Ternary atom site location in L1 2 -structured intermetallic compounds
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I. INTRODUCTION
Ternary substitutional elements are often added to binary intermetallic compounds. These additions may either be used to improve the properties of a compound—for example, hafnium additions improve the strength of the Ll2-structured compound Ni 3 Al/ or they may be used to change the crystal structure of a compound from one of low symmetry to one of higher symmetry—for example, nickel additions to Al 3 Ti transform the compound from a DO22 (tetragonal) structure to an Ll 2 (cubic) structure,2"6 shown in Fig. 1. The effect of these ternary additions may depend upon which lattice site they occupy; therefore, determination of the ternary lattice site is important. There are several methods available for determination of lattice site
occupancy. Two, which are readily available in the typical materials laboratory, are a transmission electron microscope technique called ALCHEMI,7'8 or atom site location by channeling enhanced microanalysis, and x-ray diffractometry, through measuring the relative integrated intensity of fundamental and superlattice x-ray diffraction peaks. ALCHEMI has been shown to be an effective method for the determination of site occupancy in stoichiometric compounds, but so far has not been applied widely to nonstoichiometric materials which contain anti-site defects. For further details on the ALCHEMI technique the reader is referred to a recent overview by Banerjee.9 This paper presents the results of measurements by both ALCHEMI and x-ray diffractometry of lattice site occupancy, of hafnium in Ni3Al and nickel in Al3Ti, and hence provides a comparison of these two techniques for ternary atom site location. II. EXPERIMENTAL
FIG. 1. The LI2 structure adopted by many A3B compounds. J. Mater. Res., Vol. 6, No. 5, May 1991 http://journals.cambridge.org
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Thin foils of as-cast multiphase Al 6 9Ti 23 Ni 8 (in at.%) were prepared by electropolishing 250 ^im thick, 3 mm diameter disks with a solution of 30% nitric acid in 70% methanol at 248 K and a voltage of 12 V. An Ll2-structured phase, designated as the TT phase, present in the alloy was determined to have a composition, by energy dispersive spectroscopy (EDS), of approximately Al74.2Ti19Ni6.8.10 Thin foils of an alloy, designated IC-77, which had a composition Ni76Al22Hf2 + 500 ppm B were prepared through established electropolishing routes.11 The thin foils were examined in a JEOL 2000FX transmission electron microscope (TEM), furnished with a Tracor Northern 5500II EDS system. EDS data 943
© 1991 Materials Research Society IP address: 131.170.6.51
P. R. Munroe and I. Baker: Ternary atom site location
were recorded using an accelerating voltage of 80 kV, to minimize delocalization effects (see later), using an incident beam divergence of ~5 mrad, spot sizes up to —250 nm, and acquisition times of 150 s. Pairs of data were obtained at two orientations, in a strong channeling condition directly down a zone axis, and in a random weak channeling condition —5° away from this zone axis. From these data the co
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