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Session Chairperson: J.J. Stephens, Sandia National Laboratories, Albuquerque, NM 87185
EFFECTS OF TEMPERATURE AND OXYGEN PRESSURE ON OXIDATION KINETICS OF V-4wt.% Cr-4wt% Ti AND V-5wt.% Ti ALLOYS: M. Uz, Chemical Engineering Department, Lafayette College, Easton, PA 18042; K. Natesan, Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439; N. Barbosa III, Chemical Engineering Department, Lafayette College, Easton, PA 18042
We conducted comparative investigation to determine the effects of temperature and oxygen pressure on the oxidation behavior and microstructure of V-4wt.% Cr-4wt.% Ti and V-5wt.% Cr-5wt.% Ti Alloys. The samples were cut from 1-mm-thick cold-rolled sheet stock and annealed at 1050°C for 1 h before their use in oxidation studies. The oxidation experiments were performed at oxygen pressures of 760, 0.1, 5x10-4, and 5x10-6 torr and at temperatures of 350 to 700°C. Oxidation models, oxide type and thickness, partitioning of oxygen between oxide and substrate alloy, and alloy grain size were determined as a function of temperature and oxygen pressure. The results for the two alloys are discussed and compared with one another and with those from similar studies on unalloyed V reported in the literature. Work supported by the U.S. Department of Energy, Office of Fusion Energy Research, under contract W-31-109-Eng-38.
OXIDATION BEHAVIOR OF Al3Ti INTERMETALLICS: R.G. Reddy, X. Wen, Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487
The oxidation kinetics of Al3Ti intermetallics have been studied at the temperature range of 1123K to 1273K in pure dry oxygen. The oxygen diffusion rates were investigated using TGA method. Parabolic rate constants were calculated and compared with other research results. The oxidation scales were analyzed using SEM, EDS and X-ray diffraction. A continuous Al203 scales with a little Ti02 (rutile) were formed on the intermetallic surface. The Al3Ti intermetallic exhibited excellent oxidation resistance among the Ti-Al base intermetallics.
SUPERHEATING BEHAVIOR OF NiAl: N. Kullcarni, Dept. of Materials Science & Eng., University of Florida, 245 Rhines Hall, Gainesville, Fl-32611; K.T. Hong, Division of Metals, Korea Inst. of Science & Tech (KIST), 39-1 Hawolkog-dong, Sungbook-Gu, Seoul, Korea 136-791
The intermetallic compound NiAl, when processed in a microgravity environment under electromagnetic levitation, exhibited a unique superheating effect up to 65 degrees over the melting point. This superheating behavior appeared to be related to the high supercoolings (200-250 degrees) experienced by molten NiA1 samples that rapidly solidified in the rnicrogravity levitation environment. The superheating observed appeared to be permanent and reduced asymmetrically for compositions that deviated from stoichiometry. On reaching the maximum or critical superheating temperature, an anomalous transformation occurred that caused the temperature of the sample to drop to its melting point, after which melting commenced. It appears that this transformation is an order-disorder type, that is explicitly associated with the superheating behavior in NiAl.
A STUDY OF POTENIAL EUTECTIC BRAZE ALLOYS IN THE Au-Ag-Ge TERNARY SYSTEM: P.T. Vianco, J.J. Stephens, C.A. Walker, Sandia National Laboratories, Albuquerque, NM 87185-0367
Despite the need for low vapor pressure alloys with brazing temperatures in the 400-600°C range, there are in fact very few commercially available alloys to use in this range. This study was motivated by the need to develop a brazing process with a maximum allowable brazing temperature of 500°C. A survey of ternary phase diagrams, combined with previous experience with the binary 88wt.% Au-12 Ge alloy, led us to select the Au-Ag-Ge ternary system for further study. We have found that the 77.32 wt.% Au-12.62 Ag-10.06 Ge composition is a eutectic alloy with a eutectic temperature of ~447°C. Hermetic seals to metallized alumina tensile buttons (ASTM F19 design) have been obtained with this system. This talk will present results braze joints strength results, along with a discussion of the as-solidified microstructure of this alloy. This work conducted at Sandia National Laboratories, supported by U.S. Dept. of Energy under contract number DE-AC04-94AL85000.
3:20 pm BREAK
THERMAL AND PROCESS MODELING OF MICROFIBROUS MANUFACTURING IN A SINTERING FURNACE: J.M. Shire, J.M. Khodadadi, Mechanical Engineering Department, B.J. Tatarchuck, Chemical Engineering Dept., Auburn University, Auburn, AL 36849
Mathematical modeling and numerical simulation of the thermal field for the operation of a continuous belt hydrogen furnace used for sintering applications of microfibrous materials are presented. Particular emphasis is placed on metallic fibers being sintered into a web-like matrix. Utilizing thermal energy balance and simplified heat transfer equations, the mathematical model is capable of predicting the instantaneous temperature of products as they pass through the furnace. The manufacturing process is optimized with respect to the belt speed, furnace zone temperature setting and sintering material configuration.
DIFFUSION IN SILICIDES OF Mo AND W: P.C. Tortorici, M.A. Dayananda, Purdue University, School of Materials Engineering, 1289 MSEE Building, West Lafayette, IN 47907-1289
Diffusion studies were carried out with Si in contact with Mo and W at selected temperatures between 900°C and 1350°C for the formation and growth of silicides of Mo and W. Interdiffusion between MoSi2 and Mo and between MoSi2 and W was also studied for the development of silicide layers, such as MO5Si3, MO3Si, (Mo,W)5Si3, W5Si3 and nonplanar interface morphologies. Integrated interdiffusion coefficients calculated for the various silicide layers from concentration profiles are presented. The relative diffusion behavior of Si, Mo and W in selected silicide layers will be discussed in the light of diffusion structures and marker motion.
FOCUSED ION BEAM MILLING AND MICROMANIPULATION FOR CROSS-SECTION TEM SPECIMEN PREPARATION: L.A. Giannuzzi, Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, 4000 Central Florida Blvd., Orlando, FL 32816-2450; S.R. Brown, Kirk Resources, 9333 S. John Young Parkway, Orlando, FL 32819; R.B. Irwin, F.A. Stevie, Lucent Technologies, 9333 S. John Young Parkway, Orlando, FL 32819
A technique for cross-section transmission electron microscopy (TEM) sample preparation of mutt-layered and difficult materials as described. A focused ion beam (FIB) device is used to slice an electron transparent specimen from an area of interest. Micromanipulation procedures are then used to transport the electron transparent specimen from the bulk sample to a formvar/carbon coated copper grid for subsequent TEM analysis. The versatility of this technique is demonstrated by presenting cross-sectioned TEM specimens from a wide range of materials. TEM specimens that have been produced with the FIB method include a multi-layered thin film electronic device, a metallic diffusion couple, and a ceramic fiber.
IMPROVEMENT OF COATING ADHERENCE OF HOT-DIP GALVANIZED SHEET STEELS CONTAINING SILICON: Jong-Sang Kim, Jin-Hwan Chung, Kwangyang Research Labs., Pohang Iron and Steel Company, P.O. Box 22 Suncheon, Korea
The effects of silicon in steels on the coating adherence of hot-dip galvanized steel sheets have been investigated to reduce the weight of automobile body. The presence of a stable silicon oxide formed on the steel surface has shown to be very detrimental to proper wetting by liquid zinc. A critical silicon concentration depended on the operating conditions such as mechanical pretreatments, furnace conditions and alloying composition of zinc bath.
OXIDATION AND DECARBURISATIOIN KINETICS OF AN EUTECTOID STEEL DURING STATIC SPHEROIDISATION: Sedat Ozbilen, Gazi Univ. Fac. Tech. Educ. Metal Dept., Teknikokullar, Ankara, Turkey
Eutoid steel with a composition of 0.59%C-0.69%Mn-1.19%Si-0.022%P and 0.061%S (in wt%) was austenitised at 805°C after casting. Samples were furnace and air cooled to obtain fine (0.26µm) and coarse (0.57µm) mean interlamellar spacing of pearlite. Samples of 1x10.4cm was annealed at 600°C and 700°C for varying times up to 2200 hr. x-ray diffraction, SEM investigation, wet chemical analysis and theoretical erf calculations were carried out to observe and measure the response of the eutectoid steel to oxidation and decarburisation.
EFFECT OF SEM TILTING ON MEAN INTERLAMELLAR SPACING CALCULATIONS OF PEARLITE IN EUTECTOID STEEL: Sedat Ozbilen, Gazi Univ. Fac. Tech. Educ. Metal Dept., Teknikokullar, Ankara, Turkey
Eutectoid steel with a composition of 0.59%C-0.69%Mn-1.19%Si-0.022%P and 0.061%S (in wt%) was cast before austenitisation at 850°C. Samples were furnace cooled to obtain coarse pearlitic mean interlamellar spacing of 0.57µm measured from SEM micrographs of samples. To observe the effect of SEM tilting on "mis" of pearlite samples of 1x1x0.4cm were annealed at 700°C for 115 hr. SEM micrographs with and without 30° tilt on samples were taken. Results of metallographic investigation will be presented.
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