Sherman Fairchild Professor of Engineering and Senior Associate Dean for Academic Affairs, Thayer School of Engineering, Dartmouth College, U.S.A.

Biography: Ian Baker obtained his B.A. and D. Phil. in Metallurgy and Science of Materials from the University of Oxford. He joined the Faculty of the Thayer School of Engineering at Dartmouth College, in 1982. He was the Director of the NIST-funded Center for Nanomaterials Research at Dartmouth from 2002-2005 and the Director of the NIH-funded Dartmouth Center for Cancer Nanotechnology Excellence from 2010-2016. He is a Chartered Engineer (U.K.) and a Fellow of ASM International, The Minerals, Metals and Materials Society, The Institute of Materials, Minerals and Mining (U.K.), the Materials Research Society, and the American Association for the Advancement of Science. He is Editor-in-Chief of Materials Characterization, has published around 400 papers and has given over 300 presentations at conferences, universities and to industry. Research interests include: mechanical behavior, including wear and fracture of metals, intermetallic compounds and ice; processing and recrystallization phenomena, particularly the effect of particles on recrystallization and processing by directional recrystallization; applications of electron microscopy, X-ray diffraction and X - ray topography, particularly in-situ deformation experiments; production and properties of nanocrystalline, particularly magnetic, materials; nanoparticles for biomedical applications.

Speech Title: The Microstructure and Mechanical Behavior of the Alumina-Forming Austenitic Stainless Steel Fe-20Cr-30Ni-2Nb-5Al

Abstract: There is an increasing need for affordable high temperature (~750^oC) structural alloys for both fossil fuel and concentrated solar power applications. Alumina-forming austenitic (AFA) stainless steels are a way to fulfil this need. In this presentation, we discuss the microstructure and mechanical properties of a model AFA stainless steel Fe-20Cr-30Ni-2Nb-5Al (in at. %). This alloy contains three different precipitates, i.e. Laves phase Cr2Nb, B2-structured NiAl precipitates, and L12-structured Ni3(Al,Ti) to provide strengthening, while the formation of aluminum oxide provides oxidation resistance at elevated temperature. The alloy can be completely solutionized by annealing at 1250°C. We will outline the effects of different heat treatments (0, 2.4, 24, 240 h and 1325 h at 800°C) after solutionizing on the precipitate size, including particle size-frequency distribution, and spacing, and how these affect the mechanical properties. An increase in the volume fraction of precipitates was accompanied by an increase in the room temperature yield strength from 205 MPa after the solutionizing treatment to 383 MPa after aging at 800oC for 1325h. The Laves phase and B2 precipitates form both along the grain boundaries and in the matrix, while the smaller L12 precipitates form only in the lattice. A precipitate free zone is also present along either side of the grain boundaries. Surprisingly, even when the grain boundaries are almost completely covered (93%) in Laves phase and B2 precipitates substantial room temperature ductility is still possible (~19% elongation). Creep studies have been performed at 750°C on specimens annealed at either 750°C or 800°C. The L12 precipitates did not form at 800°C but formed during subsequent creep testing. The specimens given the shortest heat treatment of 2.4 h at 800°C, which had the smallest initial particle sizes, showed both the highest yield strength and the smallest creep strain after 500 h. The extent of grain boundary coverage by precipitates did not appear to affect the creep rates. No grain boundary cracking or precipitate cracking was found after creep testing.