Wednesday, AM Room: B4
February 7, 1996 Location: Anaheim Convention Center
Session Chairperson: TBA
THE USE OF ATOMIC SIZE EFFECTS TO TUNE PHASE STABILITY IN FE3ALXSI1-x: L. Anthonv, University of Toledo, Department of Physics and Astronomy, Toledo, OH 4360-3390
It has been known for some time that the addition of certain ternary elements such as Ti and Si can result in huge increases in the D03-B2 transition temperature (TcD03-B2) of Fe3Al. Recent high-temperature calorimetry work by Anthony and Fultz [Acta. Metall. Mater., in press] has shown that the varying efficacies of early transition metal solutes in raising TcDo3-B2 in Fe3Al is related to their metallic radii. This size-effect phenomenon may prove useful in extending the stability range of other D03-ordered intermetallics. In this talk, I shall present results of such an attempt at controlling the stability range of the D03-ordered pseudobinary alloy Fe3>Aly>Si1-y. By adjusting the relative concentrations of Al and Si in Fe3AlySi1-y, it should be possible to tune the size matching to a particular early transition metal, and thereby optimize the stability of the D03 structure in this alloy system. I shall discuss the relevance of my results for alloy design and engineering in these and other intermetallic systems.
INTERDIFFUSION AND PHASE FORMATION IN THE Mo-Si SYSTEM: P. C. Tortorici, M. A. Dayananda, Purdue University School of Materials Engineering 1289 MSEE Bldg. West Lafayette, IN 47907-1289
Diffusion studies were conducted in the Mo-Si system to investigate the formation of Mo silicide compounds during interdiffusion of Mo and Si. Solid solid couples were assembled with pure Si, Mo or MoSi2 as terminal disks and isothermally annealed at selected temperatures between 900deg.C and 1200deg.C for various times. Optical Microscopy, SEM, TEM, and x-ray diffraction were utilized to characterize the resulting phase layers and microstructures. Phase layers observed include MosSi3 and MoSi2. The MoSi2 layer exhibited a columnar microstructure. Integrated diffusion coefficients were calculated for the various silicide layers. Results on the relative diffusion behavior of Mo and Si in the silicide layers will also be discussed.
ALLOYING TO ENHANCE THE SHAPE MEMORY PROPERTIES OF NiMn: M. L. Johnson, D. E. Mikkola, Dept. of Metallurgical and Materials Engineering, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931
Earlier work has shown that the shape memory behavior of binary NiMn can be altered with higher order alloying. Changes in shape recovery, transformation temperature for the B2 to LI0 transformation, and mechanical properties can be linked to changes in the c/a ratio of the low temperature phase. A series of alloys Ni50Mn50-xXx has been studied that covers a range of transformation temperatures from 500°C to below room temperature. Efforts to relate changes in shape memory behavior to changes in transformation and deformation behavior will be discussed.
FRACTURE AND FATIGUE-CRACK GROWTH RESISTANCE OF MoSi2 COMPOSITES: K. Badrinarayanan, K. T. Venkateswara Rao, R. O. Ritchie, Dept. of Materials Science and Eng., University of California, Berkely, CA 94720-1760
Recent research of MoSi2 alloys for structural use at temperatures in excess of 1200°C has been motivated by their excellent oxidation resistance; however, they invariably suffer from poor fracture resistance. In light of this, the current work was aimed at comparing the role of ductile vs brittle reinforcements in influencing the fracture toughness (Kk) and fatigue-behavior of MoSi2 at both ambient and elevated temperatures; specifically using 20 vol.% of Nb and SiC. It was found that the addition of the ductile phase as chopped Nb wire mesh resulted in a significant increase to Kk from 2MPa[[radical]]m for unreinforced MoSi2 to 5.5 MPa[[radical]]m for the composite, toughening that was associated with uncracked ligament bridging and crack deflection. However, this beneficial effect was not found under fatigue loading, as the Nb ligaments failed prematurely, thereby limiting any shielding. With brittle SiCp reinforcements, there was no detectable effect at room temperature, although at 1200°C, cyclic crack growth was observed between a threshold K of 4.1 MPa[[radical]]m. The latter phenomenon is considered in terms of a mutual competition between intrinsic cavitation damage by localized grain-boundary void formation (which promotes crack advance) and extrinsic crack-tip shielding by viscous bridging (which acts to oppose it).
THE <-> PHASE TRANSFORMATION DURING RAPIDLY QUENCHING AND HEATING IN Ti-rich TiAl ALLOY. T. Kumagai, E. Abe, M. Nakamura, National Institute for Metals, Ibaraki 305, Japan
The phase transformation from high-temperature -Ti (disordered h.c.p) to -TiAl (ordered f.c.t. with a L1o structure) and the reverse transformation from to in Ti-48at.%Al alloy have been examined. The extremely fine 2 (ordered b.c.p with D)19 structure) / lamellae together with the grains with a massive morphology (m) due to -> reaction are observed in the sample solution-treated in the phase field (1683K) followed by ice water quenching. On the other hand the similar fine 2/ lamellae due to -> reaction are formed within the m grain interiors by up-heating to the temperature range above 1473K, although these fine lamellae in the m grain interiors are not recognized in the samples up-heated to the temperature range below 1273K. The formation process of the extremely fine 2/ lamellae by rapidly quenching and heating will be discussed.
10:10 am BREAK
SMALL FATIGUE CRACK GROWTH IN -BASED TITANIUM ALUMINIDES: J. P. Campbell, S. Lillebridge, K. T. Venkateswara Rao, R. O. Ritchie, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720-1760
Recently, -based titanium alumindes have received considerable attention in the aerospace community as candidate materials for several high-temperature structural applications. However, in part due to their limited room-temperature ductility and toughness, these alloys suffer from extremely steep da/dN v. crack-growth relationships. Since this results in a severely limited crack-propagation life in damage-tolerant lifetime predictions, the structural use of -TiAl alloys may necessitate the use of fatigue threshold and near-threshold small-crack behavior in such design and life calculations. In this study, the fatigue behavior of "small" surface cracks (<500 -TiAl alloys, including the Ti-48Al-2Cr-2Nb ("GE") alloy in both equiaxed duplex and lamellar (2 + ) microstructures. In addition to delineating the principal microstructural features controlling crack growth, the intent is to characterize differences between long and small crack behavior, specifically in terms of the role of crack size on crack-tip shielding and on statistical sampling considerations. Work supported by the U. S. Air Force Office of Science Research.
MICROSTRUCTURAL INFLUENCE ON THE FATIGUE CRACK PROPAGATION OF GAMMA TIAL ALLOY: C. Choi, C. S.Lee, Center for Advanced Aerospace Materials, Pohang Univ. of Sci. & Tech., Pohang, 790-784, Korea
The role of microstructure on the fatigue crack growth resistance of Ti-48AI-1Cr-lV-2.5Nb alloy was examined in this study. Fully lamellar microstructure showed superior fatigue properties as compared to the duplex and near gamma microstructures because of large extrinsic effect such as crack closure rather than intrinsic property. In the fully lamellar microstructure, colony boundary acted as strong or weak barriers for the crack propagation depending on the relative orientation of the lamellar lath to the crack propagation direction. Lamellar structures in the duplex microstructures were not the effective obstacles due to a small colony size.
THE EFFECTS OF TI3AL AND SILICIDE PRECIPITATES ON THE TENSILE, FATIGUE CRACK GROWTH, AND CREEP BEHAVIOR AT ROOM TEMPERATURE AND 593DEG.C IN THE NEAR-ALPHA TITANIUM ALLOY, TI-1100: A. Madsen, Department of Materials Science and Engineering, University of Washington, Seattle, WA 98105; A.H. Rosenberger, H. Ghonem, Mechanics of Solids Laboratory, Dept. of Mechanical Engineering, Univ. of Washington, Seattle, WA 98105
Long term aging of near-alpha titanium alloys, at temperatures typical during service in a gas turbine engine, results in the formation of silicide precipitates along the alpha platelet boundaries and the formation of Ti3Al homogeneously distributed throughout the matrix. A post aging heat treatment that relies on the phenomenon of the critical ordering temperature is used to remove the Ti3AI precipitate while leaving the silicide intact. Three materials--unaged (precipitate free), overaged (Ti3Al+silicide). and post aging heat treated (silicide-only)--are compared, with the objective of identifying the separate effects of the Ti3Al and silicide precipitates on tensile, fatigue crack growth, and creep bchavior at room temperature and S93°C. The Ti3Al precipitate is shown to be largely responsible for the increase in tensile yield stress and for the decrease in ductility at both test temperatures. In contrast, the increase in room temperature fatigue crack growth rate associated with aging is attributed to the silicide, with Ti3A1 playing only a minor role. Aging produces a slight improvement in fatigue crack resistance at 593°C, which also appears to be due to the presence of the silicide precipitate. Accelerated creep tests indicate that the presence of Ti3Al has a beneficial influence on the creep rcsistance of Ti-1100. Since precipitation occurs over both short and long time periods (800 to 60,000 min), aging effects should be considered when characterizing high temperature mechanlcal behavior in this and similar near-alpha titanium alloys.
LOW CYCLE FATIGUE PROPERTIES OF A TITANIUM ALLOY: B. Said, A. Inchekel, N. Arakere, Wichita State University, Department of Mechanical Engineering, Wichita, KS 67208
Low cycle fatigue tests at moderately elevated temperatures were conducted on round titanium alloy specimens that were extracted from aircraft turbine disks. These tests were performed in order to investigate the effect of moderate temperatures on the cyclic stress strain curves, mechanical properties, and the cyclic life of the material. Furthermore, microstructural changes with temperature and fractographic analysis were carried out. Results showed that the testing temperatures softened the material, and reduced the fatigue life, yield point, and ultimate tensile strength.
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