Superplastic behavior of Fe-8.0 mass%Al-2.0 mass%Cr-1.3 mass%C alloy

An ultra high carbon steel containing 8.0 mass%Al-2.0 mass%Cr-1.3 mass%C was processed to obtain a fine grained microstructure and superplastic behavior. Thermomechanical processing consists of warm rolling from 1373 to 1023 K (η ≈ 90%) and then continuous rolling (η ≈ 25%) around the Ac₁ transformation temperature. The final microstructure consists of a bimodal distribution of large proeutectoid and small κ-carbides in a ferritic matrix with a grain size of about 7 μm. Two different stress exponent regimes in the temperature range from 1023 to 1193 K as a function of the strain rate have been found. At strain rates up to ≈5 × 10^-1 s^-1, the alloy was superplastic with stress exponents, n, of about 2 and tensile elongations as high as 865% at 1153 K. The plastic flow data in this regimen are in agreement with grain boundary sliding creep model. As the applied stress is increased, the stress exponent increased to about 4-5. The creep behavior is explained in this regimen by a single dislocation climb mechanism. The activation energy for creep is in both regimens comparable with that for lattice diffusion of Fe in ferrite.