Single-Crystal Elastic Constants of High-Manganese TWIP Steel Determined by a New Method Utilizing Nanoindentation James E. Wittig, Vanderbilt University, DMR INTELLECTUAL MERIT Characterizing the single-crystal elastic behavior of austenitic Fe-Mn based twinning-induced plasticity (TWIP) steels is important to make accurate stacking-fault energy (SFE) measurements. The SFE is obtained from equilibrium partial-dislocation separations (measured by transmission electron microscopy) and the elastic repulsive force acting on the two dislocations. The magnitude of the force is dependent on the dislocation habit plane ({111}), character angle and three single-crystal elastic constants C 11, C 12 and C 44. Investigations of elastic anisotropy typically rely on single-crystal material which can be difficult to synthesize or not readily available. Therefore, a major objective of this research was to develop a new method to determine single-crystal elastic constants. Nanoindentation in combination with orientation imaging microscopy (Figure 1) yields crystallographic direction specific indentation moduli, M hkl (Figure 2). A mathematical model was developed to calculate the single-crystal elastic constants from the ratio M 111 /M 100 and polycrystalline elastic data. Values of C 11, C 12 and C 44 of 174±7, 85±3, and 99±4 GPa determined for an Fe-25Mn-3Al-3Si alloy are some of the first experimental values reported for these materials. The uncertainty of this method was tested using values of M hkl available for W, Al, Cu, and β-brass and found to be within ~4% of establish literature values of C 11, C 12 and C 44. Fig. 1 – Grain orientation map of an Fe-25Mn- 3Al-3Si steel with SEM image of an indented grain oriented in. The dashed line marks an annealing twin boundary (tb) perpendicular to the surface. Grain boundaries (gb) are marked.

BROADER IMPACTS Dean Pierce, a Vanderbilt Ph.D. candidate, performed an NSF IGERT funded research collaboration with the Mechanics of Materials and Nanostructures department and Dr. Rudy Ghisleni at EMPA in Thun, Switzerland in This collaboration is ongoing and has produced the following notable impacts: Introduction to new techniques and instruments for characterizing material, including nanoindentation (see figure 2). Publication of a new method for determining single crystal elastic constants from polycrystalline specimens [Pierce et al., Mater Sci Eng A 578 (2013) 134]. Presentation of the results at Microscopy and Microanalysis 2013 in Indianapolis, IN. This research will positively impact the broader research goals of quantifying the relationship between SFE, deformation mechanisms and mechanical properties in these steels. In addition, the methodology developed in this research can be applied to other material systems. Fig. 2. (a) Indentation moduli from the, and orientations vs dihedral angle for an Fe-25Mn-3Al-3Si alloy. Indentation moduli is obtained from the slope of the unloading portion of the (b) load vs. displacement curve (Oliver-Pharr method). Single-Crystal Elastic Constants of High-Manganese TWIP Steel Determined by a New Method Utilizing Nanoindentation James E. Wittig, Vanderbilt University, DMR