| Popis: |
The realization of advanced alloy compositions in service relies on a thorough understanding of metallurgical processing variables. Within this work, the gamma prime precipitation of an advanced powder metallurgy nickel-base superalloy during controlled cooling from supersolvus temperatures is compared to prior alloy generations using a complement of characterization and modeling approaches.The on-cooling precipitation of the alloy is studied and characterized to calibrate a multi-scale precipitation model. The proposed framework incorporates a computationally efficient addition to the mean-field modeling approach that increases its ability to model dynamic, multi-modal gamma prime burst events. The gamma prime size predicted by the model shows good agreement with experimental results. The precipitation calculation is applied to the element integration points of a continuum Finite Element heat conduction simulation, where the latent heat generated from the precipitation is accounted for. The results are compared to experimental findings and indicate potential use of the model for evaluating precipitation effects at multiple length scales.The lattice misfit evolution of two commercial PM nickel superalloys during cooling from supersolvus temperatures is also characterized, using in-situ synchrotron X-Ray Diffraction (XRD). The diffraction pattern deconvolution necessary for quantifying misfit was accomplished by combining observation of the superlattice peak intensities with thermodynamic modeling to quantify the intensity relationship between the overlapping phases. The misfit from the XRD measurements was compared to the Scanning Electron Microscopy observations of gamma prime particle shapes for a subset of the experimental conditions. The trend of measured misfit agreed with the microstructural characterization. Time-resolved observations of the on-cooling lattice parameter suggest that lower-temperature changes to the peak intensity characteristics coincide with low misfit magnitudes and a plausible connection to gamma prime burst events.Variation in cooling rate for this alloy and its predecessors shows a tendency for unstable precipitate growth with slower rates. To better understand the effect of precipitate morphology on defect interaction, a series of lab heat treatments of varied cooling rates were carried out and the mechanical response characterized. A phase-field based approach is used to simulate the growth instability of gamma prime precipitates during cooling and compared to post-mortem characterization using serial sectioning and reconstruction. Phase-field modeling is then used to interrogate the interaction of the particle morphology with planar dislocation evolution. It was determined that the incipient stages of particle evolution are dictated by interface growth instability more so than elastic anisotropy effects. Planar deformation in the presence of more evolved particles tended to promote Orowan looping while smaller particles with smaller gamma channel widths showed a tendency for stacking fault formation under the conditions characterized. To further understand precipitate morphology on properties, a stand-alone spectral-based microelasticity model is employed to predict the stress field around the particles. First order deformation assumptions are assessed to understand the effect of the microelastic stress field interacting with dislocations. By comparing the fields of varied precipitate morphologies, it is observed that the magnitudes and spatial distribution of the stress tensor components vary by morphology and may contribute to differences from a defect interaction standpoint. |
