Program Organizer: Professor Carl C. Koch, Materials Science and Engineering Department, North Carolina State University, Box 7907, Raleigh, NC 27695; Dr. Robert D. Shull, NIST, Bldg. 223 B152, Gaithersburg, MD 20899
Tuesday, AM Room: Orange County 1
February 6, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: Professor Brent Fultz, Department of Materials Science, California Institute of Technology, Pasadena, CA 91125
8:30 am Invited
ATOMIC DIFFUSION IN ALLOYS: BEYOND MEAN-FIELD THEORIES: M. Atzmon, Departments of Nuclear Engineering and Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2104
The atomic flux in alloys is commonly assumed to be proportional to the gradient of the chemical potential. Such an approach is implicitly based on a mean field approximation - the migration rate of an atom is calculated based on its average environment. The effect of the average environment is expressed through the composition-dependent chemical potential, where the composition is a mean field, obtained by averaging the occupations of many lattice sites. Since the atomic jump frequencies are exponential functions of the activation enthalpy for migration, it is obvious that calculation of the jump frequencies based on average site occupation is a poor approximation. In this paper, a calculation of chemical diffusion coefficients will be presented for a dynamic Ising model. In contrast to the simple mean-field theory, the distribution of local atomic environments is taken into account. For alloys with large enthalpies of mixing, it will shown that the atomic flux is not proportional to the gradient of the chemical potential. The effect of the assumed jump dynamics on the interdiffusion coefficient will be discussed. The model will also be applied to ordering kinetics. Predictions will be compared with experimental results, where available, and the need for molecular dynamics simulations will be discussed. Work funded by NSF grant DMR- 9200132.
SOLID-STATE ALLOYING OF IMMISCIBLE ELEMENTS: E. Ma, J.-H. He, Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803; P.J. Schilling, Center for Advanced Microstructures and Devices (CAMD), Louisiana State University, Baton Rouge, LA 70803. F. Pan and B.-. Liu, Department of Materials Science and Engineering, Tsinghua University, Beijing, China
Solid-state alloying and amorphization reactions in binary systems immiscible in equilibrium, unlike those in systems exhibiting a negative heat of mixing, are not yet well understood. In this work, we have employed the Extended X-ray Absorption Fine Structure (EXAFS) technique, in addition to XRD, DSC, and TEM, to monitor the formation of thermodynamically unstable alloys induced by mechanical alloying and thin-film interdiffusion reactions in immiscible systems Cu-Fe, Cu-Ta, and Ag-Fe. The combined use of these analytical techniques yielded information regarding local microstructural evolution as well as energy (enthalpy) states of various metastable phases in systems with a positive heat of mixing. A simple model is used to explain the presence of thermodynamic driving force for alloying in certain positive-heat-of-mixing systems when the elemental domain sizes are reduced down to a few nanometers. Mechanically alloyed metastable Cu100-xFex powders were used as the starting material to produce in situ formed two-phase nanocomposites during phase separation upon sinter forging consolidation into full density bulk samples.
STRUCTURAL TRANSFORMATIONS OF Fe-Al AND Ti-Al SYSTEMS INDUCED BY MECHANICAL ALLOYING: E. Bonetti, G. Valdre, INFM - University of Bologna, Italy
The structural evolution of the Fe-Al and Ti-Al systems induced by mechanical alloying has been followed by modern techniques of analytical electron microscopy (EDS and PEELS) on cross-sectioned samples and by mechanical spectroscopy measurements (Internal Friction and Dynamic Elasticity Modulus) on cold consolidated samples. The formation of an extended solid solution was observed for both systems after the first times of milling. The the Ti3Al system collapses to a quasi-homogeneous amorphous phase through a preferential path in the destroying the host Ti lattice. Whereas, the FeAl system tranforms by a moderate thermal treatment at about 800 K in a partially long range ordered intermetallic phase. Details of both the transformations will be discussed in details.
SYNTHESIS OF METASTABLE CARBON-SILICON-NITROGEN COMPOUNDS: B. Park, C. Uslu, D.H. Lee, Y. Berta, School of Materials Science and Engineering, Georgia Institute of Technology, 778 Atlantic Drive, Atlanta, GA 30332-0245; D.B. Poker, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831
Preliminary studies have been performed on the feasibility of metastable carbon-silicon nitride formation ([[beta]]-Si1.5C1.5N4, the homologue of equilibrium [[beta]]-Si3N4 or hypothetical [[beta]]-C3N4). Thin films were formed using high dose N-implantation into SiC with varying ion doses up to 2.7 x 1018 N/cm2, and target temperatures between -196C and 980[[ring]]C. X-ray diffraction with a position-sensitive detector and cross-sectional transmission electron microscopy revealed that the as-implanted surfaces (up to 860[[ring]]C) contained 0.1 m thick buried amorphous layers. Rutherford backscattering spectroscopy showed that the peak concentration of nitrogen saturated up to approximately 54 at. % with increasing doses, suggesting formation of a new phase. Implantation to doses of 1.1 x 1017 and 2.3 x 1017 N/cm2 at 980[[ring]]C caused enhanced surface hardness. The responsible mechanisms for the observed phase formation, nucleation and phase stability will be discussed.
10:00 am BREAK
10:20 am Invited
THE KINETICS AND ENERGETICS OF SOLID STATE AMORPHIZATION: E.J. Cotts, Physics Department, Binghamton University, State University of New York, Binghamton, NY 13902-6016
Rates of solid state reactions in thin film diffusion couples are directly monitored with measurements of the heat flow during heating. Differential scanning calorimetry and x-ray diffraction analysis are utilized to investigate the kinetics and energetics of solid state amorphization reactions. This calorimetric technique also provides thermodynamic information on the amorphization of alloys during hydrogenation, such as the Zr-Rh alloys of Johnson et al's pioneering work on solid state amorphization. Further examples include studies in the Ni-Zr system of the heat of formation of amorphous alloys from Ni and Zr, and of the averaged interdiffusion coefficient. Effects of atomic mobility on solid state amorphization reactions are examined in a comparison of reactions in Ni/ZrxTi1-x composites as the Zr concentration, x, is varied. Studies in metal/Si systems, such as Ti/Si, are also discussed.
ON THE THERMODYNAMICS AND KINETICS OF BALL MILLING INDUCED COMBUSTIVE REACTIONS: L. Takacs, University of Maryland, Baltimore County, Department of Physics, Baltimore, MD 21228
A variety of highly exothermic reactions can turn into self propagating combustion during high energy ball milling. Examples include combination reactions to form metal sulfides, borides, carbides, aluminides, and silicides and displacement reactions between an oxide or halide and a more reactive metal. Thermodynamics, in particular the adiabatic temperature, is a useful guide to predict whether a reaction can sustain itself during a combustive process. However, estimating the activation time before the ignition of combustion is a much more difficult task. It requires consideration of all the relevant processes during the activation period. Our current understanding of these processes will be discussed based on the currently available experimental data. Particular attention will be paid to the formation of lattice defects and its influence on the reaction thermodynamics and kinetics.
PHASE SEPARATION OF AMORPHOUS PHASE IN MECHANICALLY ALLOYED Fe50Zr50 POWDERS: S.E. Lee, H.Y. Ra, Department of Metallurgical Engineering, Seoul National University, W.T. Kim, Department of Physics, Chongju University, Chongju, Korea; T.H. Yim, Korea Academy of Industrial Technology, Incheon, Korea
Amorphous Fe50Zr50 alloys have been successfuly manufactured by mechanical alloying of elemental Fe and Zr powders in a conventional ball mill under an Ar atmosphere. Phase formation and microstructure were studied by using X-ray diffractometery and transmission electron microscopies. Also temperature dependence of magnetization of the alloy powders was determined by using SQUID. Selected area diffraction patterns taken from the mechanically alloyed powders showed two hallow rings, indicating phase separation into Fe rich and Zr rich amorphous phases. Radial distribution funtion obtained by analizing hallow X-ray diffraction patterns also showed two separate nearest atomic distance, supporting the phase separation obserbed by TEM. From temperature dependence of magnetization of Fe-Zr alloy powders, Curie temperature of the Fe-Zr amorphous phase was determined accurately by using Arrot plot. Curie temperature of Fe-Zr amorphous phase indicated that Fe content in the ferro-magnetic amorphous phase is higher than the mean Fe composition of the alloy powders. The result also suggested that phase separation of amorphous phase into Fe rich and Zr rich phases occurred, which is in agreement with the previous results obtained from X-ray diffraction study and transmission electron miroscopy.
ON METASTABLE PHASES FORMATION MECHANISM IN ELECTRO-DEPOSITED FILMS: V.G. Shadrow, T.A. Tochitskii, A.V. Boltushkin, Institute Solid State Physics and Semiconductors, Acad. Sci. of Belarus, P.Brovki, 17, 220072 Minsk, Belarus
The mechanism of the polymorphic metal and alloy structure formation in
electrodeposited Co based films, taking into consideration the influence of the
film content, deposition conditions and included hydrogen, is proposed. In
particular, hydrogen localized in the tetrehedral pore promotes pronounced
increasing lattice parameter and probabilities of the atom successions
ABAB...and ABCABC...become equilprobable, which leads to the formation of the
fcc phase at small pH values. The addition of P or W deforms Co lattice and
promotes a heterogeneous structure formation. The succession of the atom layout
becomes random with a short-range local order. The microcrystallites with such
successon evidently are the regions of a higher mass density, where the first
crystallites appear after isothermal annealing.
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