Twinning Studies via Experiments and Theory

Huseyin Sehitoglu, University of Illinois, DMR 0803270

The intellectual focus in this work is threefold. The first is to predict the twinning stress for alloys. The second is to develop digital imaging correlation techniques at small length scales to uncover precisely twin nucleation stress, the twinning shear strain, and measure twin volume fraction. The third is to understand the role of twin-slip and twin-twin interactions on strengthening with experiments and theory.

The equation (derived from mesoscale dislocation mechanics) shows that critical twin nucleation stress depends on unstable SFE and unstable twin SFE barriers (N=3 for nucleation).

tsf isfcrit ut us isf

twin twin

22 3N 21

3Nb 4 2 3Nb

The figure (generalized plane fault energy curve)shows unstable SFE and unstable twin SFE barriers.

Planar defect energies (in mJ/m2) versus displacement along [11 2] of layers above each successive fault plane, i.e., intrinsic SF (isf) at 1, extrinsic SF (esf) at 2, twin fault (tsf) at 3, with additional layers extending the twin region. Unstable fault energies are at half units. Note the role of Al on the

results.

Predicted (squares) and experimental (circles) critical shear stress for Cu-Al and selected fcc metals. Note the decrease in critical stress with increase in Al content in Cu-Al alloys. The experimental results taken from several studies show a

variation.

Twinning Studies via Experiments and Theory

Huseyin Sehitoglu, University of Illinois, DMR 0803270

The broad impact of the work is to design new alloys with prediction of mechanical properties without lengthy experimental trials, and develop a materials design methodology that is widely applicable to various industries. For example, our focus includes Fe-Mn fcc steels that are utilized in special rail applications (for example in crossings). We are organizing an international symposium in Istanbul, Turkey where phase transformation and twinning studies will be

highlighted.

Generalized plane fault energy curve along the <110> direction in {100} plane for Cu.

The incident slip a/6[2 1 1] separates into

an a/6[-2 1 1] partial and an a/3[011] full dislocation. Schmid factor of a/6 [-211](111) orientation is 0 and in a/3[011](100) orientation is 0.477.Energy barrier for slip

in this case is expected to lead to significant strain hardening.

[111]

a/6 [2 1 1](1 -1 -1)

a/3 [0 1 1] (1 0 0)

a/6 [-2 1 1](111)

Matrix

Twin

Matrix

Twin boundary

We study the energetics of twin-slip interactions and twin growth under loading in different directions. The incident slip a/6[112] separates into an a/6[-2 1 1] partial and an a/6[301] stair rod dislocation. Schmid factor of a/6 [-211](111) orientation is 0.231 and in a/6[301](010) orientation is 0. The twin grows by one atomic layer as partial traverses across the interface.

[010]

a/6 [-2 1 1] (1 1 1)

a/6 [ 112] (-1 -1 1)

a/6 [3 0 1]](010)

Matrix

Matrix Twin

Twin boundary

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

500

1000

1500

2000

Precise experimental micro- and macro-measurements of strain hardening (slope of stress-strain curve) utilizing digital image correlation. Evolution of the strain hardening rate in {111} Fe-Mn crystals subjected to tension for the specimen with twin-twin and twin-slip intersections (blue curve), and without intersections (red curve). The dotted red line represents the maximum strain hardening rate found in a 200 micron2 region (red dotted box), and the dotted blue line represents the minimum strain hardening value obtained in a 200 micron2 region at the center primary twin-band (black dotted

box). The strain fields are scaled 0-5% to show the details.