Forced Mixing and Nanoscale Self-Organization During Severe Plastic Deformation of Complex Metal Alloys

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

TECHNICAL SUMMARY: High energy processing of metals using severe plastic deformation (SPD) by ball milling, accumulative roll bonding, or equal channel angular processing is becoming an attractive means to fabricate high-strength materials. The current proposal seeks to develop the science of this processing scheme, focusing on two critical features, forced chemical mixing and self-organization. Forced chemical mixing is the athermal rearrangement of alloy components by dislocation motion. Presently this process is only poorly understood, owing in large part to past experimental difficulties in controlling and characterizing the total strain, local temperature, and stress state during SPD. The present research will employ high pressure torsion experiments, for which the process variables are well defined. The work will test theoretical models, which predict that atomic mixing during SPD is superdiffusive rather than Fickian and that the mixing behavior should reveal strong influences of the thermochemical and thermomechanical properties of the alloy components. The second key component of the research concerns mesoscale self-organization in alloys during SPD. The models predict that many phase separating alloys should self-organize into compositional patterns on a nanometer length scale when subjected to SPD at high temperatures. These models further indicate the surprising result that such patterning may even occur at low temperatures, when diffusion processes mediated by point defects are suppressed. The research will consider at first model binary systems to test model predictions, but will then build complexity by considering ternary and even quaternary systems. Characterization of the microstructures of the nanostructured materials will include transmission electron microscopy methods, atom probe tomography and x-ray diffraction. NON-TECHNICAL SUMMARY: It has been recognized recently that materials subjected to SP often possess excellent properties for use in extreme environments; they have very high strengths and they tend to be resistant to damage by energetic particle irradiation, such as in a nuclear reactor. The goal of this research is to provide the scientific basis for processing nanocomposite structures and how to design them with properties tailored to specific applications. The mechanism by which atoms intermix and randomize during SPD is analogous to shaking a vial of oil and water, which are normally immiscible - oil floating on top of the more dense water. When the vial is shaken, the interface between the two at first roughens, and as the intensity of the shaking increases, small globules of water form in the oil and vice-versa. The sizes of the globules decrease with the intensity of shaking until finally a homogeneous emulsion is obtained. The intermixing of two immiscible metals during SPD and the length scales and patterning of the "globules" of solid phases in the newly formed nanocomposite materials are of primary interest. The proposed research has broad scientific impact for developing design strategies for new nanocomposite materials that are critical to a number of advanced materials applications: hydrogen storage, batteries, radiation-resistant nuclear materials, etc. In addition to disseminating our research through publications and scientific meetings, educational demonstrations of these new materials will be developed for the "Materials Mobile" at UIUC which introduces concepts of materials science and engineering to high school students around the State of Illinois.