K. Linga Murty, North Carolina State University Project # MET DMR  The main objectives of this project are to a) study the effect of alloying and heat treatment on the biaxial creep behavior of Ti-3Al-2.5V (wt.%) alloy, and b) investigate deformation microstructures with particular emphasis on dislocation substructure formation during biaxial creep, and develop microstructure-based creep constitutive equations.  Thin-walled tube specimens of recrystallized (Rx) Ti- 3Al-2.5V are tested in a wide range of temperatures and stresses at various stress ratios (hoop stress to axial stress, 0-2) utilizing a combination of internal pressurization and axial loading. Creep data are analyzed in the light of creep deformation theories. Transmission electron microscopy is conducted to study the microstructure before and after creep.  Fig. 1a shows that there are three distinct regimes in the plot of normalized hoop strain rate versus hoop stress, i.e. with stress exponent (n) of 1 (diffusional creep) at lower modulus-compensated stress (  /E); n=2 (grain boundary sliding) at intermediate  /E values, and n=6 (dislocation-climb controlled process) at higher  /E values. At  /E = 6  10 -4, the steady state microstructure reveals dislocations rearranging into sub-boundary formation (Fig. 1b) whereas at  /E = 2  straight dislocations are observed in the grain interior (Fig. 1c). At  /E = 5  10 -3, distinct subgrains are formed with a low dislocation density (Fig. 1d). Further creep tests at other stress ratios and detailed TEM study are under progress. Fig. 1. (a) A plot of normalized strain rate vs. stress in a Rx Ti32.5 alloy crept at a stress ratio of 2. (b)-(d) bright field TEM images showing microstructural features at various modulus- compensated stresses. Effect Of Alloying And Heat Treatment On The Biaxial Creep Behavior Of Titanium Alloys  /E=6   /E=2   /E=5  (a) (b) (c)

Broader Impact:  Potential applications for titanium alloys exist in the aerospace and chemical industries. The research will significantly augment our understanding of structure- property correlations in these alloys, and open up new opportunities for their future applications.  As a part of ‘Young Investigator’s Summer Program 2006’, three high school students (Eric, Diana and Patrick) were involved in a two-week long research project. After completion of their project, they presented their research results. We have used funding for the project from our NSF grant. This is an ideal way to recruit bright students to science and technological field.  Many undergraduate students (please see the list on the bottom right corner) have been supported through this grant. Many of them have shown interest in pursuing graduate studies. Full-time graduate students and a postdoctoral researcher have been contributing towards achieving the project objectives and at the same time, furthering their educational goals.  Many journal papers and conference presentations have been made through which results are being disseminated in the widest possible way. K. Linga Murty, North Carolina State University Project # MET DMR Effect Of Alloying And Heat Treatment On The Biaxial Creep Behavior Of Titanium Alloys Personnel: Undergraduate students: Gabe Ramos, Sev Johansson, Anthony Rice, Kyle Kooper Graduate Students: Srikant Gollapudi, Jinyuan Yan, Walid Mohamed Postdoctoral Researcher: Indrajit Charit High school students participating in the research project as a part of the Young Investigator’s Summer Program at the NC State Current laboratory personnel shown above