Nanoscale Self-Organization of Metallic Alloys under Ion Irradiation

Robert Averback and Pascal Bellon, University of Illinois, MET DMR Award# 0407958

Motivation for the research:Alloys are subjected to external forcing in a number of situations: wear, fatigue, ball milling and energetic particle irradiation, and it has been observed that these alloys can be driven far from their equilibrium phases. Theoretical efforts to understand these phenomena1 have led to the construction of “dynamic phase diagrams” like the one shown below. Here the dynamical variables are, R, the distance over which atoms are forced to move by the driving forces (i.e. shearing, or irradiation), and , the ratio of forced jumps to thermally activated jumps. Our experiments are designed to test these theoretical concepts and aid in their development. We use ion beam irradiation since the variable R and can be carefully controlled.

Significant findings:Our experiments employ thin films of the eutectic alloy, Cu85Co15, subjecting them to 1.8 MeV Kr irradiation at different temperatures.2 This procedure samples the trace through the phase diagram, indicated by the red line. We use the superparamagnetic behavior of small Co particles to determine the precipitate size. As shown in the figure below, we find that at irradiation temperatures below 300 °C, the precipitates reach a steady state size, illustrating the patterning regime in the dynamic phase diagram. By raising the temperature above 350 °C, decreasing , the growth of the precipitates does not saturate, indicating that the system has crossed into the regime of macroscopic phase separation, or coarsening. This work thus confirms the theoretical predictions, and it has begun to determine the phase boundaries in a real alloy.

References: 1. R. Enrique and P. Bellon, PRL 84, 2885 (2000).2. P. Krasnochtchekov, R.S. Averback and P. Bellon, Phys. Rev. B (in press)

DMR METALS RESEARCH PROGRAM

Nanoscale Self-Organization of Metallic Alloys under Ion Irradiation

Robert Averback and Pascal Bellon, University of Illinois, MET DMR Award# 0407958

Broader impact:

The current and future experiments of this research aim at providing critical tests of concepts and models developed in the so-called driven alloy framework. The present results provide direct evidence for compositional patterning induced by ion-beam processing, in agreement with model predictions. Because these findings are quantitative they can be directly extrapolated to other external forcing situations, and they should also be of enormous help in designing materials for nuclear energy applications.PB is currently writing a chapter on the “Relaxation on nonequilibrium alloys” for a book entitled Alloys Physics (Ed. W. Pfeiler, pub. Wiley VCH), which will highlight these results. PB has also submitted a proposal to the MRS to organize in fall 2006 a symposium on a theme that includes the present research: “Thermodynamics and kinetics of phase and microstructural evolution”.

A Materials Van to bring materials science demonstrations to local high schools:

The two graduate students supported by the grant as well as two female undergraduates from our Department have identified and prepared 10 materials science demonstrations that will be presented this fall to a science class (11th grade) at a local high school:- work hardening of an aluminum strip- heat treating and quenching of a piano wire- shape memory alloys- bulk amorphous metals for golf club heads- heat resistance of a ceramic tile- gelation of polyvinyl alcohol- synthesis of nylon- magnetic levitation with a high-Tc superconductor- conduction of light by optic fiber- grain boundary decohesion of Al by Ga wetting

Several local high schools have also been contacted and are interested in scheduling a visit from our Materials Van.

DMR METALS RESEARCH PROGRAM