Moiré fringes for misfit measurements at incoherent γ/γ′-interfaces of nickel-base superalloys

Abstract

In standard heat treated superalloys the cuboidal {gamma}{prime}-precipitates are coherently embedded in the {gamma}-matrix. The relative difference in lattice-spacings, the misfit, is compensated elastically: lattice-planes perpendicular to the coherent {gamma}/{gamma}{prime}-interface show zero misfit. During high temperature creep or long-term annealing coherency is lost: applied and misfit stresses press dislocation loops into the matrix channels between the {gamma}{prime}-cubes, leading to the formation of dislocation networks in the interfaces. Misfit measurements on such incoherent interfaces are interesting, because in long-term annealed specimens they give information about the unconstrained misfit, in deformed specimen about the internal stresses during deformation. In 1997, it was shown that the misfit in undeformed superalloys is not homogeneously distributed. Its absolute value is the highest in the primary dendrite arm (PDA) and the lowest in the interdendritic region (IR). This has to be considered when analyzing the misfit in deformed material. With the present work the authors introduce misfit analysis by Moire-fringes in incoherent interfaces as a simple method with high spatial resolution. It is applied to measure locally misfit changes during high temperature creep. The results are compared with those obtained by X-ray diffraction, transmission electron microscopy (TEM) analysis of misfit dislocation networks and theoretical considerations.