The Location of Tantalum Atoms in Ni 3 Al
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THE LOCATION OF TANTALUM ATOMS IN Ni3Al Hui Lin and David P. Pope Dept. of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104
ABSTRACT An X-ray powder diffraction method was used to determine the location of tantalum atoms in Ni 3 Al. A series of Ni 3 (Al,Ta) alloys were produced with tantalum content ranging from 0.1 to 3.0 at%. Fine powders with average particle sizes less than 80 pm were made from melt-spun ribbons by a grinding process. The values of the superlattice peak intensities normalized to the intensity of the (200) fundamental peak agreed with the calculated values assuming that tantalum atoms substitute on aluminum sites, not on nickel sites. Thus it is concluded from our experiments that tantalum atoms substitute for aluminum in Ni 3 AI, except for Ta contents around 0.2 at%where the data can not be interpreted in terms of a simple substitution scheme.
INTRODUCTION Ni3Al shows a large solubility for many ternary elements, and furthermore, the amount of strengthening produced by a given ternary addition depends upon the substitutional nature of the addition (1]. Those which substitute for Al atoms appear to have a much larger strengthening effect than those which substitute for Ni. The nature of the substitutional site is therefore an important variable in any study of solid solution strengthening of Ni 3Al. If it is assumed that the deficiency in Ni and/or Al content is compensated by the ternary element in order to maintain the stoichiometry of the alloy, the substitutional behavior of different ternary elements in Ni3Al can be determined simply by the direction of the gamma prime phase lobe of the corresponding ternary phase diagram . Recently, several microscopic techniques have been. employed to study the sites occupied by ternary elements in Ni3Al. Conflicting results obtained using atom probe field ion microscopy , ion channeling  and perturbed angular correlation  techniques have been reported. We report here our results on the sites occupied by Ta in Ni3 Al using the Xray powder diffraction method.
PRINCIPLES The intensities of powder pattern lines from a diffractometer are given by : I= IFIp Il+ COS2 20 e-2M
Mat. Res. Soc. Symp. Proc. Vol. 133. t1989 Materials Research Society
where I = relative integrated intensity (arbitrary units), F = structure factor, p = multiplicity factor, O= Bragg angle, and M = temperature factor. The trigonometric term in parentheses is the Lorentz-polarization factor. When the intensities of powder pattern lines having the same indices are compared for Ni 3AI containing different amounts of a ternary element (from 0.1 to 3.0 at%), the differences in the intensities are solely the result of differences in the structure factor, F. This is true because the small amount of ternary addition does not change the multiplicity factor, it changes the Bragg angle very little and has a small effect on the temperature factor. For Ni 3AI which has the L1 2 structure (ordered F.C.C.), the intensities of the fundamenta