Microstructural evolution and mechanisms of superplasticity in large-grained iron aluminides

Superplastic behavior has been found in Fe3Al and FeAl alloys with grain sizes of 100–600 μm. The large-grained Fe3Al and FeAl alloys exhibit all deformation characteristics of conventional fine grain size superplastic alloys. However, superplastic behavior was found in large-grained iron aluminides without the usual pre-requisites for the superplasticity of a fine grain size and grain boundary sliding. The metallographic examinations have shown that the average grain size of large-grained iron aluminides decreased during superplastic deformation. Transmission electron microscopy observations have shown that there were a great number of subgrain boundaries which formed a network and among which the proportion of low and high angle boundaries increased with increased strain. The observed superplastic phenomenon is explained by continuous recovery and recrystallization. During superplastic deformation, an unstable subgrain network forms and these sub-boundaries absorb gliding dislocations and transform into low and high angle grain boundaries. A dislocation glide and climb process accommodated by sub-boundary sliding, migration and rotation, allows the superplastic flow to proceed.