Max-Planck-Institutfür Eisenforschung, GmbH

Highly localized slip traces observed near grain boundaries

on slip systems with lower (global) Schmid factors

Slip trace maps show that

Prism slipPrism slip is most evident, then pyramidal apyramidal a, then c+ac+a;

basalbasal 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 0710570

Max-Planck-Institutfür 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 0710570

Max-Planck-Institutfür 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 0710570

Max-Planck-Institutfür Eisenforschung, GmbH

Thomas R. Bieler, Michigan State University, DMR 0710570

Prism slip stimulated mechanical twin nucleation at grain boundary ( ~ 3-8%)

Tensile Axis

1

23

4

5

6

8

9

10

1112

13

1415

100m

Luster-Morris Parameter (LMP)

m' = cos ∙ cos

NdNt

bt

Grain Bdy

bd{1012}1011

Metall Mater Trans 26A (1995) 1745

AFM

Max-Planck-Institutfür 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?

0

0.1

0.2

0.3

0.4

0.5

0.6

0.3 0.4 0.5 0.6 0.7 0.8 0.9

m' Luster-Morris Parameter

Fra

ctio

n of

obs

erva

tions

LMP twin activated

LMP no twins

• 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 0710570

Max-Planck-Institutfür Eisenforschung, GmbH

Twin interactions at grain boundary may have caused cracking at ~15% tensile strain,

100m

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 0710570