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Session Chairperson: M.F.X. Gigliotti, GE Corporate Research and Development, P.O. Box 8, Schenectady, NY 12301
8:30 am INVITED
ELEVATED TEMPERATURE MECHANICAL PROPERTIES OF THE NEAR- TITANIUM ALLOY TIMETAL 1100: J. Lindemann, A. Styczynski, L. Wagner, Technical University of Brandenburg at Cottbus, 03013 Cottbus, Germany
Fully lamellar with fine (150 m) and coarse (500 m) prior B grain sizes as well as duplex structures with p volume fractions of 20 and 60% (15 m p size) were prepared in TIMETAL 1100 by thermomechanical processing. Tensile behavior and HCF properties at R=-1 and R=0.1 were evaluated at ambient and elevated (600°C) were performed at nominal stresses of 210 and 300 MPa. The fine grained fully lamellar microstructure combines the advantages of the B annealed structures (excellent creep resistance) and duplex structures (high tensile ductility) without suffering from the disadvantages of the duplex structures, i.e., anomalous mean stress sensitivity of the fatigue strength and poor creep strength. Both properties are related to the presence of the p phase. The fatigue and creep results on the various microstructures will be interpreted in terms of phase morphologies, crystallographic textures and particular deformation mechanisms.
PRIMARY AND ANELATIC CREEP BEHVIOUR OF A HIGH TEMPERATURE NEAR -Ti-ALLOY: M. Es-Souni, Fachhochschule Kiel, Institute of Materials Technology, Legienstr. 25, Kiel 24103, Germany
The creep behavior of a high temperature near alloy Ti 6242 Si has been investigated in the stress range from 160 to 350 MPa and a temperature range from 500 to 600°C, using constant stress tensile creep experiments. Emphasis has been put on primary and anelastic creep behavior and their dependencies on stress, strain and processes microstructures. It is shown that both primary and anelastic creep strongly depend on creep strain and applied stress. Increasing stress and/or strain results in higher primary and anelastic creep strains. Furthermore, ß-heat treated specimens with a fully transformed ß-microstructure show reduced amounts of primary and anelastic creep strains, in comparison to +ß heat treated specimens with globular -grains and variable amounts of +ß mixture. The results obtained show also very similar kinetics of primary creep build up and strain recovery and suggest similar deformation mechanisms. Based on TEM investigations of dislocation structures of crept specimens, the results are discussed with respect to different mechanisms of dislocation motion under the effect of internal stress.
MINIMIZING BETA FLECKS IN THE Ti-17 ALLOY: C.E. Shamblen, GE Aircraft Engines, One Neumann Way, MD M85, Cincinnati, OH 45215-6301
Normal liquid to solid partitioning of Cr during ingot solidification for the Ti-17 alloy can result in the formation of a segregation anomaly called ß flecks. These ß flecks can exhibit a significantly lower ß transus than the alloy; they may also present mechanical property issues. While the major thrust toward minimizing ß flecks is through melt process control, this paper addresses the capability of reducing the extent of Cr segregation using a high temperature ß field homogenization heat treatment. The interdiffusion coefficients for Cr in ß phase Ti-17 were determined and used in diffusion equations to calculate the rate of dissipation of the segregated regions. Correlation is shown for the calculated, and critical experiment demonstrated, increase in the ß fleck ß transus temperature with the time and temperature of these homogenization heat treatments.
ANISOTROPIC BIAXIAL CREEP OF TEXTURED Cp-Ti TUBING: K. Linga Murty, Ratnaji R. Kola*, North Carolina State University, Box 7909, Raleigh, NC 27695-7909;*now with AT&T Laboratories, Murray Hill, NJ
Biaxial creep behavior of thin-walled tubing of cold-worked stress-relieved cp-titanium alloy was investigated using internal pressurization superimposed with axial load at ??? K. Creep anisotropy was characterized in terms of a creep locus at a constant energy dissipation with power-law dependencies of the component (hoop and axial) strain-rates under varied stress ratios. The experimental results were fit to the modified Hill's equation with anisotropy parameters, R and P which are also the contractile strain (rate) ratios. The crystallographic texture of the tubing was characterized through inverse and direct pole figures from which the crystallite orientation distribution functions (CODF) were derived. The CODF was combined with plasticity model based on power-law stress dependence of the strain-rate and dominance of basal prism and pyramidal slip systems were considered. In contrast to the recrystallized materials, experimental results deviated from the prism-model predictions albeit the deformed microstructures revealed only prism dislocations. These differences are attributed to the grain-shape anisotropy due to cold-work. Quantitative analyses of the superimposed effects of grain shape anisotropy due to cold-work. Quantitative analyses of the superimposed effects of grain shape anisotropy on grain orientation anisotropy (texture) are underway and the results to-date will be presented. This work supported by the National Science Foundation.
10:00 am BREAK
MICROSTRUCTURAL STABILITY OF A HIGH TEMPERATURE - Ti ALLOY: X.D. Zhang, J.M.K. Wiezorek, D.J. Evans and H.L. Fraser; Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210; Materials Directorate, Wright Laboratory,. WL/MLLM, Wright Patterson AFB, OH 45433
The microstructural development of Ti-6Al-2Mo-2Cr-2Sn-2Zr-0.2Si(Ti-6-22-22S) alloy after various heat treatments has been examined by optical microscopy, scanning electron microscopy (SEM), conventional transmission electron microscopy (CTEM) and high resolution electron microscopy (HREM), coupled with high spatial resolution microanalysis. Partitioning of alloying elements and the precipitation of intermetallics as a function of heat treatment are the main issues addressed in this paper. Titanium silicide, athermal and ordered 2 precipitates have been observed in the and the phase respectively. The compositional homogeneity has been studied in terms of alloying element distributions in the and the phase after different heat treatments. Similar investigations for Ti-6-4 alloy are used as a benchmark for comparison. The significance of these observations will be discussed in view of the fundamental understanding and further control of the structural stability of these type of - Ti alloys.
INFLUENCE OF ENVIRONMENT ON THE FATIGUE CRACK PROPAGATION BEHAVIOR AT ELEVATED TEMPERATURE OF THE Ti-ALLOY IMI 834: O. Schauerte,, A. Gysler, G. Lutjering, Technical University Hamburg-Harburg, 21071 Hamburg, Germany; S. Lesterlin, J. Petit, E.N.S.M.A., 86960 Futuroscope Cedex, France
The fatigue crack propagation behavior of the high-temperature Ti-alloy IMI 834 was investigated at a test temperature of 500°C under different environmental conditions: vacuum, laboratory air, and Ar=50% relative humidity. Two different microstructures were compared, a pure lamellar and a bi-modal structure, consisting of 20% equiaxed primary a phase in a lamellar matrix. Crack propagation tests were carried out on CT-specimens under constant amplitude sinusoidal loading conditions with a frequency of 30 Hz at R=0.1 and R=0.6. The results showed for example, that at R=0.6 fatigue crack propagation for both microstructures occurred at much lower near threshold K- values (4 MPa-m1/2) in the two aggressive environments (air, humid Ar) as compared to those measured in vacuum (10 MPa-m1/2). Furthermore, in an intermediate K-regime (6 to 12 MPa-m1/2) the da/dN-K curves in the aggressive environments exhibited a plateau region with a nearly constant crack propagation rate of about 5x10-8m/cycle. At high growth rates these curves approached those measured in vacuum. The results will be discussed on the basis of fracture surface studies (e.g. cleavage-like features found in the aggressive environments in comparison to a ductile fracture mode in vacuum), through thickness crack front profile analysis, and crack closure measurements.
OXIDATION RESISTANT COATINGS FOR THE O-PHASE: T.J. Jewett, Department of Materials Science and Engineering, SUNY-Stony Brook, Stony Brook, NY 11794-2275
Following the discovery of improved oxidation resistance of Ti-Al-based alloys containing second phase precipitates of Al-rich Ti(Cr,Al)2, a similar approach was suggested for the o-phase. A schematic representation of the Ti-Al-Cr-Nb quaternary system was constructed at 800°C. Subsequently, five alloys, based on the Ti-Al-Cr-Nb quaternary system were prepared in order to probe the phase equilibria between the corresponding Ti2AlX compositions of the Ti-Al-Nb and Ti-Al-Cr ternary systems. The samples were homogenized above the beta-transus then heat treated at 800°C. The resulting phase equilibria and microstructure were evaluated. Additionally, cyclic oxidation studies, at 800 and 900°C were conducted on the heat treated samples to assess their oxidation resistance. Similar behavior for the alloys was observed at 800°C, however, at 900°C the Cr rich alloys experienced significant spalling.
OXIDATION BEHAVIOR OF TERNARY Ti3Al-Nb ALLOY: R.G. Reddy, Yang Li, Department of Metallurgical and Materials Engineering, University of Alabama, P.O. Box 870202, Tuscaloosa, AL 35487-0202
The oxidation behavior of ternary Ti3Al-Nb alloy has been studied in pure oxygen over the temperature range of 750°C to 1100°C. The experiments were carried out using TGA. The oxidation products were analyzed using X-ray diffraction, SEM and EDS. Parabolic rate constants were calculated. An effective activation energy of 268 KJ/mol was deduced. The oxidation products formed are non-continuous protective scale on Ti3Al-Nb alloys. The results showed that oxidation products were mainly a mixture of TiO2 (rutile) and Al2O3 (alumina). For oxidation scale at 1100°C, it exhibited a layered scale of alternate rutile and alumina. The present results were also compared with that of binary Ti3Al.
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