Superplastic deformation in coarse-grained Fe-27Al alloys

The microstructure and high-temperature tensile properties of Fe-27Al (in a tomic percent) alloy have been investigated. Tensile tests are performed in a temperature range of 600-800 degrees C in air under an initial strain ra te of 1 x 10(-4) s(-1). Important characteristics such as up to 300% elonga tion, strain-rate sensitivities of similar to 0.3 and low how activation en ergy confirm that our Fe-27Al alloy with a coarse grain-size of similar to 700-800 mu m exhibits superplasticity at temperatures at or above 700 degre es C. The low flow activation energy, similar to 250 kJ mol(-1), indicates the mechanism operating during the superplastic deformation is not likely c ontrolled by the lattice diffusion. Our microstructural observations reveal several important features such as the low dislocation density, grain refi nement, grain-boundary migration, cavity coalescence and grain-boundary cav ity. The refined grain structure (similar to 100-200 mu m in size) is presu mably attributed to the continuous grain-boundary migration, as evidenced b y the presence of irregular curve-shaped grain boundaries. Interestingly, t his strain-induced boundary migration might actually increase the surface e nergy, as opposed to the surface-tension-induced boundary migration that re duces the surface energy during a normal annealing process. Further studies are suggested in order to establish a better understanding of the mechanis m for this grain-boundary migration and its roles on the superplasticity. ( C) 1998 Elsevier Science S.A. All rights reserved.