Strain distribution in the superplastic 7475 Al alloy
- Washington Univ., St. Louis, MO (United States). High Performance Materials Inc.
In superplastic flow, the flow stress, {sigma}, is generally related to the imposed strain-rate, {dot {var_epsilon}}, by an equation of the form: {sigma} = B ({dot {var_epsilon}}){sup m} (1) where B is a constant for a given temperature and grain size and m is the strain-rate sensitivity. The overall ductility observed in a superplastic material under any selected testing condition can be limited both by the formation and growth of localized necks within the gage length and/or by the development and interlinkage of internal cavities. The formation of macroscopic necking is determined primarily by the value of m. It can be shown that the change in the specimen cross-sectional area, A, with time, t, is given by: dA/dt = {minus}(P/B){sup 1/m} A{sup (m{minus}1)/m} (2) where P is the tensile force. It follows from Equation 2 that the probability of necking decreases as m{r_arrow}1. Generally, in the absence of failure by cavitation, a value of m{approximately}0.5 will lead to the development of diffuse necks and high elongations to failure. True superplasticity requires Newtonian viscous flow with m=1 and a consequent infinite elongation to failure. In practice, however, this condition is not attainable in real metals, and there is instead an optimal superplastic condition when m>0.5 and failure occurs by quasi-stable flow. Under these conditions, where m is significantly <1, it is important to examine the distribution of strain, and thus the extent of necking within the gage length. The 7475 Al alloy was selected for this investigation on strain distribution.
- OSTI ID:
- 64376
- Journal Information:
- Scripta Metallurgica et Materialia, Journal Name: Scripta Metallurgica et Materialia Journal Issue: 9 Vol. 32; ISSN SCRMEX; ISSN 0956-716X
- Country of Publication:
- United States
- Language:
- English
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