Two critical observations related to the nucleation of superplastic cavities are reported. Cavities preexisting in a fine grain Al-Mg alloy prior to superplastic deformation have been minimized, but not completely eliminated by a high temperature annealing treatment. This has reduced the density of emerging cavities during subsequent deformation, however nucleation of cavities occur continuously with superplastic strain, starting possibly from partially healed defects of imperceptible (nanoscale) size. The observations raise doubts about the often-used cavity nucleation criterion based on a thermodynamically stable void size. Diffusional cavity growth model, used in conjunction with this assumed "stable" size artificially enlarges its perceived contribution, but still predicts less growth than the observed cavitation levels in superplastic metals. Alternate growth mechanism for submicron size cavities is suggested.
Bibliographical noteFunding Information:
This work was performed under support from US Department 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.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics
- Mechanics of Materials
- Mechanical Engineering