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The weakest size of precipitated alloys in the micro-regime: The case of duralumin

Published online by Cambridge University Press:  15 May 2017

Kefu Gan ,
Rui Gu  and
Alfonso H.W. Ngan
Kefu Gan*
Affiliation:
Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, People’s Republic of China
Rui Gu
Affiliation:
Materials Characterization and Preparation Center, Southern University of Science and Technology, Shenzhen, People’s Republic of China
Alfonso H.W. Ngan*
Affiliation:
Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, People’s Republic of China
*
a) Address all correspondence to this author. e-mail: hwngan@hku.hk
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Abstract

In the microsize regime, all crystalline metals studied to-date exhibit a “smaller-is-stronger” size effect. Here, we report an unusual weakest-size phenomenon in the precipitated alloy duralumin 2025, i.e., below a critical size of ∼7 μm, the strength increases as the size decreases, while above this size, the strength increases toward the bulk value with increasing size. At the critical size, strain-hardening is also slowest and the room-temperature creep is fastest. Interestingly, the reduction of strength at the weakest size is more significant for the peak-aged state of duralumin 2025 than its naturally aged state. Theoretical modeling shows that at the weakest size, both strengthening mechanisms of precipitation hardening and dislocation starvation are ineffective. The present results indicate that the conventional wisdom of precipitation hardening is not applicable in the micro-regime, and the common “smaller-is-stronger” understanding is incorrect when material microstructures impose internal length scales that can affect strength.

Keywords

dislocationsstrengthalloy

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Type
Invited Feature Papers
Information
Journal of Materials Research , Volume 32 , Issue 11 , 14 June 2017 , pp. 2003 - 2013
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Copyright
Copyright © Materials Research Society 2017 

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Footnotes

Contributing Editor: Jürgen Eckert

This paper has been selected as an Invited Feature Paper.

References

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