The Role of Instabilities on Microstructural Evolution in Titanium Alloys

1. The Role of Instabilities on Microstructural Evolution in Titanium Alloys Hamish L Fraser, Ohio State University, DMR 1006487 • Intellectual Merit: • This program focuses on a fundamental understanding of the role of various structural and compositional instabilities driving fine scale second phase precipitation in high strength Ti alloys for structural applications. • Two novel findings of this study: • A newly proposed transformation mechanism [1] for the w phase in titanium alloys with a continuous change in both structure and composition from bccb to hexagonal w (refer to Fig. 1) • Development of a strategy to control fine scale intragranulara precipitation in high strength titanium alloys based on the pseudo-spinodal mechanism [2] • [1] S. Nag et. al., Phys. Rev. Lett., 106, 245701 (2011). • [2] S. Nag et. al., Acta Mater., in press (2012). 700°C t = 30 mins b w Fig. 1. Enlarged aberration-corrected HAADF-HRSTEM image of rapidly cooled Ti-9at%Mo sample showing the un-displaced and partially displaced atomic columns within the matrix and wembryo respectively. On both sides of the plot, cartoons show the arrangement of atoms as seen from < 110 > bzone axis, along with the b and partially collapsed w motifs. Small arrows show the shifts of atoms along < 111 > directions. a 10 m 650°C t = 30 mins b 10 m Fig. 2 (a) A low magnification SEM backscattered image of a b-solutionized (1000°C/30mins), step-quenched and annealed Ti5553 alloy sample at 700ºC for 30 minutes showing precipitation of grain boundary aallotriomorphsand sideplates primarily at the b grain boundaries and a small number of coarse intragranulara laths. (b) Similar step-quenched annealed sample at 650°C for 30 minutes showing orders of magnitude increase in the number density of a nucleation sites as well as a highly refined microstructure.

2. The Role of Instabilities on Microstructural Evolution in Titanium Alloys Hamish L Fraser, Ohio State University, DMR 1006487 Broader Impacts: This program aims to discover the mechanisms of nucleation of second phases in titanium alloys, thus permitting a more detailed understanding of microstructural evolution in these important alloys to be achieved. The proposed research program is part of a larger effort aimed at the development of computation tools for the prediction of microstructure/property relationships in materials, directly impacting Integrated Computational Materials Engineering (ICME), and so responding to the Materials Genome Initiative. The successful implementation of the proposed research will have a significant impact on industrial exploitation of materials and hence will make a positive contribution to the Nation’s economy. The alloys which are the subject of study are being used increasingly in commercial aerospace and automotive applications and also in the orthopedic implants sector. The educational outreach programs will have a significant influence on encouraging high school students with diverse ethnic backgrounds to enter science and engineering disciplines. At OSU, two contributions to Human Resources Development have been made. Firstly, we have been using two table-top scanning electron microscopes in our sophomore level materials characterization course. Secondly, we have been providing exposure to materials characterization, and therefore the wonders of materials science and engineering, through the Center of Science and Industry (COSI) Columbus, by providing demonstrations on the table-top SEM. At UNT, due to its geographic location, Department of MSE, are in a unique position to offer such education and training to the workforce of the Dallas-Fort Worth Metroplex. In addition, the university has a substantial segment of students of Hispanic origin. These students will gain substantially from the research and education activities associated with this proposed program.