LAUSR

Abstract The crystallography of grain boundary (GB) precipitates in a low coefficient of thermal expansion (CTE) Ni-Mo-Cr-W alloy has been investigated as a function of thermal history using transmission electron diffraction. A previously unreported cubic precipitate (φ) was found to form during aging at 1043 K for 16 h and remained after an additional heat treatment at 1143 K for 16 h. The space group of this phase is Pm 3 ¯ m with lattice parameter a∼6.42 A. In addition, the topologically close packed (TCP) phase known as μ in the literature also formed on grain boundaries when the material was heat treated at 1043 K for 16 h directly after cold rolling. Significantly, the μ phase that formed on the boundaries of unrecrystallized regions contained a high density of finely spaced (0001) nanotwins, which give rise to pseudo 6/mmm symmetry instead of the 3 ¯ m symmetry of the μ phase ( R 3 ¯ m space group) that formed on the boundaries in the recrystallized regions. After additional aging at 1143 K for 16 h, these nanotwins are absent, and the μ phase develops more widely spaced { 1 1 ¯ 02 } twins.