Local Phase Transformations—A New Creep Strengthening Mechanism in Ni-Base Superalloys

Polycrystalline Ni-based superalloys are vital materials for disks in the hot section of aerospace and land-based turbine engines due to their exceptional microstructural stability and strength at high temperatures. In the drive to increase operating temperatures and hold times in these engines, hence increasing engine efficiency and reduction of carbon emissions, creep properties of these alloys becomes increasingly important. At these higher temperatures, new deformation modes become active. Several alloy compositions and microstructure variants of commercial disk alloys are being explored, including g’ strengthened alloys as well as compositions promoting g’ and g’’ co-precipitation. Microtwinning and stacking fault shearing are important operative mechanisms in the critical 600-800°C temperature range. Advanced electron-microscopy-based techniques have been used to gain new insights into these mechanisms and the rate-limiting processes during high temperature deformation. Atomic-scale chemical and structural changes associated with stacking fault and microtwin interfaces within g’ precipitates have been identified and indicate that local phase transformations (LPT) occur commonly during creep of superalloys. Furthermore, the important deformation modes can be modulated by LPT formation, enabling a new path for improving high temperature properties. This work is part of a GOALI-DMREF program funded by the National Science Foundation in which collaborators are exploring the thermodynamic driving force for LPT formation using DFT modeling, and phase field dislocation dynamics modeling is being used to explore the interaction of dislocations with g’ microstructures under the cooperative shearing and local compositional changes associated with the LPT mechanism.