Ni-base superalloys are among the highest temperature capable alloys and are used pervasively throughout the transportation, energy, and nuclear industries. However, their microstructures have been largely limited to containing the γ´ (cubic) and γ´´ (tetragonal) phases to enable high strength at elevated temperatures, and this fixation has restricted alloy development opportunities. In the past three decades, a new set of alloys, strengthened by the γ´´´ (orthorhombic) phase, was developed by Haynes International. The alloys exhibit comparable strength to existing Ni-based superalloys and show a 25% decrease in the thermal expansion coefficient, designed for tighter clearances (thus improving engine efficiency) and help to reduce thermally induced fatigue from engine cycling.
The newest iteration of such alloys, HAYNES® 244®, has a nominal composition of Ni-22.5Mo-8Cr-6W (wt.%), and each alloying element is used to help precipitate the γ´´´-Ni2(Cr, Mo, W) phase. The deformation mechanisms of this material are currently unknown. Previous studies investigating the predecessor alloy, HAYNES® 242® alloy, showed deformation twinning to be the dominant deformation mechanism during mechanical testing, but the physical phenomena responsible for this mode of deformation were not clearly elucidated. As a result, the primary motivation of this project is to understand the deformation behavior of the 244 alloy from the atomistic level and upwards.
This work details efforts to elucidate these deformation mechanisms using an integrated computational and experimental approach. First-principles calculations were performed to determine the entire generalized stacking fault energy (GSFE) surface and slip pathways of the γ´´´ phase for dislocation slip. The various planar defects that could form from dislocation slip were predicted to provide significant barriers for dislocation motion due to their very high planar defect energies (~1000 mJ/m2), likely precluding shearing of the precipitates. We incorporated these results into phase field dislocation dynamics (PFDD) to simulate dislocation-precipitate interactions of finite size. These results showed that the planar defect energies of the γ´´´ phase largely govern the deformation behavior and critical resolved shear stress for precipitate shearing, regardless of precipitate shape, size, or orientation. Extensive mechanical testing conducted from room temperature up to 760 ºC over strain rates ranging from 10-9 s-1 to 10-4 s-1 combined with transmission electron microscopy validated the predicted deformation structures of creep and tensile samples. Shearing of individual precipitates by intrinsic and extrinsic stacking faults, as well as extensive deforming twinning, was observed. The integrated GSFE and PFDD simulations showed that the precipitates would resist dislocation shearing and favor twinning as the preferred deformation mechanism at all temperatures and strain rates investigated. These results provide pathways for microstructural and composition modification to further increase the strength of γ´´´ strengthened alloys in the future.
- Doctor of Philosophy
- Materials Engineering
- West Lafayette