Electron backscattered diffraction study on superplastic coarse-grained Fe–27 at.%Al: processing effects

An electron backscattered diffraction technique has been used to investigate detailed crystallographic features of a superplastic coarse-grained Fe–27 at.%Al alloy. Alloy samples studied have been tensile tested to failure at 800°C in air under an initial strain rate of 1×10−4 s−1. To examine processing effects, the hot isostatic pressing (HIP) has been applied prior to the superplastic deformation. The HIPed sample shows no observable pores in the fracture region while the sample without HIP reveals an elongated pore fracture structure. Nevertheless, HIP is shown to have no beneficial effects on the superplastic elongation, suggestive of the fact that the alleviation of cavity formation alone is insufficient in achieving better superplastic properties. After the superplastic deformation and the refined grains are formed, the presence of numerous small angle subboundaries in the large grain interior indicates the continuous event of recovery and recrystallization that occurs throughout the course of superplastic deformation. The post-deformation annealing yields a classic recrystallized large-grain structure, resulting from the surface-tension-induced boundary migration that reduces the grain surface-area. Conversely, the superplastic deformation of Fe–27 at.%Al involves a strain-induced boundary migration that causes the grain surface-area increase and results in a refined grain structure. The dynamic nature of recovery and recrystallization is therefore confirmed.