X-RAY DIFFRACTION ANALYSIS ON THE THICKNESS EFFECT OF γ/γ′ LATTICE MISMATCHE IN NICKEL BASE SINGLE CRYSTAL SUPERALLOY DD10

The lattice constants of the matrix phase gamma and the precipitated phase gamma' in a wafer-like nickel-base single crystal superalloy DD10 were determined by four-axis X-ray diffractometer (XRD). The variations of the lattice constants and lattice mismatches of gamma and gamma' phases in the alloy wafer with different thicknesses, i.e., different interphase restriction conditions were obtained. A critical thickness of the alloy wafer to reach an unstable stress state was defined based on the observed changes of lattice mismatch associated with a transformation of stress state from the 3D bulk state to 2D plane state. The lattice mismatches corresponding to different mismatch stress states were calculated. The lattice constants of the gamma and gamma' phases under a relaxed non-mismatch stress state were evaluated based on certain assumptions. The results showed that after the thickness of the alloy wafer reached the critical value, the lattice constant of the gamma' phase decreased linearly with the decrease of the thickness, whereas that of the gamma phase kept nearly constant, resulting in a linear increase of the absolute value of lattice mismatch. Accompanied with the thinning process, the crystal lattice misorientation and the mosaicity increased, the gamma/gamma' interphase restraint reduced and the interphase stress relaxed, and the measured stress state changed to the plane state. The estimated critical thickness and the lattice constants under non-mismatch stress state could provide references for the design of the components and the measurement of the residual stress in the superalloy.