Thermal expansions of the beta and gamma phases in a Co-Ni-Fe superalloy determined by x-ray diffraction

Commercially 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. Ln this work, relative temperature variations of the lattice constants a(T)/a (0) of the gamma (face-centered-cubic) and beta (body-centered-cubic) phase s of this alloy are determined from 97 to 773 K using analyses of their x-r ay diffraction (XRD) patterns. Plots of a(T)/a(0) for the two phases vary f rom 1.0 at 97 K to 1.012 at 773 K and show that (i) for T > 500 K, thermal expansion of the beta phase is larger than that of the gamma 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 th e oxidation of the gamma phase, whereas the beta phase is oxidation resista nt. 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 su sceptibility, is not well understood. A brief discussion on the implication s of these results is presented.