Superplasticity in large-grained intermetallics

Superplastic behavior has been found in Fe3Al, Fe3Si, FeAl, Ni3Al, NiAl and TiAl alloys with large grain sizes (>30-600 mu m). The large-grained intermetallic alloys exhibit all deformation characteristics of fine-grained superplastic alloys, however, superplastic behavior was found in intermetallics without the usual pre-requisites of a fine grain size and grain boundary sliding. Metallographic examinations have shown that the average grain size of large-grained intermetallics decreased during superplastic deformation and a much finer grain size could be obtained after superplastic deformation. Transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD) observations have shown that there were great numbers of subgrain boundaries which formed a network and among which the proportion of low and high angle boundaries increased with the increase of strain. The observed superplastic phenomenon is explained by continuously dynamic recovery and recrystallization (CDRR). During superplastic deformation, an unstable subgrain network formed and these subboundaries absorb gliding dislocations and transform into low and high angle grain boundaries. A dislocation gliding and climbing process accommodated by subgrain boundary sliding, migration and rotation, enables the superplastic flow to proceed.