HIGH-TEMPERATURE MEASUREMENTS OF LATTICE-PARAMETERS AND INTERNAL-STRESSES OF A CREEP-DEFORMED MONOCRYSTALLINE NICKEL-BASE SUPERALLOY

High-temperature X-ray line profile measurements were performed to max imal temperatures of 1050 degrees C on samples of the nickel-base supe ralloy SRR 99. The samples with rod axes near the [001] direction were investigated in the initially undeformed state and after creep deform ation at different temperatures and stresses. For the measurements of the (002) and (020) line profiles, a special X-ray double crystal diff ractometer with negligible line broadening was used which was equipped with a high-temperature vacuum chamber. The line profiles were evalua ted for the lattice parameters of the matrix phase gamma and the preci pitated gamma' phase and for values of the lattice mismatch parallel a nd perpendicular to the stress axis, respectively, which were found to be different. Elastic, tetragonal distortions of the phases gamma and gamma' could be determined between room temperature and about 900 deg rees C. These distortions are thermally induced due to the different t hermal expansion coefficients of the two phases and deformation induce d due to interfacial dislocation networks which were built up during d eformation. At the high temperatures of the X-ray measurements, at lea st partial recovery of the deformation-induced internal stresses occur red, depending on the temperature of the X-ray measurements. The resul ts are discussed and compared with data obtained by complementary tech niques.