Concurrent Improvement of Additive Manufacturing Processability and Creep Performance in a Legacy Polycrystalline Superalloy Using Grain Boundary Strengtheners

Paper in proceeding, 2024

Microcracking during processingProcessing and underperformance in creepCreep have limited wider adoption of high γ′-fraction superalloysSuperalloys in Additive ManufacturingAdditive manufacturing (AM). Certain processingProcessing issues are now understood to be related to solidification crackingSolidification cracking caused by elements such as B and Zr that are also essential for creepCreep performance, particularly with fine-grained AM microstructuresMicrostructure. A legacy γ′-strengthened polycrystalline superalloyPolycrystalline superalloy738LC738LC was the subject of the current investigation. Printing trials conducted with the legacy composition and a modified version with 10 times the initial B content revealed extensive microcrackingCracking in the legacy composition, whereas the modified alloy produced a dense crack-free microstructureMicrostructure. CALPHADCALculation of PHAse Diagrams (CALPHAD)-based solidificationSolidification simulations and crackingCracking susceptibility index calculations were performed to attempt to rationalise these findings. After hot isostatic pressingHot Isostatic Pressing (HIP) (1120 °C, 200 MPa, 4 h) and ageing heat treatmentHeat treatment (850 °C, 24 h), stress ruptureStress rupture tests showed an improvement in the rupture life of the modified alloy. Samples perpendicular to the AM building direction (typically the weaker orientation) showed a 50% increase in rupture life compared to the conventional composition, and ruptureElevated-temperature ductility ductility was also enhanced. Elevated temperature tensile ductility in the perpendicular direction increased to ≈ 11 El% for the modified alloy versus ≈ 6 El% for the conventional composition. These improvements are attributed to the presence of fine boride precipitatesPrecipitates at grain boundariesGrain boundary of the modified alloy. The findings indicate that increasing the grain boundaryGrain boundary strengthening element content may be a potential solution for both processingProcessing and mechanical performance issues in this superalloySuperalloys.