THE ROLE OF GRAIN-BOUNDARIES IN HIGH-TEMPERATURE DEFORMATION

The grain boundaries in polycrystalline materials often play a very significant role in the flow and fracture characteristics at elevated temperatures. These characteristics are reviewed by defining four distinct grain size ranges, termed macroscopic, mesoscopic, microscopic and nanoscopic respectively. The role of grain boundary sliding in the mesoscopic grain size range is examined and it is shown that sliding measurements may be analyzed to give a rate equation for the sliding process. Grain boundary sliding accounts for essentially all of the strain in metals with microscopic grain sizes in the superplastic region of flow, but in ceramic materials with microscopic grain sizes the presence of glassy phases at the boundaries is also important. Nanoscopic grain sizes provide the potential for attaining high superplastic ductilities but there are difficulties associated with achieving fully dense materials with grain sizes of the order of approximately 0.01 mum.