Variant Selection Assisted Neutron Diffraction Study on Gamma Double Prime Phase in Ni-base Superalloys

Neutron diffraction has become a power tool in studies of multi-phase materials such as Ni-base superalloys, provided that diffraction peak from each phase is distinguishable. However, for γ''-strengthened Ni-base superalloys like Inconel 718, applications of neutron diffraction have been limited to studies associated with γ matrix phase. Problems related to the γ'' phase is rarely investigated using neutron diffraction since the γ'' peaks are hardly discerned in diffraction patterns. The present research is devoted to the development of a novel method using stress-induced variant selection (SIVS) effect to make the γ'' peaks discernible in diffraction patterns, and the application of such a method in researches focused on the γ'' phase in Inconel 718.

The three γ'' variants can have preferential growth (selection) by applying a stress during aging heat treatment. This effect leads to an enhanced intensity of a γ'' peak at the expense of the other γ'' peaks, resulting in a discernible γ'' peak that can be fitted with relatively high accuracy in neutron diffraction experiments carried out in the ENGIN-X neutron diffractometer. Using this method, elastic properties of the γ'' phase are studied in tensile loading tests. The γ'' phase is found to be stiffer along its c-axis than along a-axis, as well as stiffer than the γ matrix phase. Constrained misfit strain is measured at various temperatures ranged from room temperature (RT) to 664 °C. Misfit stresses are interpreted using Eshelby’s inclusion theory. It is found that the misfit stresses in the γ'' precipitates are 3.0 GPa and 1.7 GPa compressive along the c-axis and a-axis, respectively, at RT. The γ'' phase shows a similar coefficient of thermal expansion (CTE) of its a-axis to that of the γ phase, while a smaller CTE of its c-axis than that of the γ phase, this leads to the misfit stresses decrease with increasing temperature. Evolution of lattice spacing of both the γ and γ'' phases during aging heat treatment is monitored in situ using neutron diffraction. It is indicated by the evolution of peak width that the majority of γ'' precipitates are formed within the first 1.8 hours of aging. The non-monotonic evolution of lattice strain suggests that the change in chemical composition has a dominant influence on the lattice spacing before 5 hours of aging, then the change in morphology of the γ'' precipitates (coarsening) takes the major role in affecting the lattice spacing. Deformation behaviour of the γ'' phase is studied during tensile loading tests from the lattice strain response to the applied stress. From the evolution of the lattice strain and peak width of both the γ and γ'' phases, it is revealed that dislocation shearing mechanism acts at the early stage of plastic deformation, while dislocation bowing occurs in the later stage. Deformation twinning may also happen in γ'' precipitates with disk-plane parallel to the loading direction.

The several studies associated with the γ'' phase in Inconel 718 superalloy reported in this thesis have proven that the SIVS method is a promising approach with broad applications in researches of the γ''-strengthened Ni-base superalloys including Inconel 718 and its derivatives. The presented results also contribute to the fundamental understanding of the γ'' phase in Inconel 718 and benefit the development of such a prominently used material in industry.