Superplastic properties of a δ/γ stainless steel

The high temperature deformation behavior of a duplex stainless steel has been studied by tensile tests at temperatures ranging from 850 to 1100°C. The microstructure of the as-received material consists of elongated islands of austenitic grains in a more or less continuous ferrite matrix. Metallographic studies after tensile tests showed that the material recrystallizes during heating at testing temperature and the austenite transforms into fine colonies of grains of 10–15 μm in size. The results of the mechanical tests in the temperature range 850–1100°C show a high elongation to failure (up to 300%) and a low value of n, 2–3, for strain rates up to 10−3 s−1. This result suggests that the controlling deformation mechanism is grain boundary sliding, GBS. Microstructure and texture evolution studies of deformed samples confirmed the importance of GBS. The colonies of austenitic grains decrease in size during superplastic deformation and, at the same time, only minor changes in the texture was observed in the deformed region. Finally, at high strain rates an increase of stress exponent to 5 is observed, which suggests a transition to a slip-creep controlling mechanism.