N. D’Souza

On the Roles of Oxidation and Vaporization in Surface Micro-structural Instability during Solution Heat Treatment of Ni-base Superalloys

Micro-structural instability at the surface that develops during solution heat treatment of a typical third generation Ni-base superalloy, CMSX10N has been reported. It is shown that elemental Ni vaporizes from the surface during solutioning leading to de-stabilization of γ phase. With increasing extent of vaporization, a phase mixture of β, γ′, and the refractory (W and Re-rich) precipitates occur within the surface layers resulting in the complete breakdown of the cuboidal γ/γ′ phase morphology that is usually observed. It is demonstrated that the conditions at the surface have a marked effect on the vaporization kinetics and subsequent evolution of surface phases—the presence of a continuous dense oxide such as Al2O3 or the presence of sacrificial Ni-foils interspersed in the furnace significantly suppresses elemental vaporization from the sample surface.

Role of Elemental Sublimation during Solution Heat Treatment of Ni-Based Superalloys

The role of elemental evaporation on the microstructural stability of blade surfaces has been investigated on solutioned and aged samples of Ni-based single-crystal superalloys. Evaporation of Ni and Cr at the casting surface during solution heat treatment leads to the formation of a Ni- and Cr-depleted layer at the surface. Nucleation and growth of γ′ phase occur within this layer through subsequent long-range diffusion of Re, Ta, and W between the γ′ layer and the substrate. Beyond a critical Ni and Cr loss, incipient melting initiates at the surface and principally γ′ and TCP phases are stabilized with de-stabilization of γ phase. Nucleation of TCP phases occurs at grain boundaries arising from cellular recrystallization during the ramp-up cycle. Therefore, on quenching, a range of microstructures are observed at the casting surface.

Detailed Analysis of the Solution Heat Treatment of a Third-Generation Single-Crystal Nickel-Based Superalloy CMSX-10K®

A detailed analysis of the response of as-cast third-generation single-crystal nickel-based superalloy CMSX-10K® to solution heat treatment (SHT) has been carried out, alongside an SHT optimization exercise. The analysis was conducted through microstructural characterization, differential scanning calorimetry, and compositional homogeneity measurements, quantifying (i) the dissolution and microstructural evolution of the inter-dendritic constituents, (ii) the shift in thermo-physical characteristics of the material, and (iii) the change in compositional homogeneity across the microstructure, in order to gain further understanding of these phenomena during the progression of the SHT. During the early stages of SHT, the coarse cellular γ′/narrow γ channel inter-dendritic constituents which were the last areas to solidify during casting, progressively dissolve; homogenization between these inter-dendritic areas and adjacent dendritic areas leads to a rapid increase in the incipient melting temperature TIM. The fine γ/γ′ morphology which were the first inter-dendritic constituents to solidify after primary γ dendrite solidification were found to progressively coarsen; however, subsequent dissolution of these coarsened γ/γ′ inter-dendritic areas did not result in significant increases in the TIM until the near-complete dissolution of these inter-dendritic areas. After the final SHT step, residual compositional micro-segregation could still be detected across the microstructure despite the near-complete dissolution of these remnant inter-dendritic areas; even so the TIM of the material approached the solidus temperature of the alloy.

Discontinuous Precipitation in Ni-Base Superalloys During Solution Heat Treatment

Discontinuous precipitation in single-crystal Ni-base superalloys during solution heat treatment has been studied. It is found that discontinuous precipitation occurs at temperatures approaching the solvus, where volume diffusion is dominant. Diffusion of Al ahead of the boundary leads to gamma prime precipitation and is accompanied by a loss in the driving force available for advancement of the grain boundary. The rate of gamma prime precipitation was tracked using in situ neutron diffraction during isothermal hold. Gamma prime precipitation is accompanied by super-saturation of Cr and W within the channels ahead of the interface. The driving force calculated for the initial stages of DP was [10-5 to 10-4] N/[μm2 of the grain boundary]. The results provide an insight into discontinuous precipitation during solution heat treatment of Ni-base single-crystal alloys and are useful in optimizing the heat treatment process to avoid surface defect formation.

Implications of solute super-saturation in growth of vaporisation-induced recrystallised grains during heat treatment in Ni-base superalloys

Abstract The role of vaporisation in the evolution of surface micro-structural instability has been investigated in Ni-base superalloys during heat treatment below the solvus temperature. Initial nucleation of beta (β) grains at the surface with random orientations arose from vaporisation of Ni, Co and Cr with subsequent de-stabilisation of gamma (γ) phase. Further vaporisation and inter-diffusion resulted in a discontinuous reaction (secondary reaction zone – gamma prime (γ′) matrix + topologically close packed phases with needle-like morphology). On the other hand, advancement of the secondary reaction zone grain boundary was shown to be dependent only on the solute supersaturation ahead of the boundary. The driving force for growth of the secondary reaction zone was calculated from the Gibbs energy change and nearly all of this force was available to pull the grain boundary.

The contrasting roles of creep and stress relaxation in the time-dependent deformation during in-situ cooling of a nickel-base single crystal superalloy

Time dependent plastic deformation in a single crystal nickel-base superalloy during cooling from casting relevant temperatures has been studied using a combination of in-situ neutron diffraction, transmission electron microscopy and modelling. Visco-plastic deformation during cooling was found to be dependent on the stress and constraints imposed to component contraction during cooling, which mechanistically comprises creep and stress relaxation. Creep results in progressive work hardening with dislocations shearing the γ′ precipitates, a high dislocation density in the γ channels and near the γ/γ′ interface and precipitate shearing. When macroscopic contraction is restricted, relaxation dominates. This leads to work softening from a decreased dislocation density and the presence of long segment stacking faults in γ phase. Changes in lattice strains occur to a similar magnitude in both the γ and γ′ phases during stress relaxation, while in creep there is no clear monotonic trend in lattice strain in the γ phase, but only a marginal increase in the γ′ precipitates. Using a visco-plastic law derived from in-situ experiments, the experimentally measured and calculated stresses during cooling show a good agreement when creep predominates. However, when stress relaxation dominates accounting for the decrease in dislocation density during cooling is essential.

Solidification Reaction Sequence of Co-Rich Nb-Al-Co Alloys

The freezing reaction sequence of Co-rich Nb-Al-Co ternary alloys with emphasis on the formation of Laves and Heusler phases has been examined. For Co-rich alloys, the solidification reaction sequence is observed as primary freezing of α-Co and CoAl phases, subsequent [Co + C36] and [CoAl + C36] eutectics, and the final ternary eutectic reaction [L → α-Co + C36 + CoAl]. The compositions of solidified α-Co and C36 phases agree with the corresponding vertices of the tie-triangle at the solidus temperatures. When the Nb concentration is over 20 at. pct in Co-rich alloys, the quasi-peritectic reaction [L + Co2AlNb → C36 + CoAl] does not occur as equilibrium prediction. The formation of C36 and CoAl phases occurs through solid precipitation and must be distinguished from a solidification reaction.