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

This study investigated the influence of a systematic variation in Ni:Co ratio on the lattice misfit of γ-γ' Ni-Co-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 °C for 1000 h. Neutron diffraction was performed on all seven alloy compositions at room temperature, 400 °C, 600 °C, 700 °C and 800 °C to determine the lattice parameters of the γ and γ' phases. Alloys of Co content 0–47 at% showed a reduction in lattice misfit with increasing temperature from ambient to 800 °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 °C. Neutron diffraction patterns for the 56Co alloy obtained at each test temperature indicated splitting of γ' superlattice peaks, plausibly a consequence of the elongation of the γ' 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 °C.