Max-Planck-Institut für Eisenforschung, GmbH Highly localized slip traces observed near grain boundaries on slip systems with lower (global) Schmid factors Slip trace maps show that Prism slippyramidal a c+a Prism slip is most evident, then pyramidal a, then c+a; basal basal slip is not obvious Tensile Axis 10  m Localized topography develops at boundaries Thin plane traces have lower resolved shear stress (Schmid factor) 3-D X-ray investigation Thomas R. Bieler, Michigan State University, DMR

Max-Planck-Institut für Eisenforschung, GmbH Deformed ~8% - many voxels near surface can’t be easily indexed due to large local strains 3-D x-ray map of undeformed region near triple point 3-D X-ray diffraction reveals highly localized strain Diffraction pattern from grain interior Diffraction pattern from volume near grain boundary has streaks indicating localized ~10  grain rotations Tensile axis (APS beam line 34-ID-E, G.E. Ice, W. Liu) EBSP map of surface Thomas R. Bieler, Michigan State University, DMR

Max-Planck-Institut für Eisenforschung, GmbH After ~1-3% strain, dramatic topography develops near many grain boundaries Atomic force microscopy (AFM) measurements show dramatic surface topography near grain boundaries. Mechanical twins and slip steps from slip bands are apparent –After ~8% strain, some grain boundaries show elevation changes up to several microns Thomas R. Bieler, Michigan State University, DMR

Max-Planck-Institut für Eisenforschung, GmbH Thomas R. Bieler, Michigan State University, DMR Prism slip stimulated mechanical twin nucleation at grain boundary (   ~ 3-8%) Tensile Axis  m Luster-Morris Parameter (LMP) m' = cos  ∙ cos  NdNd NtNt btbt   Grain Bdy  bdbd {1012}  1011  Metall Mater Trans 26A (1995) 1745 AFM

Max-Planck-Institut für Eisenforschung, GmbH Is cos  ∙ cos  predictive? It is a strong statistical indicator, but not deterministic Burgers vector colinearity is more strongly correlated than plane coplanarity Will Deformation twinning nucleated at grain boundary lead to a crack at larger strain? 14 cases of prism slip correlating with twinning found in two samples 20 observations of prism slip next to a grain with high twin Schmid factors Twinning always correlated with the highest m' value, but twins do not always occur with high m’ Thomas R. Bieler, Michigan State University, DMR

Max-Planck-Institut für Eisenforschung, GmbH Twin interactions at grain boundary may have caused cracking at ~15% tensile strain, 100  m The green and turquoise orientations are mechanical twins that dominate volume along grain boundary – did the twin-grain boundary interaction ultimately cause the crack? Thomas R. Bieler, Michigan State University, DMR