Program Organizer: P.W. Keefe, Special Metals Corporation, New Hartford, NY 13413; M. Sohi, Allied Signal Engines, Phoenix, AZ 85072-2118
Monday, AM Room: B4
February 5, 1996 Location: Anaheim Convention Center
Session Chairperson: P.W. Keefe, Special Metals Corporation, New Hartford, NY 13413
8:30 am
HIGH TEMPERATURE FLOW BEHAVIOR OF TITANIUM ALUMINIDES REINFORCED WITH HIGH VOLUME PERCENTAGES OF TiB2: J.S. Marte, R.J. Martin, and S.L. Kampe, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061-0237
Near- [[gamma]]TiAl and Al3Ti-based intermetallic matrices reinforced with 30, 40 and 50 v% TiB2 have been produced by XDreg. processing and densified into bulk form by powder metallurgy techniques. The compressive flow behavior has been evaluated at temperatures ranging from 800[[ring]]C to 1200[[ring]]C and strain rates from 0.0001 Hz to 0.01 Hz. The results will be analyzed by a correlation to the resulting microstructure, especially with regards to the intermetallic matrix composition, TiB2 size, and interparticle spacing. This research is motivated by the potential application of these materials in capacities currently dominated by structural ceramics.
8:50 am
PROPERTIES OF TITANIUM INTERMETALLICS-BASED PARTICULATE COMPOSITES: Satoshi Emura, Masuo Hagiwara, Yoshikuni Kawabe, National Research Institute for Metals, 1-2-1 Sengen Tsukuba Ibaraki 305 Japan
Ti-Al-Nb intermetallics, such as Ti3Al and Ti2AlNb, are considered to be very promising as a new turbine engine material. In order to improve their high temperature mechanical properties, TiB particulate-reinforced composites were developed by blended elemental (BE) powder metallurgy method. A new technique was introduced, whereby matrix forming powders were blended with TiB2 ceramic powder to form TiB in-situ in the matrix. Processing conditions such as the sintering temperature/time for obtaining homogeneous composites were discussed in detail. Microstructural observations and high temperature mechanical tests, particularly tensile and creep tests, were performed. The results were also compared with unreinforced matrix material.
9:10 am
SYNTHESIS OF TITANIUM ALUMINIDES BY MECHANICAL ALLOYING: F.H. Froes, C. Suryanarayana, Institute for Materials and Advanced Processes, University of Idaho, Moscow, ID 83844-3026; C.M. Ward-Close, P. Goodwin, DRA Farnborough, R50 Building, Hampshire GU14 6TD, United Kingdom; F. Biancaniello, F. Gayle, National Institute of Standards and Technology, Gaithersburg, MD 20899
Synthesis of the titanium aluminides (TixAl, where x = 1 or 3) using a mechanical alloying (MA) approach allows production of novel materials with potentially enhanced behavior. This results from the ability to achieve novel constitutional and microstructural effects by MA, including solubility extension, modification of the crystal structure, and refinement of the grain size. Recent results will be discussed for both Ti3Al and TiAl-type compositions. Consideration will also be given to the influence of the milling atmosphere on phase selection; particularly the effect of nitrogen and oxygen contamination.
9:30 am
REACTIVE SINTERING AND REACTIVE POWDER FORGING OF A TiAl-BASED ALLOY: David E. Alman, Materials Science Division, U.S. Bureau of Mines, Albany Research Center, Albany, OR 97321
High melting temperature, low density, and oxidation resistant intermetallic compounds, such as TiAl, are desirable for use as structural materials at elevated temperatures and in aggressive environments. Recently its has been recognized that these compounds may have applications in industries, such as the automotive industry, where cost is frequently a major factor in material selection. For intermetallic compounds to be widely used for these types of applications, low cost fabrication methods will be a requirement. One potential economical method for component product is coupling Reactive Syntheses with traditional powder metallurgy techniques (e.g. press and sinter of powder forging). Reactive Synthesis involves forming compounds insitu from elemental powders. Discussed will be the effect of alloy composition and processing parameters on the microstructure and properties of a TiAl-based alloy fabricated by reactive sintering and reactive powder forging.
9:50 am BREAK
10:10 am
EFFECTS OF W, Mo AND Si ON THE CREEP BEHAVIOR OF A CAST GAMMA TITANIUM ALUMINIDE: P. R. Bhowal, H. F. Merrick, Allied Signal Engines, Phoenix, AZ 85034
Gamma titanium aluminides exhibit attractive high temperature specific strength and stiffness characteristics, and oxidation resistance. However, improvements in the creep resistance of gamma aluminides are needed if the promise for more general application in gas turbines is to be realized. A new investment cast gamma titanium aluminide, nominally Ti-47Al-2Nb-1Mn-0.5W-0.5Mo-0.2Si(At.%), has been identified which exhibit significantly higher creep resistance over first generation cast aluminides such as Ti-47Al-2Nb-2Cr and Ti-47Al-2Nb-2Mn. Creep strength improvement over these latter aluminides was achieved by the addition of small amounts of W, Mo and Si. In this study, the effects of these alloying elements on tensile and creep behavior have been evaluated in more detail in order to optimize the new gamma aluminide chemistry. Tensile tests were performed on material in the HIP and heat treated conditions at temperatures from RT to 815[[ring]]C and creep tests were conducted in the temperature range 650-815C. The influence of the W, Mo and Si alloying additions on microstructure and mechanical properties of the new gamma aluminide will be described.
10:30 am
MICROSTRUCTURAL REFINEMENT OF THE Ti46Al2W0.5Si CAST ALLOY BY STATIC HEAT TREATMENT: E. Evangelista, W. J. Zhang, L. Francesconi, E. Cerri, S. Spigarelli, Department of Mechanics, University of Ancona, Via Brecce Bianche, I-60131 Ancona, Italy
Investment casting may be the most feasible and cost effective approach for the manufacture of near-net-shape components such as blades and vanes, compared to wrought or powder metallurgical route. But the key shortcoming of cast TiAl alloy is the remain of coarse ingot microstructure in the final product, which deteriorates the property performance. In this paper, static heat treatment; quenching/tempering process, was performed in an attempt to refine the microstructure of a cast Ti46Al2W0.5Si alloy. It was found that a fine near-fully lamellar structure with the colony size of about 30 um, or a duplex structure comprising of equaixed [[gamma]]grains and fine [[alpha]]2 plates or spheroids, can be obtained by tempering the quenched massive-transformed structure at 1280-1170C. Preliminary compression tests and hardness measurements confirmed that the ductility and strength of the studied alloy are greatly improved with the fine tempered microstructure. The microstructure change during heat treatment were examined by TEM, and the existence of [[beta]] and Ti5Si3 phase was confirmed due to the addition of W and Si.
10:50 am
HALL PETCH BEHAVIOR IN GAMMA-BASED TITANIUM ALUMINIDES: IMPLICATIONS FOR PROCESSING AND APPLICATIONS: W.O. Soboyejo, C. Mercer, Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210-1179
The inverse square root dependence of yield/fracture properties on the average equiaxed/lamellar packet size in gamma-based titanium aluminides is demonstrated in this paper. Room- and elevated-temperature yield/ultimate tensile strength and ductility, and room-temperature fracture toughness are shown to exhibit such grain size dependence in binary (Ti-48Al) and ternary (Ti-48Al-1.4Mn, Ti-48Al-2Mn and Ti-48Al-1.5Cr) gamma alloys. The strong effects of the average equiaxed equiaxed/lamellar packet size in duplex alloys with a wide range of lamellar volume fractions is attributed to the controlling influence of soft deformation modes. The observed trends are also rationalized by considering the critical conditions associated with dislocation pile-ups in lamellar and equiaxed grains. The implications of the observed grain size dependence are discussed for future processing and alloy development that may impact the near/midterm applications of gamma alloys.
11:10 am
DEFORMATION AND FRACTURE UNDER COMPRESSIVE LOADING IN LAMELLAR TiAl MICROSTRUCTURES: Ming Dao, Bimal K. Kad, Robert J. Asaro, Department of Applied Mechanics and Engineering Sciences, University of California-San Diego, LaJolla, CA 92093-0411
A rate dependent elasto-viscoplastic, finite strain, crystallographic slip
model is applied to study the mechanical behavior of lamellar and nearly
lamellar TiAl polycrystals. Orientation dependent mechanical behavior of single
PST is modeled using soft-mode and hard-mode slip systems at the single crystal
level. At the polycrystal level, finite element method is used to catch the
non-uniform deformation modes developed. Stress-strain curve, texture
development and especially the failure modes under the compressive loading are
examined. Intergranular fracture and internal buckling are found to be the
most important failure modes under compression. The computational results are
shown to be in very good agreement with our proof of the concept' experiments.
Results under compressive loading are also compared to the results under
tensile loading.
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