Processing-Structure-Property Relationships in Directly Aged Alloy 718

V. Vieira Rielli


This thesis elucidates, via high-resolution microscopy, how chemical and morphological aspects of the gamma prime and gamma double prime precipitates are affected during conventional and direct ageing, and the impacts of the cooling rate from forging on precipitation. Additionally, radial property heterogeneities in an industrially forged turbine disk inherited from processing are correlated with a strengthening model of the contributions from grain boundaries, solid solution, dislocation density, and precipitation. Finally, through a systematic approach, changes in hot deformation temperature and strain rate enable the clarification of the effects of geometrically necessary dislocations on the co-precipitation phenomenon, where gamma prime and gamma double prime precipitate simultaneously during direct ageing. The effects of the micro- to nanostructure changes on mechanical properties are evidenced with hardness testing, previously shown to correlate well with high-temperature yield strength