Initiation and early stages of cavity growth during superplastic and hot deformation

The formation and growth of internal voids in metallic alloys are of considerable concern in components produced by superplastic and hot forming processes. Grain boundary cavitation, observed under conditions where boundaries are weaker than grain interior, results from concentration of strain around non-deformable particles and hard second phases located at grain boundaries. Careful examination of microstructures of aluminum and titanium alloys show that most voids are "nucleated" due to incompatible deformation between phases, although a few cavities might preexist in certain alloys. Increased levels of local hydrostatic stress arising from plastic constraints can activate growth of interfacial defects leading to rapid dilatation, followed by a slower growth rate due to reduced constraint as voids expand. An interface-constrained plasticity growth model described in this work explains why cavity growth rate is enhanced by increased strain rate, larger particle size, decreasing forming temperature and lower m value (strain rate sensitivity parameter) in metals, not by the diffusional growth mechanism. Experimental studies, utilizing image analysis revealed how the population of cavity nuclei also increase with increasing strain, strain rate and decreasing temperature, supporting the predictions of this new model.