Ultra High-Temperature Deformation in a Single Crystal Superalloy: Meso-Scale Process Simulation and Micro-Mechanisms

Abstract

A mesoscale study of a single crystal nickel-base super-alloy subjected to an industrially relevant process simulation has revealed the complex interplay between micro-structural development and the micro-mechanical behavior. As sample gauge volumes were smaller than the length scale of the highly cored structure of the parent material from which they were produced, their subtle composition differences gave rise to differing work hardening rates, influenced by varying secondary dendrite arm spacings, γ′ phase solvus temperatures and a topologically inverted γ/γ′ micro-structure. The γ′ precipitates possessed a characteristic ‘X’ morphology, resulting from the simultaneously active solute transport mechanisms of thermally favored octodendritic growth and N-type rafting, indicating creep-type mechanisms were prevalent. high resolution-electron back-scatter diffraction (HR-EBSD) characterization reveals deformation patterning that follows the γ/γ′ micro-structure, with high geometrically necessary dislocation density fields localized to the γ/γ′ interfaces; Orowan looping is evidently the mechanism that mediated plasticity. Examination of the residual elastic stresses indicated the 'X' γ' precipitate morphology had significantly enhanced the deformation heterogeneity, resulting in stress states within the γ channels that favor slip, and that encourage further growth of γ' precipitate protrusions. The combination of such localized plasticity and residual stresses are considered to be critical in the formation of the re-crystallization defect in subsequent post-casting homogenization heat treatments.