Interfacial phenomena and microstructural evolution during superplastic deformation

Abstract

AbstractCertain polycrystalline materials of fine equiaxed grains of size <10µm exhibit tensile ductility of several hundred to a few thousand per cent, called superplasticity, at intermediate strain rates and high temperatures. Experimental and theoretical results on mechanisms for superplastic deformation and concurrent microstructural evolution are reviewed to understand the role of grain boundary phenomena towards enhancement of grain growth and cavitation, etc. Grain boundary sliding (GBS) dominates superplastic flow, the extent of which varies between the grain and interphase boundaries in two‐phase alloys. Accommodation to GBS is provided by diffusional and dislocation mechanisms, which are manifested in the form of grain boundary migration (GBM), grain rotation and grain switching events. Although GBS helps in retaining the initial equiaxed grain shape of superplastic materials, the accommodation processes accelerate grain growth. In the lack of other accommodation processes the cavities are formed, being more pronounced at the boundaries of maximum sliding. The ppm‐level impurities present in superplastic materials do not influence the sliding and flow characteristics under typical superplastic conditions but they do promote cavitation and limit the ductility. The presence of precipitate or hard particles in quasi‐single‐phase superplastic materials inhibits GBM but provides sites for cavitation. Such materials exhibit more dislocation activities than anticipated for accommodation of GBS. The nature of microstructural changes noted during superplastic deformation supports the operation of cooperative GBS, in addition to a large variety of combinations between sliding and accommodation processes. Copyright © 2001 John Wiley & Sons, Ltd.