Temperature and composition dependence of the elastic constants of Ni 3 Al
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INTRODUCTION
THE ordered alloy Ni3Al (L12 structure) is technically important, because it is the so-called g 8 phase that occurs as precipitates in nickel-base superalloys. It is generally believed that the differences between the elastic constants and lattice parameters of the matrix and precipitates strongly influence the morphology and spatial correlations of the g 8 precipitates, which in turn can affect the mechanical properties of the alloys. We have recently measured the lattice constants of the equilibrium matrix and g 8 phases in four binary nickel-base alloys to temperatures up to ,1000 K.[1] We have also measured the cij of a monocrystalline NiAl solid solution alloy containing 12.69 at. pct Al to 1300 K.[2] These data, in conjunction with previous measurements of the cij on solid solutions of other compositions,[3] provide a thorough characterization of the cij as functions of temperature and composition for the Ni-Al solid solution over nearly the entire range of its phase stability. What is missing from a complete picture of the elastic behavior of two-phase NiAl g/g 8 alloys are measurements of the cij of Ni3Al as a function of composition and temperature. The single-crystal elastic constants cij of Ni3Al have been measured by four groups of investigators.[4–7] The data of Kayser and Stassis[4] and Wallow et al.,[5] who measured the cij of Ni3Al of stoichiometric composition, are in good agreement, but the results reported by the other two investigators are not. For example, the room-temperature values of the cij measured by Ono and Stern[6] and Dickson et al.[7] vary from 15 to 40 pct lower than those of Kayser and Stassis. However, the Ni3Al phase in the alloy of Ono and Stern contained 24 pct Al, while that in the alloy of Dickson et al. contained only 22.5 pct Al. These differences in the values of the cij are rather large and suggest that the cij could S.V. PRIKHODKO, Spectroscopist, H. YANG, Undergraduate Student, and A.J. ARDELL, Professor, Department of Materials Science and Engineering, and J.D. CARNES, Graduate Student, and D.G. ISAAK, Associate Researcher, Institute of Geophysics and Planetary Physics, are with the University of California, Los Angeles, CA 90095-1595. Manuscript submitted January 28, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
be a very strong function of composition, as shown in Figure 1. In addition to the data in Figure 1, Franse et al. measured the Young’s modulus, E, of two polycrystalline alloys containing 24 and 25 pct Al and reported that E was much smaller in the alloy with the smaller aluminum concentration.[8] The apparent dependence of the elastic constants of Ni3Al with composition was a major factor prompting the present investigation. The possible dependence of the cij of Ni3Al on aluminum content, especially at compositions lower than 25 pct, is important because, in two-phase Ni-Al g/g 8 alloys, the Ni3Al phase contains only a little more than 23 pct Al.[9] Additionally, the previously measured cij[4,5] are reliable over the temperature range from 29
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