THE INFLUENCE OF HYDROSTATIC-PRESSURE ON GRAIN-BOUNDARY SLIDING IN SUPERPLASTICITY - IMPLICATIONS FOR CAVITATION

It has been established that the occurrence of cavitation during superplastic deformation may be reduced substantially by the superimposition of a hydrostatic pressure of the order of the flow stress. Careful inspection of the available data suggests that both cavity nucleation and cavity growth are retarded by the superimposition of hydrostatic pressure. The present experimental study demonstrates that the grain boundary sliding activity occurring during superplastic deformation is not affected by hydrostatic pressure. Experimental measurements also reveal that the grain boundary sliding contribution to superplastic strain is more than 75%. both at low elongations of approximately 50% and at large elongations of over 200%. It is concluded that the imposition of hydrostatic pressure retards cavity nucleation during superplastic deformation by increasing the size of a stable cavity nucleus.