Sponsored by: EPD Process Fundamentals Committee, MSD Thermodynamics & Phase Equilibria Committee, Japan Institute of Metals
Program Organizers: Prof. R.Y. Lin, University of Cincinnati; Prof. Y. Austin Chang, University of Wisconsin-Madison; Prof. R. Reddy, University of Navada-Reno, Dr. C.T. Liu, Oak Ridge NL
Tuesday, AM Room: B2
February 6, 1996 Location: Anaheim Convention Center
Session Chairperson: C. T. Liu, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6093
8:30 am Invited
ENVIRONMENTAL EFFECTS AND INTRINSIC MECHANICAL PROPERTIES OF Ni3Al: E. P. George, C. T. Liu, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6093
The effects of environment, strain rates, temperature, and boron doping on the mechanical behavior of Ni3Al are reviewed. Environmental embrittlement now appears to be the main reason for the grain boundary brittlement of B- free Ni3Al. The embrittlement mechanism involves the generation of atomic hydrogen at the grain boundaries, by either the reductiol1 of water vapor by Al, or the direct dissociation of molecular H2. Consistent wlth this ductility increases with increasing strain rate, decreasing temperature, and decreasing amounts of H2O/H2 in the environment. When the embrittlement is suppressed-by testing in H2O/H2 free environments (approximately 10-10 torr) the tensile ductility of B-free Ni3Al increases dramatically (to more than 40deg./O), and the fracture mode changes to predominantly transgranular. Boron alleviates environmental embrittlement in Ni3Al. Possible mechanisms include, slowing down H diffusion, and altering the surface chemistry to interfere with the generation and retention of atomic H. Research sponsored by the Division of Material Sciences, U.S. Department of Energy under contract DE- AC05- 840R21400 with Lockheed Martin Energy Systems, Inc.
8:55 am Invited
INTERACTIONS OF WATER AND OXYGEN ON Ni3(Al,Ti) AND Ni3Fe SURFACES-EVIDENCE FOR THE CRITICAL ROLE OF COMPETITIVE REACTIONS: Yip-Wah Chung, Department of Materials Sci. & Eng., Robert R McCormick, School of Engineering and Applied Science, Evanston, IL 60208
Recent studies indicate that much of the observed room temperature embrittlement occurring in intermetallics is due to environmental effects. Our research has focused on the specific interactions between water vapor and oxygen (the two key active components in air) and Ni3(Al, Ti) and Ni3Fe surfaces, using standard tools of surface science. Our studies show that when absorbed on to Ni3(Al, Ti) surfaces, water indeed can dissociate to produce atomic hydrogen, a key proposed step in the embrittlement process, but only on specific crystallographic planes and above a certain critieal temperature. This result may explain the observation that Ni3Al does not suffer moisture-induced embrittlement at room temperature when solidified in certain orientation. Water also absorbs on Ni3Fe surfaces, dissociating to produce hydrogen above 200K. However, the water absorption process is approximately 1000 times slower than those of oxygen. In a normal room temperature ambient (the partial pressure ratio of oxygen to water being 15 to 1), oxygen will out-compete water vapor when exposed to fresh Ni3Fe surfaces, thus quenching the hydrogen production process. This may explain its high ductility in regular air ambiance.
9:20 am Invited
AN OVERVIEW OF ENVIRONMENTAL EFFECTS IN IRON ALUMINIDES: I. Baker, Thayer School of Engineering, Dartmouth College, Hanover, NH 03755
Iron aluminides hold great promise for intermediate temperature applications (to approximately 600[[ring]]C) because of their excellent oxidation and sulphidation resistance and reasonable strength. However, they have poor low temperature ductility which is exacerbated by water-containing environment. This paper will present an overview of the environmental effect on iron aluminides. It will cover the effects of iron aluminum ratio; ternary alloying additions, particularly boron; strain rate and temperature. The mechanisms involved in the water-induced embrittlement will be discussed.
9:45 am Invited
PREPARATION OF A Ti-Ni INTERMETALLIC COMPOUND BY USING A DROP SHAFT: Yoshikazu Suzuki, Katsuyoshi Shimokawa, Yoshinobu Ueda and Jiro Nagao, 2-17 Tsukisamu-Higashi, Toyohira-ku, Sapporo Hokkaido 062, Japan
A Ti-Ni intermetallic compound could be formed rapidly from Ti and Ni powder mixed compact with Self-propagating High-temperature Synthesis. Microgravity experiments using a drop shaft are carried out for the very short available time. This report shows the effect of microgravity or high gravity on formability of an intermetallic compound by using a drop shaft. It is concluded that the melted speeimen forms a porous macrostructure and a nearly amorphous alloy by rapid cooling under microgravity. Otherwise, when the capsule of the drop shaft stops after falling and the melted speeimen solidifies with rapid cooling under high gravity, the specimen forms a uniform and dense structure with better crystallinity.
10:10 am BREAK
10:20 am Invited
HIGH TEMPERATURE DEFORMATION AND TERNARY ALLOYING OF NbCr2 LAVES INTERMETALLICS: T. Takasugi, Institute for Materials Reserarch, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980 Japan
A study has been conducted to examine the effect of ternary alloying elements on high temperature compressive deformation property of the C15 NbCr2 based intermetallics, which have attractive properties as high temperature structural materials because of their high strength, high melting point, low density and good chemical stability. The high temperature deformation of the C15 NbCr2 intermetallics can be improved by two principal alloying methods. One method is based on consideration of site occupation and size factor of the additive elements; in the C15 lattice, addition of Mo, V and W can promote the synchro shear and resultant dislocation movement. Other method is based on modification of microstructure through alloying; moderate addition of these elements results in a variety of duplex microstructure between the C15 phase and bcc solid solution without forming any intermediate phases, and can enhance the high temperature deformability. Being concurrent with mechanical observation, lattice property of the additive atoms in the C15 structure was investigated by XRD, TEM HREM and ALCHEMI observations and then discussed in association with the observed mechanical behavior. Also, phase relation between the C15 structure and the bcc solid solution in Nb-Cr-X systems was investigated by OM, XRD and TEM observations.
10:45 am Invited
EFFECT OF INTERFACIAL MODIFICATION ON MECHANICAL BEHAVIOUR OF INTERMETALLIC MATRIX COMPOSITES: J. M. Yang, Department of Material Science and Engineering, University of California, Los Angeles, CA 90095-1595
Intermetallic matrix composites are currently being developed for variety of high temperature structural applications. One of the most critical issue is the chemical, physical and mechanical incompatibilities between the fiber and matrix. Fiber coating appears lo be one of the most promising approaches for designing a functional interface to improve the fiber/matrix incompatibility. In this paper the effect of fiber coating on the mechanical behaviour of several intermetallic matrix composites including Ti3Al, Ti2AlNb and NiAl would be discussed.
11:10 am
HIGH TEMPERATURE OXIDATION BEHAVIOR OF TiAl-BASED INTERMETALLICS: M.F. Stroosnijder, J.D. Sunderkotter, V.A.C. Haanappel, Institute for Advanced Materials-Ispra Site, Joint Research Center-European Commission, 21020 Ispra(VA), Italy
In spite of their high aluminum content, TiAl-based intermetallics do generally not form long lasting protective alumina scales. After longer exposure times, the scales, which are initially rich in alumina, deteriorate and scales consisting of mixed alumina and titania predominate, with similar high growth rates as pure titania. A great deal of research work has been undertaken to improve the oxidation behavior of these alloys by adding ternary and quartenary elements. Among other candidates, especially niobium was found to improve the oxidation behavior of titanium aluminides. Nitrogen from the air appears to change the oxidation resistance dramatically. Despite this, in the literature the oxidation mechanism in air is often treated on the basis of Ti-Al-O thermodynamics and kinetics only. In the present contribution the influence of alloying elements, in particular niobium, and the effect of nitrogen in the oxidizing environment on the high temperature oxidation behavior of TiAl-based intermetallics will be considered.
11:35 am
MECHANISM OF DISINTEGRATION DURING METAL DUSTING: L. Levin, A.Katsman, L. Klinger, T. Werber, Department of Materials Engineering, Technion-Israel Institute of Technology, Technion, Haifa 32000, Israel
The process of catastrophic carborization, i.e. "metal dusting", which
converts a compact metallic material into fine particles, was analyzed. It was
shown that disintegration of the compact metal matrix to a powder is the result
of plastic deformation and subsequent fracture accompanying diffusional phase
formation in the near-surface layer. The process is controlled by internal
stresses arising during phase transformation. Competition between stress
generation and relaxation may result in attaining the ultimate strength in the
near-surface layer and its fracture. Such a mechanism of metal disintegration
can occur in a certain temperature interval dependent on the kinetic and
geometrical parameters of the system diffusivities of the alloy's components,
the ratio between specific volume of the new and the old phases, and the
ultimate plastic strain. The temperature interval for carbon steel
disintegration is the carburizing atmosphere, the rate of disintegration, and
the period of the disintegration cycle were evaluated and compared with
experimental data.
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