Single-Crystal Elastic Constants of High- Manganese TWIP Steel Determined by a New Method Utilizing Nanoindentation James E. Wittig, Vanderbilt University, DMR 0805295 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. Fig. 1 – Grain orientation map of an Fe-25Mn-3Al-3Si steel with SEM image of an indented grain oriented in <101>. The dashed line marks an annealing twin boundary (tb) perpendicular to the surface .