On the existence of a low strain rate regime during high temperature deformation of a superplastic 25.7% Cr - 6.6% Ni steel

Numerous reports are now available showing the occurrence of superplastic deformation in high and ultrahigh carbon steels, low carbon low alloy steels, austenitic stainless steels, and duplex stainless steels or alloys. In this review we tabulate the more significant results in this area and examine in detail the current status of structural superplasticity in steels and ferrous alloys.

An experimental stainless steel containing 14Cr20Mn was found to exhibit moderate superplastic behaviour in the temperature range 800–1000 °C at strain rates of 10⁻⁴–10⁻³⁻¹. The strain rate sensitivity index was found to be insensitive to test temperatures between 825 and 875 °C but varied with strain rate, reaching a maximum of about 0.5 at 3 × 10⁻⁴s⁻¹. A model is proposed to estimate the rate of grain growth that occurred during superplastic deformation in terms of static grain growth, which is a function of time and the activation energy of grain boundary self-diffusion, and strain-enhanced growth, which is a function of strain rate and volume self-diffusion. Cavitation accompanied superplastic deformation and increased with increasing local strain, increasing temperature and decreasing strain rate. The cavitation results were analysed using strain-based and time-based models. The latter gave a more consistent estimate of the cavities present in the prestrain condition.