force. For martensitic transformation, it is chemical driving force. For deformation twin, it is mechanical driving force.