Strain-rate effects on superclastic 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. © 2002 Elsevier Science B.V. All rights reserved.