Annealing twins, as the name implies, are formed after solution annealing FCC metals. If you color etch the color is different on either side of the twin boundary. If you have EBSD, you will see that a twin boundary is a low-angle boundary, generally 60 degrees. Mechanical twins can be formed by heavy deformation of FCC metals and they are more lens like in shape, and cold work affects can be seen within the grains. They are visible in the LOM. If you go to my web site I have Power Points on the structure of stainless steels and you can see twin boundaries and mechanical twins.

In addition to different morphology. The mechanical twins are fine formations that could be seen only in TEM while annealing ones are usually the size of a grain.

FCC metals are not ordinarily deform by mechanical twinning, although gold-silver alloys twin fairly when deformed at low temperature.

Annealing twins often form in FCC metals such as Cu, Al, Ni that have been cold worked and annealed. As the crystal regrows a stacking fault leads to the formation of twins.

Annealing twins, as the name implies, are formed after solution annealing FCC metals. If you color etch the color is different on either side of the twin boundary. If you have EBSD, you will see that a twin boundary is a low-angle boundary, generally 60 degrees. Mechanical twins can be formed by heavy deformation of FCC metals and they are more lens like in shape, and cold work affects can be seen within the grains. They are visible in the LOM. If you go to my web site I have Power Points on the structure of stainless steels and you can see twin boundaries and mechanical twins.

If the annealing happens after the deformation leading to the mechanical twins, I would expect the annealing twins to be almost dislocation free compared to the mechanical twins.

Very true!

Yes, I worked on brass and aluminium before. Mechanical twins are very different from annealing twins. The former is a deformed structure consisting of layers of thin and elongated crystallites, whereas the latter is a recrystallized structure.

Mechanical or deformation twins as the name suggests form during deformation and albeit at low homologous temperatures. As the above answers also suggests, they form in order to accommodate strains during deformation. In most cases (read polycrystalline materials) the deformation twins are extremely fine with lamella thickness under 50 nm depending on the SFE of the material. The deformation twins usually have a lens or slightly curved shape due to its mechanism of formation and are resolved by TEM or by recent methods such as HR-EBSD or transmission kikuchi diffraction.

Annealing twins  form in metals of low to moderate SFE (SFE < Ni) after annealing heat treatment of the deformed material. These low energy boundaries are created to decrease the overall interfacial energy of the system. Annealing twins in cubic materials usually have long parallel twin boundaries (coherent) which may or may not be terminated by an incoherent twin boundary. These boundaries can be of various sizes depending on the final grain size after annealing (from nanometer scale to hundreds of microns).

A common feature of these both types of boundaries are that they share the same CSL relationship (Σ3).

Dear Chidambaram,

Could you explain how the stereographic projection can be used to differentiate between mechanical and annealing twins? I believe the crystallography is the same for both, though coherency and strain within the twin would be different.

Annealing twin boundaries are formed by errors in the stacking pattern of the close packed planes (ABCABCABCCBACBA...). They are low angles boundaries, usually

You are correct. Annealing twins tend to be straight lines, althought the line can take a left or right :turn: for a short distance and then rotate 90 degrees and go stright again in the same direction as at the other end. Mechanical twins are more lenticular in shape and evidence of cold work in the matrix around them is visible. But, you can see mechanical twins much better using Nomarski differential interference contrast (DIC) than in bright field immumination. They will stand in relief above the matrix and the deformation of the matrix will be more visible also.

I am wondering if the deformation twin has higher free energy than the annealing twin?

I would assume so, but I do not recall any data on that point (but I am sure it exists).

GFV

Dear Preetish D'Silva,

I would like also to add to the previous answers that if you measure the misorientation of crystal lattice across the twins using the EBSD (that is plotting only the misorientation angles of 60° around the (111) axis). It is possible to distinguish between the annealing and mechanical twins based on their width; the mechanical twins are narrower than the annealing ones, with values around 0.5 µm and 5.5 µm, respectively.

Please see the attached file