Ultra high-temperature deformation in a single crystal superalloy: Meso-scale process simulation and micro-mechanisms

TL;DR

This study uses mesoscale simulations and micro-mechanical analysis to understand high-temperature deformation mechanisms in a single crystal superalloy, revealing complex microstructural interactions and stress effects influencing deformation and defect formation.

Contribution

It provides new insights into the micro-mechanisms of deformation and microstructural evolution in superalloys under high-temperature conditions, combining process simulation with advanced characterization.

Findings

Gamma-prime precipitates exhibit 'X' morphology due to active solute transport mechanisms.

Dislocation activity is localized at gamma/gamma-prime interfaces, primarily via Orowan looping.

Residual stresses and microstructural features promote deformation heterogeneity and defect formation.

Abstract

A mesoscale study of a single crystal nickel-base superalloy subjected to an industrially relevant process simulation has revealed the complex interplay between microstructural development and the micromechanical behaviour. 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, gamma-prime phase solvus temperatures and a topologically inverted gamma/gamma-prime microstructure. The gamma-prime precipitates possessed a characteristic `X' morphology, resulting from the simultaneously active solute transport mechanisms of thermally favoured octodendritic growth and N-type rafting, indicating creep-type mechanisms were prevalent. High resolution-electron backscatter diffraction (HR-EBSD) characterisation reveals deformation patterning that follows the gamma/gamma-prime microstructure, with high geometrically necessary dislocation density fields localised to the gamma/gamma-prime interfaces; Orowan looping is evidently the mechanism that mediated plasticity. Examination of the residual elastic stresses indicated the `X' gamma-prime precipitate morphology had significantly enhanced the deformation heterogeneity, resulting in stress states within the gamma channels that favour slip, and that encourage further growth of gamma-prime precipitate protrusions. The combination of such localised plasticity and residual stresses are considered to be critical in the formation of the recrystallisation defect in subsequent post-casting homogenisation heat treatments.