The Role of Instabilities on Microstructural Evolution in Titanium Alloys

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AbstractTECHNICAL 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.