Lattice misfits in four binary Ni-base gamma/gamma' alloys at ambient and elevated temperatures

inHigh-temperature X-ray diffractometry was used to determine the in situlattice parameters, a(gamma) and a(gamma'), and lattice misfits, delta = (a(gamma') - a(gamma))/a(gamma), of the matrix (gamma) and dispersed gamma'-type (Ni(3)X) phases in polycrystalline binary Ni-Al, Ni-Ga, Ni-Ge, and Ni-Si alloys as functions of temperature, up to about 680 degrees C. Concentrated alloys containing large volume fractions of the gamma' phase (similar to 0.40 to 0.50) were aged at. 700 degrees C to produce large, elastically unconstrained precipitates, The room-temperature misfits are 0.00474 (Ni-Al), 0.01005 (Ni-Ga), 0.00626 (Ni-Ge), and -0.00226 (Ni-Si), with an estimated error of +/-4 pct. The absolute values of the lattice constants of the gamma and gamma' phases, at compositions corresponding to thermodynamic equilibrium at about 700 degrees C, are in excellent agreement with data from the literature, with the exception of Ni3Ga, the lattice constant of which is much larger than expected. In Ni-Ge alloys, delta decreases to 0.00612 at 679 degrees C, and in Ni-Ga alloys, the decrease is to 0.0097. In Ni-Si and Ni-Al alloys, delta exhibits a stronger temperature dependence, changing to -0.00285 at 683 degrees C (Ni-Si) and to 0.00424 at 680 degrees C (Ni-Al). Since the times required to complete the high-temperature X-ray diffraction (XRD) scans were relatively short (2.5 hours at most), we believe that the changes in delta observed are attributable to differences between the thermal expansion coefficients of the gamma and gamma' phases, because the compositions of the phases in question reflect the equilibrium compositions at 700 degrees C. Empirical equations are presented that accurately describe the temperature dependences of a(gamma), a(gamma'), and delta over the range of temperatures of this investigation.