Accurate measurement of lattice misfit between gamma and gamma' phases in nickel-base superalloys at high temperatures

摘要

Accurate measurement of lattice misfit between gamma and gamma' phases in Ni-base single crystal superalloys at high temperatures has successfully been achieved using a parallel beam mode X-ray diffractometer with a high temperature specimen stage. The superalloys have gamma' precipitates, an ordered FCC structure based on Ni_3Al, in a gamma-matrix having a disordered FCC structure (Ll_2). The parallel beam mode, the optics of which is made from a Ge (111) channel cut crystal generates a superior monochromatic and parallel CuK_(alphal) beam, which makes it possible to resolve a cluster peak in Ni-base superalloys into individual gamma and gamma' peaks. This peak separation was almost impossible using the CuK_(alpha 1+2) doublet beam from the standard type para-focusing mode, due to the very small difference in lattice parameters between the two phases. The lattice misfit between the gamma and gamma' phases, which dominantly affects the creep properties of Ni-base superalloys, can be measured within an error of 0.01percent up to 1200 deg C. In addition, the coherency strain of the gamma' phase in the gamma-matrix was estimated by measuring the lattice parameter of the isolated gamma' phase particles as a reference.