TECHNICAL: Austenitic Fe-Mn alloys have exceptional mechanical properties. Recent trends in the automotive industry towards improved safety standards, reduced weight, and cost effective manufacturing have led to a renewed interest in these high strength and ?super tough? steels. Alloys of Fe-Mn exhibit extensive ducility and strength over a large temperature range (-200 to 250 C) where low temperature deformation can show elongation >75% and ultimate tensile strength over 1000 MPa. These outstanding mechanical properties are produced by the transformation induced plasticity (TRIP) effect, which is related to the low intrinsic stacking fault energy (SFE) in austenitic steels. The additions of elements such as Al, Si, C, and N have large influences on both the SFE and deformation mechanism. Temperature dependent plasticity can change from TRIP to twinning induced plasticity (TWIP) when the SFE is affected. Although extensive work has been performed in this area over the last 50 years, there is still a considerable lack of understanding and disagreement for the underlying factors that determine whether the temperature dependent deformation mechanism will be dislocation glide, strain induced martensite, or mechanical twinning. The research is a systematic study with model FeMn(AlSi) and FeMnC alloys as well as more complex high Mn-N stainless steel materials to investigate the influence of alloys elements (Mn, Al, Si, C, N) on the SFE for these austenitic materials. A major objective of the research is the development of structure property relationships between quantitative measurements of SFE as a function of composition and temperature and the temperature dependent mechanical deformation mechanisms for austenitic Fe-Mn alloys. These fundamental studies will provide the basis for optimizing the Fe-Mn alloy composition to achieve improved mechanical properties in new commercial alloys. NON-TECHNICAL: The research involves collaboration between Vanderbilt University, the Max-Planck Institut fur Eisenforschung, and Oak Ridge National Laboratory. This collaboration will not only serve to make a strong scientific approach to the research, it will also provide an educational component to the program where both graduate students and undergraduate students will be exposed to international interactions and state-of-the-art facilities at a national laboratory. Undergraduates will be involved in this program with both senior independent research opportunities and summer internships. Graduate students involved in this program will receive the training required to pursue a career in Materials Science and Engineering. A second objective of the current project is to train underrepresented groups as future scientists. The P.I. of this project (co-PI for the Vanderbilt/Fisk IGERT program and director of the Vanderbilt Interdisciplinary Materials Science Program) is dedicated to increasing the diversity of the Vanderbilt Ph.D. program. He is currently the principal advisor for one of the Vanderbilt/Fisk IGERT minority students, and this NSF funded program would provide for future support of new Fisk masters students interested in a Vanderbilt Ph.D. The research program has the potential to make major contributions to the education, training and knowledge of future scientists.
High-manganese austenitic transformation- and twinning-induced plasticity (TRIP/TWIP) steels are a new class of advanced high strength steels (AHSSs) that have received significant research interest over the last decade owing to their superior combination of strain-hardening, strength, toughness, and ductility compared to other types of steel. The automotive industry is extremely interested in these steels as a means to reduce vehicle weight (through down gauging), increase crash worthiness and improve manufacturing efficiency from better room temperature formability. In the current research, the stacking fault energy (SFE), which is an important parameter that controls the mechanical properties of the steel, was experimentally measured for the first time in three high-Mn austenitic steels (Fe (22,25,28)Mn3Al3Si) using electron microscopy methods. In this process, a new method to determine single crystal elastic constants was developed that employed nano-indention and electron backscattered diffraction methods. SFE values of 15±3, 22±3 and 39±5 mJ m-2 were measured for the 22, 25 and 28%Mn alloys respectively. Tensile testing over a range in temperature was performed to determine the mechanical properties. Microstructural analysis of the deformed materials using transmission electron microscopy allowed for a basic understanding of the influence of SFE on the deformation mechanisms and mechanical properties of these materials. A better understanding of the influence of the SFE on the changes in the deformation mechanisms for these alloys will assist in the future development of these new high-Mn steels with improved performance.
