Materials Week '97: Monday PM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Monday afternoon, September 15.

ALLOY MODELING AND DESIGN: Session II: Short-Range Order and Phase Stability in Alloys

Session Chair: Prof. G. Ceder, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139

MEASUREMENT AND MODELING OF SHORT-RANGE CORRELATIONS IN METAL ALLOYS: Gene E. Ice, Cullie Sparks, Xiaogang Jiang, Ernest Epperson, Lee Robertson, Oak Ridge National Laboratory, Oak Ridge, TN 37831

Precision measurements of pair correlations in binary metallic solid solutions challenge the understanding of atomic size and bonding in local alloy structure. Measurements with tunable synchrotron X radiation can control the scattering contrast between atoms in a solid solution to produce a minimum and maximum contrast with elements nearby in the periodic table. We present measured pair-correlations for a series of binary Fe-Ni, Cr-Ni and Cr-Fe alloys. The response of the atoms to the local forces in terms or their displacements and ordering is discussed. Models are described which combine chemical correlations and static displacements. Theoretical calculations, of static atomic displacements, are needed to compare with observations. New efforts to statistically approach the modeling of SRO in alloys are also discussed. Research sponsored by the Division of Materials Sciences, U.S. Department of Energy under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corporation.

2:40 pm INVITED

CALCULATION OF ALLOY PHASE DIAGRAMS BY CONTINUOUS CLUSTER VARIATION METHOD: K. Masuda-Jindo, Department of Materials Science and Engineering, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 227, Japan; R. Kikuchi, Department of Materials Science and Engineering, University of California, Los Angeles, CA 90024-1595

A new formulation of the CVM which allows atomic displacement from lattice points is used to calculate the phase stability and phase diagrams of binary alloys. We formulate continuous atomic displacements around lattice points using the pair approximation of the CVM. We study the phase stabilities of binary alloy systems taking into account the local lattice distortions, with the use of the continuous displacement treatment of CVM, in terms of three kinds of pair probability functions g_ij and three pairwise potentials _ij. We focus our attention on model 2D alloys as well as fcc binary alloys, like Cu/Au systems. It is shown that the continuous displacement lowers the ordering temperature, and size mismatch in the constituent atoms increases the transition temperature. The local lattice-distortion effects in a disordered Cu-25 at. pct Au solid solution have also been investigated using the pair distribution functions determined by the continuous CVM.

3:20 pm BREAK

3:40 pm INVITED

FIRST-PRINCIPLES STUDY OF SITE OCCUPATIONS IN Ti-AlNb B₂ ALLOYS: M. Asta, D.D. Johnson, Sandia National Laboratories, Livermore, CA 94551

The characterization of any B2 structure in a ternary alloy requires, in addition to the average compositions, two independent order parameters. For Ti-Al-Nb alloys, these may be defined as _Al=(c_I^Al -c_II^Al)/2c_Aland _Nb = (c_I^Nb-c_II^Nb)/2c _Nb, in terms of the compositions on sublattices I and II. Recent experiments in Ti-Al-Nb have shown that the ratio _Al/_Nb is strongly concentration dependent. We investigate the structure of the B2 phase using two different electronic-structure-based methods. In the first, cluster-variation method calculations of _Al and _Nb are performed with interactions derived from the formation energies of ordered bcc compounds. In the second, based on the inhomogeneous coherent-potential approximation, an ordering susceptibility is calculated within the disordered bcc phase and used to determine _Al/_Nb as a function of composition. Both sets of calculated results will be compared and contrasted with experimental findings. Work is supported by the U.S. Department of Energy, OBES, Division of Materials, contract #DE-AC04-94AL85000.

4:20 pm INVITED

COMPARISON OF THE ELECTRONIC STATES OF ALLOYS FROM THE COHERENT POTENTIAL APPROXIMATION AND ORDER-N METHODS: J.S. Faulkner, Nassrin Moghadam, Alloy Research Center and Department of Physics, Florida Atlantic University, Boca Raton, FL 33431

The Coherent Potential Approximation (CPA) was derived using infinite-order perturbation theory and tested against one-dimensional models at a time when the calculation of the electronic states of disordered alloys was deemed impossible. Today, order-N methods such as the locally self-consistent multiple-scattering (LSMS) method are capable of obtaining the electronic structure of models made up of thousands of atoms, each described by self-consistent LDA atomic potentials. These models have been shown to be large enough to give realistic results for infinite alloys. We compare the predictions of the two approaches for the total energy, density of states, and other properties of disordered alloys. This research was sponsored in part by DOE grant number DE-FG05-89ER45392.

5:00 pm

X-RAY AND NEUTRON SCATTERING STUDY OF THE ORDER-DISORDER TRANSITION IN AlMnCu₂: J.J. Hoyt, Computational Materials Science Department, Sandia National Laboratories, Livermore, CA 94550; B.C. Chakoumakos, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6393; S.T. Misture, High Temperature Materials Lab, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6064; R. McCormack, National Institute of Standards and Technology, Gaithersburg, MD 20899; M. Asta, D.D. Johnson, Sandia National Laboratories, Livermore, CA, 94551; J.D. Althoff, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720

The long range order parameter in binary alloys, such as CuZn, can be measured as a function of temperature using x-ray diffraction. In ternary systems however, an extra degree of freedom exists such that two order parameters describe the transition and the measurement of both requires separate diffraction experiments using different incident radiations. In this study we have measured both order parameters in the B2-disorder transition of the Heusler alloy AlMnCu₂ using a combination of high temperature x-ray and neutron powder diffraction, analyzed by the Rietveld method. For temperatures just below the critical point (.88 < T/Tc < 1.0) it is shown that the concentrations of Al and Mn on a given sublattice are not equivalent. The results are compared with two separate first-principles, electronic structure calculations. This work is supported by the NSF under contract #DMR 93301220. ORNL is managed by Lockheed Martin Energy Research for the DOE under contract #DE-AC05-96OR22464. Work at Sandia was supported by the U.S. DOE, Office of Basic Energy Science, Division of Materials Science.