Dear Ajing,

Thanks for mentioning your work on nano-twinned structures. I have read your papers and find them to be very insightful. Regarding your comment about the strengthening-softening transition, there is also a recent experimental study by Lu et al (Science, 2009) that reveals that the transition may be due to "twin-twin" interaction or rather the interaction of pre-existing defects within the TBs when the TB spacing falls below a critical distance. We have also explored this issue by way of simulations in our work (Kulkarni and Asaro, Acta Mater, 2009) and observed a similar interaction.

Yashashree

Dear Shailendra,

The strengthening-softening transition in nanotwinned metals is indeed quite intriguing and it would be insightful to investigate various possible governing mechanisms. Thanks for mentioning the work by Shabib and Miller. Interestingly, the softening due to dislocation transmission across TBs after deformation that they observe occurs at lower twin density as compared to higher twin density. It would also be interesting to see how much of the coherent TB structure is retained after profuse absorption and transmission due to deformation.

Yashashree

Dear Shailendr,

Yes, you're right. Theortically one can study the difference for a variety of nano-twinned FCC metals. However, some of them are not easy to twin and Cu is the only one can be syntheszed with tunalbe twin sizes reported so far.

Regards,

Ajing

Dear Yashashre,

Thanks for your insightful comments.

Actually, we started to approach the twinning strengthening mechanism issue by MD simulations three years ago. Due to the computational limitation, we employed a quasi-two-dimensional simulation model, and we have elucidated the twin-dislocation interaction mechanisms and the resulting effects on their macroscopic stress-strain response. You can find our work in the following papers.

Title: Molecular dynamics simulation of plastic deformation of nanotwinned copper
Author(s): CAO, AJ; WEI, YG
Source: JOURNAL OF APPLIED PHYSICS Volume: 102 Issue: 8 Pages: - Published: OCT 15 2007

Interestingly, we found this kind of twin strengthening effect is also valid in metal nanowires. Here is the representative paper of our work.

Title: Deformation mechanisms of face-centered-cubic metal nanowires with twin boundaries
Author(s): CAO, AJ; WEI, YG; MAO, SX
Source: APPLIED PHYSICS LETTERS Volume: 90 Issue: 15 Pages: - Published: APR 9 2007

Regarding the strengthening-softening transition, it is really intriguing. Although the dislocation nucleation may become dominant mechanism in governing the yield strength, it is hard for me to imagine why there is such a transition, since the dislocation-twin interaction is the latter on process influencing the "strain-hardening" effect not the yield strength. I think there are still some unsolved issues in this topic and hope more work continues to appear.

Regards,

Ajing

Dear Yashashree,

Thank you for initiating a very interesting topic. There is also some recent work by Shabib and Miller reported in MSMSE (09) and Acta Materialia. They indicate that with deformation, TB's become weaker and fail to resist dislocation pile-up.It would be interesting to understand the implications of such a mechanism on the softening response.

Another aspect that may be studied is the nature of strengthening-softening transition for various nano-twinned FCC materials.

Thanks,

~Shailendra

Dear Ajing,

Thanks for your comments and for pointing out the interesting work by E. Ma. Yes, their study shows the formation of large micron-sized grains within a matrix of nanocrystalline grains leading to a bimodal grain size distribution. Twinning is observed within these large grains which contributes to the strengthening, while the large grains enhance the ductility by accommodating large plastic strains as you mention.

Yashashree

Yashashree
Thanks for introducing this fascinating topic. To my understanding, firstly coherent twin boundary (TB) is one type of grain boundaries (GBs) with mirror symmetry of the matrix and twin. It however differs mostly from the conventional GBs in that there is no disordered atoms, vacancy, voids in the TB interfaces. So naturally introducing large amount of nano-scale twins into metals would greatly enhance yield strength, as they block the motion of dislocations, while the plasticity is little ruined, which is expected to be related to the dislocation storation. This is a similar idea of "bi-model", which was proposed by E. Ma at JHU for nanocrystalline metals. High density of growth twins imparts high yield strength while preserving the capacity for efficient dislocation storage in large grains. so taking davantage by both small twins and large grains, high yield strength and reasonalbe tensile ductility can be achieved simutaneously.
Ajing