TECHNICAL SUMMARY: The proposed effort consists of five tasks, each bearing upon the stated aim of the proposal, namely to develop a detailed understanding of the role of various structural instabilities on microstructural evolution in titanium alloys. The five tasks are: determination of the factors governing intra-granular nucleation of alpha in the beta matrix, precipitation of allotriomorphic alpha-titanium on prior beta grain boundaries and formation of alpha sideplates in titanium alloys, characterization of the formation of the basket-weave microstructure, characterization of the formation of refined laths of (secondary) alpha-titanium in ribs of beta-titanium, and a summarizing task involving the development of a unified mechanistic model of nucleation and phase separation in titanium alloys. The research will involve primarily the application of novel state-of-the-art characterization tools to study critical issues related to microstructural evolution in multi-phase titanium alloys. Emphasis will be placed on determining the mechanisms underlying the early stages of second phase nucleation. In addition, the elemental partitioning between the different phases and compositional profiles at interphase boundaries in these alloys will be determined at the highest achievable accuracy. A concurrent theme will be to determine the accuracy, fidelity and interpretability of data and information obtained from the two different types of analytical procedures, namely scanning transmission electron microscopy-based electron energy-loss spectroscopy and 3D local electrode atom probe tomography.

NON-TECHNICAL SUMMARY: The proposed program focuses on a study aimed at formulating a detailed understanding of the role of various structural and compositional instabilities on microstructural evolution in titanium alloys. These alloys have been applied in a number of product areas, including commercial aircraft jet engines and, in contrast, bio-medical engineering, largely because of a combination of attractive properties and because they are of relatively low density. The full exploitation of titanium alloys requires manipulation of their microstructures to affect a range of combinations of properties, and this in turn requires a detailed understanding of the processes involved in the evolution of these microstructures. There are many significant unanswered questions concerning the evolution of microstructures in titanium alloys, especially regarding the early stages of second phase nucleation. It is necessary to focus effort on developing a more thorough understanding of these processes, and this is a central objective of the present proposal. Recent significant increases in the level of sophistication of characterization tools present a real opportunity to make detailed observations of the early stages of microstructural evolution, and much of the work proposed here is based on the application of these new and improved techniques of materials characterization.

Project Report

" Award Number: DMR- 1006487 Expiration: August 31st, 2013 Nature of the project: In most structural materials, for example, ones that find application in jet engines and planes, automobiles, and metallic prostheses, the microstructure of a given material plays a significant role in determining the mechanical properties. Hence, it is very important that a detailed understanding be achieved of the ways in which microstructure can be varied, so that various combinations of mechanical properties may be obtained. The present project primarily focused on developing a fundamental understanding of the role of instabilities on microstructural evolution in titanium alloys. These instabilities are inherent in one of the constituent phases of titanium alloys, and a major goal of this project was to develop a better understanding of these instabilities and their consequent influence on the precipitation sequences in these alloys, and so the various forms of the microstructure. This developed understanding leads to new microstructures, and so new interrelationships between microstructure and properties. Outcomes of the project: This is one of a very few programs of research aimed at characterizing nucleation mechanisms by use of two state-of-the-art techniques, the first, aberration-corrected (Scanning) Transmission Electron Microscopy (OSU) and the second, Atom Probe Tomography (UNT). This has resulted in the discovery of two new modes of nucleation, one involving a mixed-mode mechanism for nucleation of the omega phase and the second involving the pseudo-spinodal mechanism for precipitation of the alpha phase, both in titanium alloys. The omega phase is thought to influence the precipitation sequences in titanium alloys, and so a detailed understanding of the formation of that latter phase is extremely important. The discovery of the new pseudo-spinodal pathway for phase transformations in titanium alloys is important because it leads to the formation of very refined distributions of the alpha phase, and this results in enhanced mechanical properties. Intellectual merit: The intellectual merit involves the determination of nucleation mechanisms in titanium alloys. In materials sciences, nucleation is one of the most difficult phenomena to understand as it occurs on such a refined scale (i.e., atomic scale). However, it is one of the most important to understand as it controls the evolution of microstructures. The detailed mechanistic understanding derived in this program now permits computational materials scientists to develop more accurate and physically-based models to predict the evolution of microstructure – a key part of Integrated Computational Materials Engineering (ICME), which, being at the core of the Materials Genome, is an activity of national importance. Broader impacts: Regarding impacting the field of materials sciences, firstly, we have coupled two state-of-the-art techniques, aberration-corrected scanning transmission electron microscopy with 3D atom probe. Secondly, we have discovered a novel mixed-mode coupled displacive-diffusional mechanism of solid-solid transformations involving a composition-dependent displacive component, based on the study of omega phase precipitation in titanium alloys. Thirdly, we have discovered a novel mechanism of fine scale alpha precipitation in titanium alloys based on the concept of pseudo-spinodal decomposition. Regarding impacts on human resources, at the Ohio State University, two table-top scanning electron microscopes (SEMs) have been introduced in the sophomore level materials characterization course at OSU. This has permitted an increased personal use by the students that has resulted in a very significant increase in the level of excitement and interest. New teaching modules are being developed, based on experiments done using these table-top microscopes, that can be used by other institutions. Furthermore, children are being exposed to the wonders of materials science and engineering, via materials characterization, through the Center of Science and Industry (COSI) Columbus, by providing demonstrations on the table-top SEMs. These SEMs have been set up at the Center of Science and Industry (COSI) in Columbus. Thus, graduate research assistants (GRAs) from the PI’s research group at the Ohio State University brought FEI Phenom scanning electron microscopes to the Center of Science and Industry (COSI) for Nano Science Day in April of 2012. The present program has had a direct impact on educational outreach activities being carried out in materials science and engineering at the University of North Texas. Thus, high-school students in the DFW metroplex are regularly participating in the Materials Day program wherein these students are exposed to fascinating techniques such as the 3D atom probe, and its ability to probe composition of materials at nanometer scales.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Diana Farkas
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Ohio State University
United States
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