Temperature dependence of the lattice parameter of the γ and γ′ phases in the nickel-based superalloy CMSX-2 Part I: Observation of a tetragonal distortion of the γ′ phase at high temperature by γ-ray diffractometry

A tetragonal distortion of the γ′ phase of the CMSX-2 superalloy has been evidenced in situ at high temperature by an uncommon use of γ-ray diffractometry. The value of |c – a|/a at 1075°C is 1·8 × 10−3 and has the same order of magnitude as the lattice parameter mismatch between the γ and γ′ phases. Its evolution with temperature depends on the thermal history of the sample which modifies the γ′ precipitates' morphology. This shows that the tetragonal distortion is strongly influenced by the internal stress state of the materials.