Superplastic properties of a delta/gamma stainless steel

The high temperature deformation behavior of a duplex stainless steel has b een studied by tensile tests at temperatures ranging from 850 to 1100 degre esC. The microstructure of the as-received material consists of elongated i slands of austenitic grains in a more or less continuous ferrite matrix. Me tallographic studies after tensile tests showed that the material recrystal lizes during heating at testing temperature and the austenite transforms in to fine colonies of grains of 10-15 mum in size. The results of the mechani cal tests in the temperature range 850-1100 degreesC 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 mechani sm is grain boundary sliding, GBS. Microstructure and texture evolution stu dies of deformed samples confirmed the importance of GBS. The colonies of a ustenitic grains decrease in size during superplastic deformation and, at t he same time, only minor changes in the texture was observed in the deforme d region. Finally, at high strain rates an increase of stress exponent to 5 is observed, which suggests a transition to a slip-creep controlling mecha nism. (C) 2001 Elsevier Science B.V. All rights reserved.