Structural superplasticity at higher strain rates of hypereutectoid Fe-5.5Al-1Sn-1Cr-1.3C steel

Abstract

A fine-grained ultra-high-carbon steel—UHC steel—containing 1.35 wt pct carbon, 5.5 wt pct aluminum, 1 wt pct tin, and 1 wt pct chromium exhibits fine-structure superplasticity in the temperature regime between 775 °C and 900 °C at higher strain rates up to 10⁻² s⁻¹. Thermomechanical processing was performed in order to achieve a fine-grained equiaxed microstructure consisting of κ-carbides of about 0.7 to 2.5 µm in size finely distributed within the ferritic Fe(Al, Sn, Cr) solid solution matrix with a linear intercept grain size of 3 to 5 µm. Superplasticity occurred in the strain rate regime of 10⁻⁴<-$\dot \varepsilon$≤10⁻² s⁻¹ with m values of 0.5 to 0.6 (stress exponent n=1.6 to 2). Tensile elongations of more than 900 pct were recorded. From thermal activation analysis, activation energies of Q=230 to 243 kJ/mole were determined, which clearly reveal a contribution of the alloying elements Al and Sn to the lattice diffusion of iron. The governing deformation mechanism is grain boundary sliding accommodated by dislocation climb controlled by lattice diffusion sustained by chemical diffusion. At very high strain rates of $\dot \varepsilon$≳2 · 10⁻² s⁻¹, the strain-rate-sensitivity exponent decreases to about 0.2≤m≤0.27, which indicates class II solid solution behavior of this material.