The Microstructure and Mechanical Properties of Alumina- Forming Austenitic Stainless Steels
Prof Ian Baker
Thayer School of Engineering, Dartmouth College, Hanover, U.S.A.
Time & Place
Mon, 24 Jul 2017 15:00:00 NZST in E14
In order to achieve energy efficiencies of >50 % in power generation systems, materials are required that are both strong and corrosion-resistant at temperatures >700 oC, and economically viable. Austenitic steels strengthened with Laves phase, NiAl and Ni3Al precipitates, and alloyed with aluminum to improve oxidation resistance, are potential candidate materials. The microstructure and microchemistry of the alumina-forming austenitic (AFA) stainless steels Fe-20Cr-30Ni-2Nb-5Al (in at.%) and of the more complex AFA stainless steels DAFA26 and DAFA29 that were recently-developed at Oak Ridge National Laboratory (ORNL) have been characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction and atom probe tomography. Both the relationship between the crystallographic orientation of the precipitates and the f.c.c. parent matrix, and the increase of precipitate size and volume fraction after various thermo-mechanical treatments (TMTs) have been studied. Different TMTs were performed on these steels to improve their mechanical performance. Tensile tests were performed at both room temperature and elevated temperature on the materials at different aging conditions in order to understand the influence of the TMTs on the material’s mechanical properties. Appropriate TMTs were shown to reduce the grain size to the nanoscale, raise the yield strength to >1000 MPa, and significantly increase the ductility. It was found that TMTs can also produce finer and more uniformly-distributed Fe2Nb and NiAl precipitates. Both the nanoscale grains and a high volume fraction of Ni3Al precipitates contribute to the high yield strength. Interestingly, even with Laves phase and NiAl precipitates almost completely covering the grain boundaries, significant room-temperature tensile ductility was observed. The results of preliminary creep studies will also be outlined.