Influence of Ni:Co ratio and temperature on the lattice misfit of ?-?' Ni-Co-Al-Ti-Cr alloys

This study investigated the influence of a systematic variation in Ni:Co ratio on the lattice misfit of gamma-gamma' NiCo-5Al-5Ti-15Cr (at%) alloys over a broad temperature range. Seven model superalloys denoted by their nominal Co content, 0Co, 9Co, 19Co, 28Co, 38Co, 47Co and 56Co (at%), were hot-rolled to achieve a fine grain size and then aged at 800 degrees C for 1000 h. Neutron diffraction was performed on all seven alloy compositions at room temperature, 400 degrees C, 600 degrees C, 700 degrees C and 800 degrees C to determine the lattice parameters of the gamma and gamma' phases. Alloys of Co content 0-47 at% showed a reduction in lattice misfit with increasing temperature from ambient to 800 degrees C, with misfit values remaining positive throughout this temperature range. These lattice misfit-temperature profiles demonstrated an almost systematic increase in misfit with increasing alloy Co content from 0 to 38 at%. In contrast, the 47Co alloy exhibited lower lattice misfit values than the 38Co alloy at temperatures ambient to 700 degrees C. Neutron diffraction patterns for the 56Co alloy obtained at each test temperature indicated splitting of gamma' superlattice peaks, plausibly a consequence of the elongation of the gamma' precipitates observed following ageing and of their spatial distribution. The lattice misfit data acquired were compared with elemental partitioning data for the same alloy compositions of the Ni-Co-5Al-5Ti-15Cr (at%) system similarly aged at 800 degrees C. (c) 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).