The Influence of Stacking Fault Energy on the Phase Transformations and Deformation Mechanisms in Iron-Manganese Alloys James E. Wittig, Vanderbilt University, DMR INTELLECTUAL MERIT Recently developed iron-manganese austenitic twinning- induced plasticity (TWIP) steels possess a superior combination of strength and ductility. The automotive industry is interested in these alloys as a means to reduce vehicle weight and increase crash worthiness. The outstanding mechanical properties result from the occurrence of mechanical twinning during deformation which is only activated at specific values of stacking fault energy (SFE), a composition dependent materials property. A major objective of this research is to experimentally determine the SFE of various Fe-Mn based steels and correlate these data to the deformation mechanisms and mechanical properties: The SFE was determined by measurement of the spacing (fault width) of multiple pairs of partial dislocations, using transmission electron microscopy (TEM), Fig. 1a. The experimental data in Fig. 1b show SFE values of ~14, 20 and 39 mJ m -2 for three Fe-(22, 25 and 28)Mn-3Al-3Si wt.% alloys. These SFE measurements represent the first experimental values from these new materials and will aid in their optimization by providing important information used in thermodynamic modeling. Fig. 1 (a) TEM micrograph of two partial dislocations in an Fe-14Cr-16Mn-0.3C-0.3N steel (b) Experimental SFE data from partial dislocation measurements for three Fe- XMn-3Al-3Si alloys. a b

The Influence of Stacking Fault Energy on the Phase Transformations and Deformation Mechanisms in Iron-Manganese Alloys James E. Wittig, Vanderbilt University, DMR BROADER IMPACTS Dean Pierce, a Vanderbilt Ph.D. candidate, who in 2011 participated in an NSF-IGERT funded international internship at the Max-Planck-Institut für Eisenforschung (MPIE) in Düsseldorf, Germany, returned to Germany in 2012 sponsored by this NSF-DMR award and Prof. Dierk Raabe from the MPIE. This second visit resulted in a number of new collaborations: Prof. Wolfgang Bleck and Dr. Alireza Saeed-Akbari, Institut für Eisenhüttenkunde, RWTH University, Aachen Prof. Joachim Mayer and Dr. Maryam Beig, Gemeinschaftslabor für Elektronenmikroskopie RWTH and the Ernst-Ruska-Centrum (ERC) in Jülich. Specifically, working together as a team, some of the first atomic-resolution images of mechanical twins, Fig. 2, were produced using the aberration-corrected Titan TEM at the ERC. Dean Pierce also provided training for members of the ERC staff concerning experimental methods for SFE measurements. These newly established collaborations, which will continue into the next year, provide a stronger research program for understanding the influence of SFE on the mechanical properties of Fe-Mn steels. Fig. 2. The image displays atomic resolution of a mechanical twin, ~2 nm wide in an Fe-16Mn-14Cr- 0.3C-0.3N sample deformed 20%. The atomic positions and the interface between the matrix and the twined crystal are directly observed.