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mmercially useful multiphase alloy Inconel 783 is made up of Ni(28%), Fe(25%), and Co(35%) with smaller amounts of Cr, Al, Nb, Mo, Ti, C, and Si. In this work, relative temperature variations of the lattice constants a(T)/a 0 of the gamma (face-centered-cubic) and beta (body-centered-cubic) phases of this alloy are determined from 97 to 773 K using analyses of their x-ray diffraction (XRD) patterns. Plots of a(T)/a 0 for the two phases vary from 1.0 at 97 K to 1.012 at 773 K and show that (i) for T > 500 K, thermal expansion of the  phase is larger than that of the ␥ phase; and (ii) an abrupt jump is observed near 300 K. The appearance of new lines above 700 K in XRD representing Co2CrO4 and CoCr2O4 is interpreted in terms of the oxidation of the ␥ phase, whereas the  phase is oxidation resistant. The anomalous change in a(T)/a 0, observed near 300 K and accompanied by a similar change in the temperature variation of the initial magnetic susceptibility, is not well understood. A brief discussion on the implications of these results is presented.
I. INTRODUCTION
Fe–Ni–Co base superalloys with low coefficient of thermal expansion (CTE, ␣) are useful for turbine casting for high-temperature applications. Their low CTE results from the Invar effect in which the normal CTE is compensated by the contraction associated with the loss of ferromagnetism on heating. (For the Invar effect, see the review by Wasserman.1) However these alloys tend to be sensitive to stress-assisted grain-boundary oxidation (SAGBO) embrittlement, because of their low Cr content. Because Cr depresses the ferromagnetic Tc of the face-centered-cubic (fcc) Ni-based ␥ phase2 at the rate of about 50 °C/at.% of Cr, more Cr cannot be added without losing the benefits of the Invar effect. The above considerations have been employed to develop the commercial alloy Inconel 783 by Inco Alloys International, Huntington, WV.3 Subsequent studies of alloy 783 have been reported by Mannan and deBarbadillo,4 Higginbotham et al.5, and Lyons et al.6 Scanning electron microscopy (SEM), optical microscopy, and transmission electron microscopy (TEM) studies reported in these references show that alloy 783 contains the face-centered-cubic (fcc) ␥ phase, the  phase with the NiAl body-centered-cubic (bcc) structure, and a finely dispersed strengthening ␥⬘ phase with the ordered a)
II. EXPERIMENTAL DETAILS
The sample of alloy 783 was kindly provided by Inco Alloys International and it was used as received without any additional heat treatments discussed in Refs. 4–6.
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 15, No. 8, Aug 2000
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Ni3Al fcc structure. The distribution and to some extent the amounts of these phases can be affected by different heat treatments.4–6 These heat treatments followed by high-temperature crack-growth experiments showed that the presence of the  phase is beneficial against notch brittleness and SAGBO.4–6 However, the reasons for the beneficial effects of the  phase ar
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