The objective of this paper is to investigate Resonance Ultrasound Spectroscopy (RUS) measurement models to more precisely connect changes in the resonance frequencies of nickel-based super-alloy material to the macro/microscopic state. RUS models using analytical solutions and the finite element method (FEM) were developed to address varying elastic properties, grain structures and creep. Experimental studies were performed investigating the effect of exposure to high temperatures and stress for varying part shape and three grain structure classes: single crystals, directionally-solidified and polycrystalline structures. Inversion using both traditional analytical models was enhanced in order to simultaneously estimate varying material properties and changes in part geometry due to creep. Inversion using surrogate models from FEM simulations was also developed, addressing varying crystal orientation and complex geometries. Results are presented comparing the forward model trends and inversion results with nickel alloy parts under various test conditions.