Twinning and slip are two competing deformation mechanisms. When the critical resolved shear stress of slip is much smaller than twinning, deformation will be dominated by dislocations and you will see significant dislocation activities that lead to the formation of subgrains and dislocation cells.

Many factors can affect the twinning propensity. These include (1) temperature -- low temperature promotes twinning; (2) strain rate -- high strain rate favors deformation twinning; (3) grain size -- reducing grain size usually make twinning more difficult. However, grain size effect is very complicated in FCC structures. I am not going to discuss this here as your sample is of a BCC structure; and (4) stacking fault energy -- lowering stacking fault energy makes twinning easier.

Grain size should play a significant effect in your case. If you want to get deformation twins, you can deform your materials at a very low temperature and/or under a very high strain rate.

One issue that you need to pay attention is that twins can only be seen in some specific crystallographic orientations in high-resolution TEM. If you are interested in getting a better statistics on twinning propensity in nanostructured materials, transmission Kikuchi diffraction is a better and much easier technique than TEM.

Regards

Xiaozhou Liao

as we know that..dislocation cells and deformation twins or twin faults contribute to subgrain refinement....the observation of dislocation, can use XRD to know the orientation. then used a Hall-Petch relation when considering dislocation cell size d, intertwin spacing Δ and grain size D.

Sorry Komang I did not get you. Using Hall-Petch to calculate what?How can it help to explain whether twins form or not? Actually there is no twin in my material, so how intertwin spacing can be used to explain the issue?

Texture (grain orientation) and the deformation rate can be a significant factor. Orientation effect was observed in the fcc metals (e.g. Archives of Metallurgy,47, 2002,157) and hcp (zinc, Phil. Mag., A90, 2010, 1873).

Greetings,

 Mirek

Dear Wrobel, 

I agree that grain orientation plays an important role in twinning, but during SPD grain size can reach 5-10 nm (in my material), and SAED patterns I got show complete ring patterns, indicating creation of randomly oriented nano-grains. If there were a particular texture in the material, I would get broken rings in the SAEDPs. 

I understand that twins were absent in samples with relatively large grains (how look SAED for such material). The nanostructure is a different story - mayby the microstructure is recrystallized. I think for nanostructure study you can try to use HRTEM . However, a more simple  X-ray textures measurement may be also helpful. By the way, what SPD method you used?

Greetings,

Mirek

Wrobel: 

Thanks for your comment. I actually used HRTEM and could not find any twins in the shear bands studied. Grain size could reach as small as ~5 nm (again observed by HRTEM as it is impossible to see such fine grains using dark field or bright field imaging). The relatively large grains contained a large number of dislocation cells and SAED pattern showed some diffraction spots slightly spread (please see attached the SAEDP). I think the formation of subgrains (low angle grain boundaries) led to the elongated diffraction spots. Texture also creates such stretched diffraction spots, and this is quite possible in one of the shear bands formed at very high speed (10 mm/s) with the SAED attached, where grains oriented along shearing direction, but it cannot justify the absence of twins in the rest of the shear bands studied were SAED patterns were complete rings without any sings of texture. 

I found some papers arguing the effect of pre-straining on low temperature twinning in Cr and Nb (both bcc). These studies suggest that slip and small dislocation cells can suppress twinning. Do you think same can happen in my material? 

Unfortunately I cannot use XRD since I am focusing on shear bands. As you know shear bands are too thin to be studied using XRD. Even micro XRD does not work. 

I worked in early late 70s on the kinetics of twin formation. It is one mode of deformation favored by the propagation of partial dislocations -hence related to SFE as you mentioned, and is suppressed if the other modes of deformation takes over. If you try to deform at low temp. then twinning can be observed, if cracking does not occur before twinning. Yes activation of other deformation modes will suppress the twin formation. You can anneal the sample and try to deform at very low temps. You may see twinning.

In beta alloys twinning is typically not activated due to the presence of beta stabilizing elements (see e.g. The effect of aluminium on twinning in binary alpha-titanium, by Arnas Fitznera, b, D.G. Leo Prakasha, c, Joao Quinta da Fonsecaa, Matthew Thomasd, Shu-Yan Zhange, Joe Kellehere, Pascal Manuele, Michael Preussa).

Frederik,

I totally disagree with you. Deformation modes of beta Ti alloys are strongly dependent on beta phase stability. There are many examples in the literature where twins were observed as main deformation mode of beta Ti alloys. Gum metals are clear cut examples in this regard. 

As for the paper suggested, I should say that Al is a strong alpha stabilising element not beta. Also, the paper argues twinning in alpha-Ti-Al with hcp crystal structure. For the alloys containing > 7 wt.% Al Gama phase also plays a role. This paper is totally irrelevant to my work.

Thanks anyway for your answer.

Oops, answering too quickly there... Mixed up the phases there... Shouldn't try to answer questions while lunching ;-)

How did you check for twins? Normally, metastable beta alloys twin on the {332}113 system, whereas stable beta alloys twin via the {112}111 system, next to regular slip. If you have subgrain boundaries that are strong enough they will also suppress twinning (the difference between grain boundaries and subgrain boundaries can be at least partially arbitrary).

Dear Ahmad,

You mentioned that your alloy is supposed to be deformed by twinning because of its low SFE and Bo and Md values. The problem is that if this prediction is correct?

Have you already measured the SFE? Even if it’s done, as far as I know in contrast to fcc alloys, there is no quantitative criterion regarding the effect of SFE on deformation mechanisms of beta titanium alloys.

On the other hand, the Md-Bo diagrams give us only a rough prediction of deformation mechanisms which may be far from the real observations. For example, the effect of several parameters such as grain size, strain rate and deformation temperature is not considered in this diagram. Anyway, where is exactly the position of your alloy in the Md-Bo diagram?

Dear Ahmad Zafari,

A) In fcc: metals:

1) the shear bands intersect mechanical twins and destroy them this way ( as I remember: JOURNAL OF MATERIALS PROCESSING TECHNOLOGY,  53, 1995, 75), so I can imagine that  twins disappear when shear bands density is large.

2) prestrain is always need for deformation twinning

3) when the SFE is very low,  the phase transition effect is observed instead deformation twins.

Some like, it should be for hcp.

B) In large grains with special orientations you can not see deformation twins, even when they are there. It is important how the thin foil plane  is located in the deformed sample. Looking for the twins I am always looking for the double reflections on the diffraction pattern (are you sure they are not on the SAD attached).

I can hardly something to help when I do not know the details of the deformation process, which you use and the value of SFE.

Greetings,

Mirek

Twinning and slip are two competing deformation mechanisms. When the critical resolved shear stress of slip is much smaller than twinning, deformation will be dominated by dislocations and you will see significant dislocation activities that lead to the formation of subgrains and dislocation cells.

Many factors can affect the twinning propensity. These include (1) temperature -- low temperature promotes twinning; (2) strain rate -- high strain rate favors deformation twinning; (3) grain size -- reducing grain size usually make twinning more difficult. However, grain size effect is very complicated in FCC structures. I am not going to discuss this here as your sample is of a BCC structure; and (4) stacking fault energy -- lowering stacking fault energy makes twinning easier.

Grain size should play a significant effect in your case. If you want to get deformation twins, you can deform your materials at a very low temperature and/or under a very high strain rate.

One issue that you need to pay attention is that twins can only be seen in some specific crystallographic orientations in high-resolution TEM. If you are interested in getting a better statistics on twinning propensity in nanostructured materials, transmission Kikuchi diffraction is a better and much easier technique than TEM.

Regards

Xiaozhou Liao

We seemed to have twins for pure calcium metal at 77k in Article Tensile Properties of High-purity Ca Metal

Dear Ahmad Zafari ,

Severely deformed samples are always difficult for characterization.

(1) the usual straight twins will be distorted and present strange shape, leading to difficult recognition.

(2) severe deformation can induce rotations and stress concentration, which should be accommodated by the GNDs (geometrically necessary dislocation). And GNDs make both TEM and (especially) EBSD to be tough works.

(3) as we can see, beta-Ti alloys are very easy to be twinned by {332} or {112} twins. Thus, the abundant twinning, cross twinning, second twinning and their interaction with slipping dislocations will enhance the difficulty.

(1)-(3) could be the source of the subgrains and disslocation cells in your sample.

(4) As Xiaozhou Liao mentioned above, temperature and strain rate are the very original influencing factors, and in your work, lower temprature maybe be the better.

Best luck!

Xianbing Zhang