Some thoughts on future directions for research and applications in superplasticity

Achieving superplasticity in fine-grained materials at high strain rates and at low temperatures are objectives that will contribute to expanding commercial application of superplastic forming. Success in high strain rate superplasticity (HSRS) (10^-2 s^-1 and greater) has been achieved in mechanically-alloyed and fine-grained metal-matrix composites. The basis for this success is not fully understood. Studies are required to improve understanding of the contribution of grain boundary sliding, grain boundary structure, subgrains, random dislocations and second phase particles in achieving HSRS and in explaining the threshold stresses that are observed. In the area of ceramics, recent efforts have been focused on the understanding and improvement of superplastic properties in the presence of grain boundary glassy phases. In addition, ingot processing routes have been successfully demonstrated to produce two-phase, fine-grained superplastic ceramics. The concept can be extended to the processing of eutectic ceramics, such as ZrO₂-Al₂O₃, into fine-grained materials. In the area of superplastic bulk forming, novel methods are being pursued by utilizing localized incremental deformation for forming axisymmetric components. In a related area, fine grains are being created in bulk material by generating a large amount of deformation, without changing the shape of the bulk sample, through localized plastic flow, using equiangular extrusion techniques. In alternative approaches to achieve superplasticity, alloys deforming by solute-drag controlled dislocation creep are under evaluation; in these alloys a fine-grain size is not required, the strain-rate sensitivity exponent is about 0.33, and elongations in excess of 300% are achieved, often accompanied by only a small amount of cavitation.