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Session Chairperson: P.J. Bania, Timet, P.O. Box 2128, Henderson, NV 89015
PHASE EQUILBRIA IN Ti-Al-Nb ALLOYS: K. Muraleedharan, Dept. of Materials Science & Eng., Carnegie Mellon University, Pittsburgh, PA 15213; D. Banerjee, Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500058, India
The phase equilibria in the technologically important Ti-Al-Nb ternary system are presented. Special emphasis is made in the composition regime where the Orthorhombic (O, Ti2AlNb, Cmcm) phase has considerable presence in the microstructures. Isothermal sections at temperatures at and above 700°C are presented along with three vertical section at 22.5, 25 and 27.5 atomic % Al levels. The difficulties experienced in determining the phase diagrams, arising out of low diffusivities and the nature of the alpha2+B2O peritectoid transformation, are highlighted. The presence of a large solubility range for the single phase O phase field close to an Al content 27.5% is discussed in the light of the experimental results. Finally a clear definition of the areas wherein there exists a lack in our current understanding of the Ti-Al-Nb system are highlighted.
PROCESS/MICROSTRUCTURE/PROPERTY RELATIONSHIPS IN AN ORTHORHOMBIC TITANIUM ALUMINIDE ALLOY: A.P. Woodfield, GE Aircraft Engines, 1 Neumann Way, Cincinnati OH 45215; S.K. Srivatsa, V.K. Vasudevan, University of Cincinnati, Cincinnati, OH 45215
An orthorhombic titanium aluminide alloy ingot with a nominal composition of Ti-22Al-26Nb (atom percent) has been produced and converted to 6" diameter billet. A continuous cooling transformation diagram was generated using compression samples, and TEM performed to understand phase evolution during cooling. A series of melts were cut from the 6" billet and either forged into pancakes or rolled into rings using a variety of process routes. The effects of process variables and final heat treatment on the microstructure and mechanical properties of the pancakes and rings have been evaluated. Mechanical properties included tensile (room temperature to 1300°F), creep (1000° - 1200°F), fracture toughness (room temperature) and fatigue (room temperature to 1200°F).
THE ORDERING BEHAVIOR OF THE ORTHORHOMBIC PHASE IN Ti-Al-Nb ALLOYS: K. Muraleedharan, D. Banerjee, Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderbad 500058, India
The ordering behavior of the orthorhombic (O, Ti2AlNb, Cmcm) phase are presented. The presence of the O phase in two distinct forms with different site occupations is discussed. They are (1) the O1 phase Nb atoms occupying the Ti sublattice randomly and (2) the O2 phase with a specific sublattice for the Nb atoms. Both O1 and O2 exhibit the same space group and lattice periodicity and this makes their identification from one another a difficult task. We present in this paper, the various diffraction effects which arise due to the different ordering behavior of the O phase, for both X-ray and electron diffraction. Specific reflections are singled out. A definition of the order parameter and its experimental determination are presented. Possible transitions between the two forms are presented in the spirit of the Landau-Lifshitz theory for order-disorder transformations.
EFFECT OF EXPOSURE ON FATIGUE OF COATED AND UNCOATED ORTHORHOMBIC Ti- ALUMINIDES: J.R. Dobbs, M.F.X. Gigliotti, GE CR&D, P.O. Box 8, K-1, MB105, Schenectady, NY 12301; M.J. Kaufman, Dept. Materials Science and Engineering, Univ. of Florida, 201 Rhines Hall, Gainesville FL 32611
Both uncoated and coated fatigue samples of the alloy Ti-22Al-26Nb were subjected to elevated temperature exposures in air. Oxygen ingress and interdiffusion (and reaction) of elements between coating and substrate cause changes in both microstructure and properties (i.e., embrittlement). After exposure, the microstructures of the substrate, the reaction zone and the oxidation products were examined, and fatigue tests were conducted. The suitability of selected coatings for protection of orthorhombic alloys against property degradation from exposure at elevated temperatures will be discussed.
3:30 pm BREAK
3:50 pm INVITED
ORTHORHOMBIC TITANIUM ALUMINIDE ALLOY DEVELOPMENT: C.G. Rhodes, Rockwell Science Center, Thousand Oaks, CA; J.C. Chesnutt, General Electric Aircraft Engines, Evendale, OH; J.A. Hall, Allied Signal Inc., Phoenix, AZ; J.R. Porter, Rockwell Science Center, D.A. Miracle, M.L. Gambone, Materials Directorate, USAF, Wright-Patterson AFB, OH
The development of a Ti-based metal matrix composite (MMC) material, capable of sustained and repeated exposure to service temperatures in the range of 650-760°C (1200-1400°F), has been undertaken in a U.S. Air Force sponsored program. Several performance characteristics qualify as "critical" for specific turbine engine component applications and service environments. To define these component-critical MMC characteristics, we selected a turbine engine compressor rotor, in which transverse tensile and creep strengths were found to be critical issues. Improvements in MMC transverse tensile and creep strengths can be made by increasing the matrix properties through alloy chemistry modifications, as well as through microstructural manipulation. This program, and others, have examined, in a systematic way, the effects of elemental additions, such as Mo, Ta, Si, V, and O, on mechanical properties, environmental resistance, and processability of Ti-Al-Nb alloys. These efforts have led to the state-of-the-art MMC matrix orthorhombic titanium aluminide alloy. This work was supported by Contract F33615-91-C-5647.
THE EFFECT OF HEAT TREATMENTS ON THE MECHANICAL BEHAVIOR OF AN ORTHORHOMBIC Ti-ALLOY: S. Lutjering, D. Eylon, University of Dayton, Dayton, OH 45469-0240; P.R. Smith, Materials Directorate, Wright Laboratory, Wright-Patterson AFB, OH 5433-7817
Titanium aluminide alloys containing the ordered orthorhombic phase, based on Ti2AlNb, are considered as potential materials for the compressor section of aerospace engines both in their monolithic form and as matrices in metal matrix composites. The microstructure (volume fraction of the ordered alpha-2, B2 and orthorhombic phases, grain size, lath size and spacing) has a significant effect on the mechanical properties of these materials. In this study two heat treatments of the orthorhombic alloy Ti-22A1-23Nb resulting in a fully transformed and a duplex microstructure were investigated. Room temperature and elevated temperature tensile and cyclic behavior including fatigue life, fatigue crack initiation and propagation will be discussed with respect to these microstructural conditions.
EFFECT OF MICROSTRUCTURE ON THE CREEP AND RT TENSILE BEHAVIOR OF O+B2 ALLOYS: C.J. Boehlert, B.S. Majumdar, V. Seetharaman, UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432-1894
The phase evolution, room temperature tensile, and elevated temperature creep behaviors of Ti-25Al-25Nb and Ti-23Al-27Nb have been examined. A variety of microstructures, consisting of different volume fractions of B2 and O were achieved through deformation processing and heat treatments. Supertransus heat treated microstructures suffered from intergranular attack and exhibited low ductility. Subtransus heat treated microstructures exhibited a balance of RT tensile and elevated temperature creep performance. Creep samples showed evidence of microstructural instability, involving cellular/widmanstatten precipitation of O phase within the B2 grains. The kinetics of O phase precipitation at the creep temperature, 650°C, and the effect of this instability on creep are presented. In addition, overall effects of microstructural features on tensile, creep, and damage accumulation are discussed.
PRODUCTION OF HIGH TEMPERATURE TITANIUM ALUMINIDE FOILS VIA PLASMA SPRAY PROCESSING: R.S. Thakur, Mohit Sisodia*, M.K. Bhargava, Materials Procurement Division, Hindhustan Motors Company, Indore, India; *Dept. of Metallurgical Engineering, Malaviya Regional Engineering College, Jaipur 302 017, India
Plasma Spray Forming is a upscaled version of droplet deposition method from the melt, which combines the steps of melting, rapid solidification, and consolidation into a single operation. The present paper critically reviews the versatility of this process which is successfully applied to the synthesis of Ti alloys and Ti- aluminide preforms for subsequent cold rolling into thin foils. These Plasma Sprayed preforms are transformed into dense wrought Ti-aluminide foils by a roll consolidation process. It has been observed that production of this dense foils are as continuous coil, which would improve process efficiency and yield high quality at low cost. Actually these high strength Ti alloys and Ti-aluminide foils are required for fabricating composite structures for advanced aerospace technology and space shuttles. In addition to it, various parameters are discussed which primarily includes designing of optimum interface, growth kinetics etc., and became necessary in enhancing service life of these foils at elevated temperatures.
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