Morphological evolution of γ′ precipitates in superalloy M4706 during thermal aging

After a standard heat treatment, the morphological evolution of γ′ precipitates in the directionally solidified Ni-base superalloy M4706 with time is investigated during thermal exposure at 800 °C. In contrast to previous results, it is found that as the average particle size increases from 241 to 368 nm with increasing aging time up to 10,000 h, the shape of γ′ precipitates gradually changes from cuboidal to sphere-like. X-ray diffraction experiments reveal that the constrained γ/γ′ lattice misfits of M4706 fall into the range of −0.226% to −0.223% between 800 and 850 °C, whereas the value is −0.529% at 1050 °C. Based on the experimental results, the morphological transition of γ′ precipitates is discussed.