Force chemical mixing during severe plastic deformation

Robert S. Averback, University of Illinois at Urbana-Champaign, DMR 1005813

Understanding how atoms reorganize during severe plastic deformation is fundamental to developing new high-strength structural materials that are resistant to extreme environments. Here we show, using High Pressure Torsion (HPT) experiments, that atoms move superdiffusively in two-phase Cu-Ag alloys during severe plastic deformation.

The key findings are:

•The dissolution rate of precipitates with strain depends linearly on precipitate size, as shown on the right, and not on its square, as expected for dissolution by ordinary diffusion mechanisms.

•These experimental results on Cu-Ag are well explained by our dislocation glide mechanism of chemical mixing.

These findings will expedite the development of new processing schemes for advanced nano-composite materials with unique properties. They also provide new insights into such processes as sliding wear, erosion, and fatigue.

Z-contrast STEM and atom probe tomography show the dissolution of Ag precipitates during HPT processing. Model predictions and experimental data show good agreement: a linear dependence on particle radius and approximately correct magnitude.

Increasing Strain

Increasing Strain

Cu Ag

Force chemical mixing during severe plastic deformation

Robert S. Averback, University of Illinois at Urbana-Champaign, DMR 1005813

Undergraduate laboratory on non-equilibrium processing of advanced metals:The procedures developed in this research have been transferred to the Senior Metals Laboratory to teach non-equilibrium processing of nano-crystalline and amorphous materials. Students use our milling tools to prepare non-equilibrium alloys by ball-milling methods. Two examples are homogeneous Cu-Ag alloys, as shown on the left, and Cu-Y metallic glasses. Students then measure the properties of these non-equilibrium materials as they move toward equilibrium during thermal annealing. Such properties include, grain size, hardness, or the activation energy of crystallization. Importantly, this laboratory is open-ended so students can develop their own ideas. A graduate student supported by this proposal works closely with the students.The Laboratory has two primary goals:

•Provide a deeper understanding of the concepts of non-traditional processing and the properties of advanced metals.

•Develop interest and excitement amongst undergraduate students for careers in materials research

An example of an undergraduate’s work is illustrated above, showing the change in the diffraction pattern as a Cu-Ag powder is balled-milled (left), and subsequently annealed (right). Solubility and particle size were investigated as a function of milling time and annealing temperature.

X-ray diffraction

time

tem

p

An undergraduate and graduate student working together to press ball-milled samples into dense pellets. The laboratory educational activities are a direct extension of the current research. This project has allowed us to integrate education and research in a meaningful way.