Superplastic deformation in coarse-grained Fe–27A1 alloys

The microstructure and high-temperature tensile properties of Fe–27A1 (in atomic percent) alloy have been investigated. Tensile tests are performed in a temperature range of 600–800°C in air under an initial strain rate of 1 × 1 10−4 s−1. Important characteristics such as up to 300% elongation, strain-rate sensitivities of ∼ 0.3 and low flow activation energy confirm that our Fe–27A1 alloy with a coarse grain-size of ∼ 700–800 μm exhibits superplasticity at temperatures at or above 700°C. The low flow activation energy, ∼ 250 kj mol−1, indicates the mechanism operating during the superplastic deformation is not likely controlled by the lattice diffusion. Our microstructural observations reveal several important features such as the low dislocation density, grain refinement, grain-boundary migration, cavity coalescence and grain-boundary cavity. The refined grain structure (∼ 100–200 μm in size) is presumably attributed to the continuous grain-boundary migration, as evidenced by the presence of irregular curve-shaped grain boundaries. Interestingly, this strain-induced boundary migration might actually increase the surface energy, as opposed to the surface-tension-induced boundary migration that reduces the surface energy during a normal annealing process. Further studies are suggested in order to establish a better understanding of the mechanism for this grain-boundary migration and its roles on the superplasticity.