Strain-rate effects on superplastic properties of coarse-grained iron aluminides: a microstructural examination

Strain-rate effects on superplastic properties of coarse-grained Fe–Al based alloys have been examined. Alloys studied were Fe–28.4 at.% Al–2.0Cr and Fe–24.5Al–1.4Ti, with initial grain sizes ranging from ∼0.9 to 1.7 mm. For comparison, a binary alloy of Fe–26.5Al was also studied. Alloy samples have been tensile tested in air under initial strain rates of 1×10−4, 1×10−2, and 1×10−1 s−1 at 800 and 900 °C. Elongation-to-failure obtained for the alloys studied are at least 167%, with the maximum of 420% for Fe–26.5Al at 800 °C, 1×10−4 s−1, thus confirming the superplasticity. At 900 °C, the elongation shows a positive strain-rate dependence while a negative strain-rate dependence of elongation is observed at 800 °C. At 900 °C, grain growth is likely to occur at the low strain rate. A larger elongation is thus obtained when a higher strain rate is applied. Nevertheless, at 800 °C, high strain-rate deformation results in a grain-migration structure and poor elongation is obtained. Superplastic deformation is better performed at 800 °C with a strain rate of 1×10−4 s−1 or at 900 °C with 1×10−2 s−1, in order to yield a greater elongation and achieve refined grain structures. Based on the microstructure observed, the effect of a strain rate increased by two orders of magnitude is approximately equivalent to that of a 100 °C decrement in the test temperature.