C.M.F. Rae

The precipitation of topologically close-packed phases in rhenium-containing superalloys

Abstract Most modern single crystal alloys are thermodynamically unstable with respect to the formation of topologically close-packed (TCP) phases due to the addition of solid solution strengtheners, such as rhenium, molybdenum and tungsten. The occurrence, nucleation and growth of TCP phases in a number of rhenium-containing second generation alloys have been studied. It has been found that all the alloys form the σ phase; in some cases as a meta-stable intermediate. Most go on to form either the μ of P phase, depending on the alloy composition, the second phase nucleating from the σ phase. The structure of the σ phase is shown to depend on the fit between the γ matrix and the σ phase and it is suggested that alloys which show good lattice fit suffer accelerated TCP precipitation.

On the kinetics of rafting in CMSX-4 superalloy single crystals

Abstract The kinetics of γ′ rafting in CMSX-4 single crystals at 950°C has been studied using scanning and transmission electron microscopies. The crept material was subjected to further heat treatment in the absence of applied stress, in order to deduce the threshold strain ϵth for rafting to continue; this is shown to be 0.10±0.03%. For strains smaller than ϵth, rafting occurs exceedingly slowly; once ϵth is exceeded the kinetics are largely unaffected by the absence of applied stress. The magnitude of ϵth confers a reduction in γ/γ′ interfacial coherency and a relaxation of interfacial misfit stresses. It is concluded that the threshold strain is a quantity suitable for distinguishing between rafting occurring in the “elastic” and “plastic” regimes. The results confirm conclusively that plasticization of the horizontal γ-channels by (a/2)〈1 1 0〉{111} creep dislocations and their subsequent adsorption at the γ/γ′ interfaces, with a concomitant loss of perfect coherency and reduction in elastic misfit strains, is responsible for providing the kinetic path which enables rafting to occur at a reasonable speed. An argument is put forward to explain the magnitude of ϵth.

Creep of CMSX-4 superalloy single crystals: Effects of misorientation and temperature

Abstract In order to better understand the creep performance of CMSX-4 superalloy single crystals, creep strain testing has been carried out on various specimens systematically misaligned by up to 20° from 〈001〉. At 750°C, significant primary creep is observed, the extent of which depends strongly upon small misorientations away from the 〈001〉/〈011〉 symmetry boundary. In this regime there is evidence of secondary creep which is associated with primary creep strain sufficient for two or more further {111}〈11 2 〉 systems to become active. At 950°C, tertiary creep is prevalent, there being little primary creep exhibited under the stress levels tested. Measurements of the shape change and the lattice rotation, as deduced from analysis of electron backscatter patterns in the scanning electron microscope (SEM), confirms that the macroscopic shape deformation is due to {111}〈11 2 〉 at 750°C but that this changes to {111}〈1 1 0〉 at 950°C. This conclusion is supported further by observations made using transmission electron microscopy (TEM). The extent of creep as a function of misorientation away from 〈001〉 has been rationalised with a numerical model which takes account of the underlying deformation mechanisms. In the case of primary creep, the model accounts for the initial orientation of the tensile axis, the lattice rotation, the hardening on the primary slip system, the extent of primary creep, the onset of secondary creep and the evolution of the macroscopic creep strain. There appears to be evidence to suggest that the maximum creep rate in the primary regime can be correlated to the secondary rate, which is invariant with increasing creep strain, i.e. a steady state is reached. At higher temperatures where tertiary creep is prevalent, the misorientation dependence of creep deformation is less strong; its extent is rationalised on the basis of Schmids law. For the purposes of engineering design, it is suggested that the occurrence of {111}〈11 2 〉 and {111}〈1 1 0〉 deformation be associated with primary and tertiary creep; these should be modelled as two separate curves which are associated with hardening and softening, respectively.

Recrystallisation of single crystal superalloy CMSX–4

Abstract A series of experiments investigating the recrystallisation of single crystal superalloy CMSX-4 have been carried out. Indentation atroom temperature has been used to study the effects of annealing time and temperature, and it has been found that a very strong dependence upon temperature is evident. Annealing above the γ′ solvus temperature results in very rapid growth of recrystallised grains whereas annealing below the γ′ solvus greatly suppresses the advancing grain boundaries. Additionally experiments have been carried out using an electrothermal mechanical test (ETMT) machine, to study the effects of degree of plastic strain and the temperature at which the strain is introduced. The strain threshold for recrystallisation under various annealing conditions has been determined and it has been found that recrystallisation occurs more readily if strain is introduced above 950°C. Finally, apparent activation energies for recrystallisation have been determined by measuring the change in resistivity that occurs during recrystallisation.

The role of stacking fault shear in the primary creep of [001]-oriented single crystal superalloys at 750°C and 750 MPa

The structure of the a〈112〉 dislocation ribbons in CMSX-4 single crystal superalloy crept at 750°C and 750 MPa has been characterised using transmission electron microscopy. Calculations are performed which confirm that the macroscopic creep deformation occurring under these conditions, i.e. the accumulation of primary creep, is principally a result of the movement of the ribbons through the γ/γ′ structure. Emphasis is placed on clarifying the factors which control the movement of the fault: it appears that the rate controlling step is the entry of the final a/3〈112〉 dislocation into the γ′ phase, which has the effect of removing the superlattice extrinsic fault. Further observations suggest that the a/2〈110〉 dislocations play a vital role not only in nucleating the ribbons, but also in terminating their long-range movement through the microstructure. It is proposed that this latter effect results in the breakdown of primary creep and thus the transition to secondary ‘steady-state’ creep.

Kinetics of rafting in a single crystal superalloy: effects of residual microsegregation

Abstract The kinetics of directional γ′ coarsening, known as rafting, are examined in the nickel based single crystal superalloy CMSX-4. Electron microscopy and image analysis are used to characterise heat treated specimens, prestrained in creep to a plastic strain beyond the threshold required to initiate rafting. The kinetics of rafting are found to be sensitive to the presence of microsegregation from casting, incompletely removed by the heat treatment process; thus the dendritic regions raft appreciably faster than the interdendritic ones – this despite rhenium enrichment at the dendrite cores consistent with its preferred partitioning during solidification. Estimates of the activation energy required for rafting in the two regions are established and Avrami-type equations developed. Since the important γ′ forming elements aluminium and tantalum are known to partition interdendritically causing an enhanced γ′ fraction, it is suggested that the correspondingly smaller γ channel width so caused is responsible for the greater resistance to rafting displayed by the interdendritic regions.

Solution Heat Treatment Optimization of Fourth-Generation Single-Crystal Nickel-Base Superalloys

The optimization of the solution heat treatment (SHT) of fourth-generation single-crystal nickel-base superalloys LDSX-6B and LDSX-6C is presented. The methodological approach to optimizing the SHT process is particularly highlighted. Differential scanning calorimetry (DSC) measurements and electron-probe microanalysis (EPMA) mapping were carried out to investigate material properties in the as-cast condition and after SHT. The DSC equipment was also adopted as a vacuum furnace to evaluate the suitability of the SHT ramp profile and to check the safety margins with regard to incipient melting during SHT. SHT trials were carried out in a laboratory-scale vacuum furnace, after which the heat-treated samples were subjected to DSC experiments, microstructural analysis, and EPMA mapping to assess the effects of SHT peak temperatures and soak periods. From the DSC and EPMA results, as-cast LDSX-6B shows a lower degree of elemental microsegregation; hence, this alloy is relatively easier to homogenize in the SHT trials. In contrast, as-cast LDSX-6C was found to have a higher degree of elemental microsegregation; therefore, it is much more difficult to homogenize and is highly prone to incipient melting. The results of this study indicate that an increase in the SHT peak temperature and/or soak period will lead to an improved compositional homogeneity in the material as expected. After SHT, both alloys retained some residual elemental microsegregation and the LDSX-6C alloy showed precipitation of topologically close-packed (TCP) phase at the dendrite cores. The most appropriate and economic SHT process may be determined based on the methodological approach presented in this study and the requirements of the materials during service.

Extrinsic and intrinsic nodes in the gamma prime phase of a single crystal superalloy

Abstract Transmission Electron Microscopy has been used to examine the interaction of superdislocations to form nodes and networks in the γ′ phase of the cold deformed and annealed single crystal superalloy, SRR99. The formation of these “super nodes” is traced through various metastable configurations to form, eventually, extended “super nodes” which contain superlattice stacking faults. Several dislocations interacting are observed to form networks of such nodes having alternately intrinsic and extrinsic character. The extrinsic supernodes are found to be larger than the intrinsic supernodes with the implication that the superlattice extrinsic stacking fault energy is lower than the superlattices intrinsic fault energy.

On the movement of grain boundary dislocations in recrystallizing interfaces

Abstract The movement of grain boundary dislocations in a recrystallizing boundary in aluminium has been shown by the observation of oblique slip traces left behind a migrating interface. This indicates that the moving grain boundary retains the structure of the stationary boundary. The observed movement cannot, however, completely account for the migration using a dislocation mechanism for migration.

On the precipitation of delta phase in ALLVAC ® 718Plus

ALLVAC 718Plus is a new commercial superalloy derived from Inconel 718, but possessing a higher temperature capability whilst employing the same philosophy regarding the microstructure. Many articles have been published describing various heat treatments exploiting the precipitation of intermetallic phases at grain boundaries to optimize the mechanical properties over a range of testing conditions. The requirement to further improve the mechanical properties of this alloy drives our interest in the precipitation mechanism of the delta and eta phases found in this alloy. We report the presence of finely layered structures composed of two phases, delta and eta, with distinct structures and chemistries. Possible pathways to explain this precipitation in 718Plus are considered as follows: (i) the sequential formation of the delta from eta phase and (ii) the simultaneous precipitation of both eta and delta facilitated via solute rejection. Both can result in the formation of those small delta layers observed in HRSTEM. We discuss which is most likely by comparing the relative alignment of the phases by image processing and the analysis of the HRSTEM images, and propose formation mechanisms consistent with the distinctive dislocation structures observed at the interface.