Microstructural evolution of superplasticity in large-grained Ni–48Al intermetallics

Superplastic behavior has been found in large-grained Ni–48Al intermetallics with grain size of 200 μm. The large-grained Ni–48Al alloy exhibit deformation characteristics of most fine-grained superplastic alloys without the usual pre-requisites of a fine grain size and grain boundary sliding (GBS). Metallographic examination (OM) has shown that the average grain size of large-grained intermetallics decreases 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 are great numbers of subgrain boundaries which form a network and among which the proportion of low and high angle boundaries increases 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 forms 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.