The extent of cavity growth estimated from a combination of diffusional and plasticity based growth models generally underestimates the actual cavity growth in superplastic alloys. It has been shown that in a fine grain Al-Mg alloy, cavity growth begins by matrix/particle debonding at grain boundary particles (Mat. Sci. Forum, Trans. Tech. Pub. 304-306 (1999) 609), and also from pre-existing voids. In this study, cavity growth beyond interface decohesion is modeled in which deformation of the matrix surrounding the cavity is free from interface constraint, but it still experiences an accelerated local deformation rate. Stress and strain-rate in this region are intensified due to the perturbed flow field near the cavity, and not relaxed during the time frame for superplastic forming. This local deformation around the cavity is a function of strain-rate sensitivity, m, the level of strain concentration, and the cavity spacing. Two important effects not previously considered: (i) local stress concentration around the cavities, and (ii) continuous nucleation of new cavities, have been included in this work. Using this model that is suitable for low overall cavity volume (i.e. no cavity coalescence), faster growth rate is predicted for single cavities when strain-rate sensitivity is low and/or the population density of cavities is low (generally at slow strain-rates). By combining the predicted growth rate of individual cavities with the emerging cavity population density determined experimentally, a quantitative understanding of the various complex dependencies of cavitation has been obtained.
Bibliographical noteFunding Information:
This work was performed under support from US Dept. of Energy under grant FG02-96ER45608-A000, and a contract from General Motors R&D Center. Acknowledgement is also due to the US Air Force Contract F33615-94-C-5804 for the appointment of A.K. Ghosh during his sabbatical leave at the Air Force Research Laboratory at WPAFB, Ohio. The authors would like to thank Prof. William Nix for his useful comments.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Polymers and Plastics
- Metals and Alloys