Determination of Activation Energies and Modeling of Low Temperature Creep of Alpha, Alpha-Beta, and Beta Titanium Alloys S. Ankem, FASM, University of Maryland, College Park, DMR MET# OVERVIEW Titanium alloys have many applications including aerospace, naval, biomedical, and nuclear. Often, titanium components are subject to loading for long periods of time at low temperatures. Titanium has long been known to creep, or time-dependent permanent deformation at low temperatures. Therefore, it is important to understand the mechanisms of creep deformation to predict the mechanical reliability of the structures and to design advanced materials with improved creep resistance. APPROACH This investigation involves experimental, analytical, and theoretical components. Mechanical testing is enhanced by scanning and transmission electron microscopy. RESULTS A model was developed to explain the unusual phenomenon of slow twin growth during creep of single phase  and  Ti alloys. It was shown that dislocation pileups in large-grained alloys give the stress-concentration necessary for twin nucleation. Twin growth is rate-limited by the diffusion of oxygen away from the growing twin. The activation energy for twin growth based on this model compares favorably with the experimentally measured activation for creep of  and  Ti alloys. TEM micrograph of twins growing from a dislocation pileup at a grain boundary

EDUCATION One graduate student and one undergraduate student were involved with research under this grant. The graduate student, Dr. Greg Oberson, completed his PhD in November The undergraduate student, Mr. Max Grace, was the first undergraduate to receive this prestigious scholarship award from the ARCS Foundation for his ongoing work with Dr. Ankem. OUTREACH The outcomes of the research are being communicated to the professionals in the field and to the general public in the form of various national and international conferences and by publishing the papers in prestigious journals. Publications and presentations in the past year include: -S. Ankem, et al, “Mechanical Properties of Alloys Consisting of Two Ductile Phases”, Prog. Mater. Sci., Vol. 51, pp A. Jaworski, Jr. and S. Ankem, “Influence of the Second Phase on the Room-Temperature Tensile and Creep Deformation Mechanisms of  Titanium Alloys, Part I. Tensile Deformation”, Metall. and Mater. Trans. A, Vol. 37A, pp A. Jaworski, Jr. and S. Ankem, “Influence of the Second Phase on the Room-Temperature Tensile and Creep Deformation Mechanisms of  Titanium Alloys, Part II. Creep Deformation”, Metall. and Mater. Trans. A, Vol. 37A, pp P.G. Oberson and S. Ankem, “Dynamics of Twinning in Beta-Titanium Alloys”, Presented at the TMS Annual Meeting, March 2006, San Antonio, TX. Determination of Activation Energies and Modeling of Low Temperature Creep of Alpha, Alpha-Beta, and Beta Titanium Alloys S. Ankem, FASM, University of Maryland, College Park, DMR MET# BROADER IMPACT Mr. Max Grace presenting his research findings