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Plastic Deformation Recovery in Freestanding Nanocrystalline Aluminum and Gold Thin Films


Science 30 March 2007:
Vol. 315. no. 5820, pp. 1831 - 1834
DOI: 10.1126/science.1137580
Jagannathan Rajagopalan, Jong H. Han, M. Taher A. Saif*
In nanocrystalline metals, lack of intragranular dislocation sources leads to plastic deformation mechanisms that substantially differ from those in coarse-grained metals. However, irrespective of grain size, plastic deformation is considered irrecoverable. We show experimentally that plastically deformed nanocrystalline aluminum and gold films with grain sizes of 65 nanometers and 50 nanometers, respectively, recovered a substantial fraction (50 to 100%) of plastic strain after unloading. This recoverywas time dependent and was expedited at higher temperatures. Furthermore, the stress-strain characteristics during the next loading remained almost unchanged when strain recovery was complete.These observations in two dissimilar face-centered cubic metals suggest that strain recovery might be characteristic of other metals with similar grain sizes and crystalline packing.

Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.

* To whom correspondence should be addressed. E-mail: saif@uiuc.edu

 

http://www.sciencemag.org/cgi/content/full/315/5820/1831?rss=1

Comments

Xiaofei Zhu's picture

Dear Prof. Saif,

      It is interesting that the FCC nanocrystalline film ( Al and Au) can recover 50 to 100% of plastic strain after unloading. I have a question needing your help. In the Supporting Online Material, you mentioned that stresses in the specimens after buckling are negligible (< 0.1 MPa). What's method do you used to measure the stress in the film?

Sincerely,

 Xiaofei

Ying Li's picture

Maybe it is Stoney formula. We can measure the stress in the film according to the curvature of the film.

Dear Xiaofei

The stress estimate comes from the Critical buckling load of the free standing sample .

It is anchored at both ends. Its cross section and the length are known.

Elastic modulus of the metal is also known. After buckling, the sample is subjected to the critical buckling load plus a negligible load (compared to the

critical buckling load). The stress is simply the buckling load divided by the critical buckling load.

 

Taher Saif 

 

Taher Saif

Xiaofei Zhu's picture

Dear Prof. Saif

Thanks for your reply. I have no knowledge of the buckling method to estimate residual stress. Is there any classic references on this method that you can recommend to me?

sincerely,

Xiaofei Zhu

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