Probing the Early Stages of Second Phase Nucleation and Phase Separation in Titanium Alloys

Project Summary
The proposed program focuses on a study of the early stages of second phase nucleation and phase separation in Ti alloys. These alloys have been applied in a number of product areas, including commercial aircraft and 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 Ti alloys requires manipulation of microstructure to effect 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 Ti alloys, especially regarding the early stages of second phase nucleation and phase separation. 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 nucleation, and much of the work proposed here is based on the application of these new and improved techniques of materials characterization.
The proposed effort consists of five tasks, each bearing upon the stated aim of the proposal, namely to develop a detailed understanding of the nucleation of a-Ti in a b-matrix including the influence of the w phase andwphase separation and the formation of colony and basketweave microstructures. The five tasks are: characterization of the precipitation of allotriomorphic a-Ti on prior b grain boundaries, characterization of the formation of Widmanstätten plates from allotriomorphic a-Ti, characterization of early stages of formation of basketweave microstructure, determination of the influence of metastable phases, and, forming a unified view of nucleation and phase separation in Ti alloys.
In the main, the research will involve the application of novel state-of-the-art characterization tools to the study of critical issues related to microstructural evolution in multi-phase Ti alloys. Emphasis will be placed on determining the mechanisms underlying the early stages of second phase nucleation and phase separation in the b matrix. 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 the 3D local electrode atom probe tomography.

Intellectual merit of the proposed program
The proposed research program involves critical experimentation aimed at formulating detailed understandings of the early stages of second phase nucleation and phase separation during solid-state transformations in Ti alloys. The proposed research effort brings together state-of-the-art characterization tools for addressing these fundamental problems associated with Ti alloys. Progress is expected to be made in a wide variety of areas, including a detailed understanding of microstructural evolution in Ti alloys, and the program will lead to the development of useful new research tools for nanoscale characterization which will be applicable to a wide range of metallic materials beyond Ti alloys.

Broader impacts of the proposed research
The proposed research program is part of a larger effort aimed at the provision of computation tools for the prediction of microstructure/property relationships in materials. The successful implementation of the proposed research will result in new science and have a significant impact on industrial exploitation of materials and hence will make a positive contribution to the Nation’s economy. The provision of research tools capable of prediction of properties in these alloys will have a marked impact on industry. The educational outreach programs will have a significant influence on encouraging high school students with diverse ethnic backgrounds to enter science and engineering disciplines. Due to its geographic location, the College of Engineering and the Department of Materials Science and Engineering at UNT, are in a unique position to offer such education and training to the workforce of the Dallas-Fort Worth metroplex.