Using Variant Selection to Facilitate Accurate Fitting of γ″ Peaks in Neutron Diffraction

Abstract

c¢¢ diffraction peaks are hard to discern in neutron/X-ray diffraction patterns, hindering studies on the c¢¢-strengthened superalloys using in-situ diffraction. In this study, we propose a variant selection method to increase the intensity of c¢¢ peaks and to facilitate accurate fitting. The specific variants of c¢¢ are controlled by applying a 300 MPa tensile stress during aging at 790°C for 5 hours. The interaction energy between the applied stress and the transformation strain of each c¢¢ variant differs, leading to an increase in the amount of the variants with a greater energy reduction at the expense of other variants. The enhanced variants result in greater c¢¢ peak intensities in neutron diffraction patterns, allowing both the Pawley refinement and single peak fitting to be performed. Lattice parameters of c¢¢ and c phases, and lattice misfit between the two phases and volume fraction of c¢¢ are acquired. The uncertainties associated with the fitting maintain an acceptable level corresponding to 150 microstrains. The proposed variant selection method shows potential for studying the role of c¢¢ phase in Ni-base superalloys.

Evolution of Lattice Spacing of Gamma Double Prime Precipitates During Aging of Polycrystalline Ni-Base Superalloys: An In Situ Investigation

“…The purpose of the applied tensile stress in the first experiment was to introduce the SIVS effect on the c¢¢ variant distribution to facilitate the measurement of c¢¢ lattice spacing. [17] In addition, the selected c¢¢ variant distribution will lead to anisotropic evolution of lattice spacing during aging, providing extra information for studying the relationship between the evolution of lattice spacing and precipitation process.…”

“…Round c¢ particles with a smaller size than the c¢¢ are also observed in both grains. Our previous study, [17] in which the IN718 sample underwent a 300 MPa tensile-stress aging at 790°C for 5 hours, has estimated the c¢/c¢¢ volume fraction ratio to be 0.36. In the current study, the sizes of both c¢ and c¢¢ precipitates are similar to those in the previous study; therefore, it is reasonable to consider the c¢/c¢¢ volume fraction ratio to be 0.36.…”

“…The c¢¢ variant distributions in differently oriented grains after tensile-stress aging agree with our previous observation on the same material after aging heat treatment with a 300 MPa tensile stress applied for 5 hours. [17] B. Diffraction Pattern Diffraction spectra obtained by bank 1 and bank 2 detectors for the stress-free aging at the beginning and end of aging are shown in Figure 4. Both bank 1 and bank 2 spectra show the peaks shifted to a lower d-spacing at the end of aging, indicating the lattice parameter of the c matrix phase decreased due to the aging.…”

“…Such SIVS effect resulted in a measurable intensity of the {004} c¢¢ peak that allowed accurate peak fitting. [17] The {200} peak obtained by bank 2 detector were contributed from the family of grains with {100} plane normal to the transverse direction of the sample (also parallel to the diffraction vector for bank 2 detector). Due to the SIVS effect, the c¢¢ variant with disc-plane parallel to the longitudinal direction was absent in this family of grains, resulting in a larger intensity of the {200} c¢¢ peak in the spectrum for bank 2 detector.…”

“…Such a stress-induced variant selection (SIVS) is attributed to the interaction between the applied or residual stresses and the misfit strain. [4,17,18] Coarsening of the c¢¢ precipitates is of a great concern to IN718 since the size of precipitates has significant influences on the strengthening, and the continued growth of precipitates leads to loss of coherency and transformation from metastable c¢¢ phase to stable d phase which is detrimental to the strengthening. Coarsening behavior of the c¢¢ precipitates has been studied by a few researchers, [3,12,13,[18][19][20] primarily confirming Lifshitz-Slyozov-Wagner (LSW) theory of particle coarsening and showing that this is controlled by the volume diffusion of Nb in the c matrix.…”