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.
At high stresses bulk immiscible or insoluble metals can be forced to mix. The process shares similarities to mixing of insoluble liquids. Consider, for example, shaking a vial of oil and water, which are normally immiscible and phase separate – oil floating on top of the more dense water. When the vial is shaken the interface between the two at first roughens, and then 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 intensity until finally a homogeneous emulsion is obtained. Similarly, metals that typically do not like to mix can be forced to form solutions when mixed vigorously by high strain deformation. While the two processes share some common qualitative features, the processes occurring at the atomic level that lead to mixing and phase separation in the solid and liquid systems differ dramatically. Mixing in crystalline solids, such as metals, has not been studied in the great detail that mixing in liquids has and thus this work was undertaken to expand our understanding of mixing in solids. In a similar way that emulsions formed from two immiscible liquids results in an emulsified liquid with properties distinct from the two components, mixing metals through deformation provides opportunities to create alloys with unique properties. The scientific framework for understanding how metals mix during mechanical shearing, however, must be developed before predictive engineering can be utilized to optimize materials properties. Our work made major contributions to understanding this problem. We quantified the rate at which atoms mix during shearing of crystals for systems with different chemical properties. We also investigated the importance of processing temperature in affecting the competition between shear induced mixing and the de-mixing stemming from equilibrating forces.. We found that temperature can be used to precisely tune the nanoscale structure of the materials and that the shearing rate has limited effect on the evolution of the materials. Computational simulations and theory were developed to understand and interpret the experimental results. We specifically demonstrated how these materials have unique mechanical properties and high wear resistance. The project resulted in the training of two PhD students and several undergraduate researchers. The PhD students successfully defended their theses and are currently gainfully employed in their field of study. It also enabled the development of computational educational tools to complement university coursework. These tools have been made publically available.
