Relating micro-segregation to site specific high temperature deformation in single crystal nickel-base superalloy castings

Thermo-mechanical deformation of the solid on cooling following solidification has been studied quantitatively in a Ni-base single crystal superalloy, CMSX-4 used in turbine blade applications. In the as-cast state, the alloy has location specific properties due to micro-segregation of alloying elements during solidification; this effect become increasingly important with smaller specimen cross-section in thermo-mechanical tests. Accordingly, normalised resistance/resistivity tests have been used to classify samples with varying micro-segregation, given the different γ and γ′ phase resistivities. Increased normalised resistance corresponds to increased local solvus temperature, which determines the plastic strain and stress evolution during cooling. Upon cooling from above the γ′ solvus temperature, dislocation creep occurs within the γ phase accompanied by a small increase in stress. A critical precipitation γ′ volume fraction is reached as the material cools, leading to precipitation hardening as measured by a dramatic resistance change and thereby stress increase at lower temperatures. Short-term creep tests capturing the history-dependent deformation, as demonstrated by controlled cooling experiments, gives steady-state creep, enabling parameter measurement for a Norton-type constitutive equation in a given temperature range. Implications of these results to modelling of plastic strain and stress during cooling from close to solvus temperature during casting has been discussed.<br

Ultra-high temperature deformation in a single crystal superalloy: Mesoscale process simulation and micromechanisms

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, γ′ phase solvus temperatures and a topologically inverted γ/γ′ microstructure. The γ′ precipitates possessed a characteristic butterfly 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 γ/γ′ microstructure, with high geometrically necessary dislocation density fields localised to the γ/γ′ interfaces; Orowan looping is evidently the mechanism that mediated plasticity. Examination of the residual elastic stresses indicated the butterfly γ′ precipitate morphology had significantly enhanced the deformation heterogeneity, resulting in stress states within the γ channels that favour slip, and that encourage further growth of γ′ 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

Ultra-high temperature deformation in a single crystal superalloy: Mesoscale process simulation and micromechanisms

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, γ′ phase solvus temperatures and a topologically inverted γ/γ′ microstructure. The γ′ precipitates possessed a characteristic butterfly 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 γ/γ′ microstructure, with high geometrically necessary dislocation density fields localised to the γ/γ′ interfaces; Orowan looping is evidently the mechanism that mediated plasticity. Examination of the residual elastic stresses indicated the butterfly γ′ precipitate morphology had significantly enhanced the deformation heterogeneity, resulting in stress states within the γ channels that favour slip, and that encourage further growth of γ′ 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

Influence of cooling rate on the precipitation kinetics of nanoscale isothermal omega-phase in metastable beta-Ti alloy, Ti-5Al-5Mo-5V-3Cr

In metastable β-Ti alloys, nanoscale isothermal ω-phase (ωiso) precipitates are regarded as the nucleation sites for the α strengthening phase. Here we investigate the precipitation kinetics of the ωiso precipitates as a function of cooling rate (air cooling and water quenching) after β-solutionising. A combined in situ small-angle neutron scattering (SANS) and electrical resistivity measurement approach was used during ageing of Ti–5Al–5Mo–5V–3Cr wt% (Ti-5553) alloy at 300 °C and 325 °C up to 8 h. The SANS modelling was consistent with ellipsoid shaped particles for the ωiso precipitates, for both air-cooled and water-quenched samples. The precipitates attained a maximum size (equatorial diameter) of ∼21 nm and ∼17 nm after 2 h and 4 h of ageing the water-quenched and air-cooled samples respectively. Although the air-cooled samples showed delayed nucleation in comparison to water-quenched sample, the volume fraction became approximately the same (∼11%) after ageing for 8 h. The average value of the activation energy for ωiso nucleation from the β-phase matrix was determined as 122 kJ mol−1 from electrical resistivity data using a modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The hardness increased with ageing time, with water quenching leading to a higher final value of hardness than air cooling