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Instability of Superplastic Aluminium Alloys

Published online by Cambridge University Press:  16 February 2011

H. Iwasaki ,
S. Hayami ,
K. Higashi  and
S. Tanimura
H. Iwasaki
Affiliation:
Department of Materials Science, Himeji Institute of Technology, Shosha, Himeji, Hyogo, 671-22, Japan
S. Hayami
Affiliation:
Department of Materials Science, Himeji Institute of Technology, Shosha, Himeji, Hyogo, 671-22, Japan
K. Higashi
Affiliation:
Department of Mechanical Engineering, University of Osaka Prefecture, Mozu, Sakai, Osaka, 561, Japan
S. Tanimura
Affiliation:
Department of Mechanical Engineering, University of Osaka Prefecture, Mozu, Sakai, Osaka, 561, Japan
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Abstract

The influence of the strain hardening coefficient,γ,and the strain rate sensitivity exponent, m, on the onset and growth of necking has been pointed out in many models about plastic instability controlled by the mechanical properties of materials. In this paper, the instability parameter I (=(l-γ-m)/m) proposed by Hart is used to relate the values of m (=∂lnσ/∂lnέ) and γ (=∂σ/(σ·∂ε)) for some superplastic aluminium alloys. These values of m and γ were calculated from the load-elongation curve at various strain-rates in constant strain-rate tests with periodic m determinations by strain-rate departures of small magnitude. It will be shown that the strain hardening due to grain growth contributes to an increase in the stability of plastic deformation during superplastic flow. However the decrease of m due to dynamic grain growth with strain causes superplastic deformation to be unstable near the onset of fracture. The transition of I values from negative to positive coincides with a decrease in m value and flow stress.

It is concluded that the development of necks can be characterized by an instability parameter I which include the effect of strain hardening caused by strain induced grain growth.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 196: Symposium T – Superplasticity in Metals, Ceramics, and Intermetallics , 1990 , 233
Copyright
Copyright © Materials Research Society 1990

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References

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