Fig. 1. Illustration of the angle θ between stress axis and c-axis of the γ″ precipitate [30].

Fig. 2. Schematic illustration of in situ neutron diffraction tensile deformation experiment in ENGIN-X, ISIS neutron source, UK.

Fig. 3. (a) IPF along the z-axis on the cross-section of the TA sample, (b)-(d) γ″ variant distribution in grains selected from locations indicated in (a), (e) IPF on the cross-section of the CA sample, (f)-(h) γ″ variant distribution in grains selected from locations indicated in (e). Insets show the orientation of existing γ″ variants (indicated as either coloured ellipse or circle) with habit planes lying on different faces of the grey cube according to the {100}γ″//{100}γ and [100]γ″//<100>γ relationship with the matrix phase.

Fig. 4. Diffraction patterns obtained along the loading direction at room temperature (RT) before loading both tensile-aged (TA) and compressive-aged (CA) samples. Small {116}, {204}, and {004} γ″ peaks are discerned in TA sample but not in CA sample.

Fig. 5. Schematic illustration of diffraction from an <100> oriented grain in TA and CA samples, respectively, viewing along the transverse direction of the bar sample. Non-overlapping {004} γ″ peak is contributed from the variant with a c-axis parallel to the diffraction vector while the {200} overlapping peak is from the variant with an a-axis parallel to the diffraction vector.

Fig. 6. Fitting of diffraction peaks for (a)-(c) non-overlapping peaks for TA sample, and deconvolution results for (d)-(f) overlapping peak for CA sample, showing the accuracy of the fitting.

Fig. 7. Stress-lattice strain plots for each hkl reflection of both the γ and γ″ phases in the elastic regime. (a)-(c) for TA sample, (d)-(f) for CA sample. Diffraction elastic constants were obtained by linearly fitting to each of the plots. It is shown that the γ″ is stiffer along its c-axis (<004> direction) than a-axis (<200> direction), as well as stiffer than the matrix phase. Errors bars associated with the fitting uncertainties are about the size of the symbol and are not shown for clarity.

Fig. 8. Applied stress-strain curves for quasi-static in situ neutron tensile deformation and in-house laboratory tensile deformation of the two differently aged samples. The ‘x’ marker shows the point where the extensometer slipped.

Fig. 9. Evolution of hkl-specific lattice strain of the γ phase along the longitudinal direction for (a) TA and (b) CA samples. The horizontal lines indicate the onsets of nonlinearity, which coincide with the 0.2% yield strength obtained from macro stress-strain curves. The error bars associated with uncertainties from peak fitting are about the size of the symbol and are not shown here for clarity.

Fig. 10. Lattice strain evolution of the γ and γ″ phases for (a) TA and (b) CA samples during tensile deformation. The horizontal lines indicate the onsets of nonlinearity. Error bars associated with uncertainties of peak fitting are about the size of the symbol in the elastic regime for both phases, and increase from 200 to 1200 microstrains in the plastic regime for the γ″, 100 to 360 microstrains for the γ phase. Error bars are not shown for clarity.

Fig. 11. Evolution of normalized peak broadening of the γ and γ″ phases in the {200} and {220} oriented grains for the (a) TA and (b) CA samples. And peak intensity evolution for (c) TA and (d) CA samples. Error bars for peak width associated with uncertainties of peak fitting are about the size of the symbol in the elastic regime and increase from 0.07 to 0.44 in the plastic regime for both phases. Error bars for peak intensity are about the size of the symbol. Error bars are not shown for clarity.

Fig. 12. Schematic illustrations of active dislocations (arrows in blue and red) and existing γ″ variants in grains oriented with [011] parallel to loading direction in the (a) TA sample and (b) CA sample.