Knowledge of the exact physical mechanism of cavity formation and early growth is important for the prediction of the extent of internal damage following superplastic deformation. To this end, the early stages of cavitation in a superplastic Al-Mg-Mn-Cu alloy have been experimentally studied and reported here. Small cavities (<0.5 μm) were detected by scanning electron microscopy and the number of cavities per unit volume was monitored by image analysis through optical microscopy. Before deformation, some cavities were seen at the particle-matrix interfaces. However, during tensile deformation in the temperature range of 450-550°C (and strain rates ∼ 10-4 to 10-2 s-1), additional cavities emerge and grow. Most cavities are observed at the interface between particles and the matrix from submicrometer size range, and grow initially along the interface. This suggests that early cavity growth is by matrix/particle decohesion, possibly starting from interfacial defects, and this growth has rapid kinetics. The density of observable cavities increases with strain, i.e. "nucleation" is continuous. The number of cavities increases at higher strain rates and at lower test temperatures. This is due to the higher flow stresses, reduced strain-rate sensitivity and poorer diffusional accommodation process, which assist in the initial growth of the submicrometer and nanoscale interface defects. But the evidence for diffusional cavity growth in the initial stages was not found.
Bibliographical noteFunding Information:
This work was performed under support from the 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
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Polymers and Plastics
- Metals and Alloys