Haruo Nozaki1, Yuji Uesugi1, Yukio Nishikawa1 and Imao Tamura2
1Production Engineering Laboratory, Matsushita Electric Industrial Co., Ltd., Osaka
Many models were suggested for the phase transformation superplasticity. Most of them provide only qualitative pictures of the mechanism. The internal stress theory by Greenwood et al. gives a quantitative explanation of experimental results, but neglects the compatibility of strain and the balance of stress. In the present study, we proposed the equation between transformation plastic strain ε tp, and applied stress by using continuum mechanics which satisfied the conditions above mentioned. Main results obtained are as follows.
(1) The equation proposed shows a qualitative good agreement with experimental results that ε tp, increases linearly in the range of low applied stress σ A and rapidly in the range of high σ A, and that ε tp increases as \dotT decreases. This suggests that the deformation during phase transformation approaches the normal creep, since the contribution of internal stress decreases relatively in the condition of high σ A or low \dotT.
(2) The eigen strain due to volume chance in a newly transformed region arrises to relieve the applied stress σ A and creates internal stress. The matrix is deformed plastically by internal stress and σ A. It seems that the internal stress decreases soon due to mechanisms proposed up to present, for instance, stress relaxation by plastic deformation. It is, therfore, suggested that the phase transformation superplasticity results from the repeating occurrence and decrease of internal stress.
superplasticity, transformation, contniuum mechanics, high temperature properties, internal stress
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