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A new model for the yield stress in superalloys accounting for unimodal and multimodal γ′ size distributions is presented. A critique of the classic models on γ′ shearing is presented ...and important features not previously considered are incorporated in our model. This is extended to account for multimodal particle size distribution effects by weighting the individual particle contribution to the total strength. This analysis is focused on powder metallurgy alloys. The yield stress and particle strengthening are predicted for eight superalloys containing wide variations in initial microstructure, composition and at temperatures up to 700°C. We demonstrate through a theoretical approach that the strength of alloys with multimodal γ′ is lower than those with unimodal γ′ radius in the vicinity of 10–30nm. For the first time, a parameter-free physics-based model is able to predict the yield stress in superalloys with complex microstructures, including unimodal and multimodal γ′ size. This has been possible by removing limitations inherent to the classical models. Such approach also enables critical evaluation of the relevant factors contributing to the yield strength of polycrystalline superalloys.
Creep deformation of the CMSX-4 nickel-base single crystal superalloy is studied in the range 750–850°C. Emphasis is placed on elucidating the factors causing primary creep when the tensile stress is ...orientated within 20°C of the technologically important 〈001〉 direction. It is demonstrated unambiguously that primary creep occurs only if a threshold stress of approximately 500MPa is exceeded; thereafter the accumulated primary creep strain is proportional to the magnitude by which the threshold stress is surpassed. A very high dependence of the strain rate on the applied stress is observed. Transmission electron microscopy confirms that the stress threshold is associated with the movement of a〈112¯〉 dislocation ribbons through the γ′ precipitates by so-called stacking fault shearing. The conditions necessary for the nucleation and propagation of a〈112¯〉 dislocation ribbons and the cessation of their movement are considered; it is demonstrated that the anisotropy of creep deformation for tensile loading within 20°C of 〈001〉 can be rationalised.
High-resolution scanning transmission electron microscopy (STEM) has been used to study the structure of dislocations in single crystal superalloy samples that have been subjected to conditions that ...favour the primary creep regime. The study has revealed the detailed structure of extended a2〈112〉 dislocations as they shear the γ′ precipitates during creep. These dislocations dissociate in a manner that is consistent with predictions made using the phase-field model of dislocations and also suggests the importance of the reordering process during their movement. The shearing done by the a〈112〉 dislocations was also found to distort the γ/γ′ interface, changing its appearance from linear to a “saw tooth” pattern. Another important observation was the segregation of alloying elements with a high atomic mass to the stacking faults, presumably to reduce their energies during shear. Numerous a2〈110〉 dissociated dislocations were also observed in the γ channels of the superalloy. The high resolution provided by the STEM imaging enables one to study the high-energy faults that are usually difficult to observe in conventional weak-beam TEM, such as complex intrinsic and extrinsic stacking faults in the γ′ and intrinsic stacking faults in the γ, and to make estimates of their energies.
The Alloys by Design project aims to develop and refine an integrated array of models to assist in the rapid development of nickel based superalloys. A viable alloy must be stable, creep resistant, ...castable and coatable. By incorporating the complex interactions between composition, microstructure and processing of the alloys into models, a wider range of compositions can be explored enabling more innovative design strategies. Each of these issues is addressed at a range of appropriate levels from the atomic scale to the continuum.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Allvac 718Plus is a newly developed nickel-based superalloy designed to replace Inconel 718 in static and rotating applications in gas turbine engines. Fine γ′ precipitates act as its principal ...strengthening phase and a second plate-like phase is generally present at grain boundaries. This has been reported to be the δ phase (Ni3Nb, D0a, orthorhombic), and is used to pin grain boundaries and improve resistance to intergranular fracture. In Allvac 718Plus non-uniform precipitation of a δ-like phase was observed along the grain boundaries; however, no relation was found between grain-boundary misorientation and the occurrence of precipitation. The crystal structure and chemistry of this phase was found to be different from the orthorhombic Ni3Nb δ phase reported previously in Inconel 718 and Allvac 718Plus. The true structure was found to be consistent with the hexagonal η-Ni3Ti D024 structure, but its chemistry was close to Ni6AlNb with partial ordering of Al and Nb over the prototype Ti sites. It was found that the serrated boundaries observed in the alloy were a result of discontinuous precipitation of η-Ni6AlNb, which was a predominant precipitation mechanism throughout the microstructure.
A dislocation network which formed during selective laser melting (SLM) of a Ni-base superalloy was analyzed using scanning transmission electron microscopy (STEM). This network traverses an ordered ...Gamma'-phase domain, in between two adjacent Gamma-solid solution regions. The Gamma’-phase region has formed when two Gamma’-phase particles have started to coalesce, trapping the dislocation network in this ordered region so that it formed two dislocation families with pairs of anti-phase boundary (APB) coupled super partial dislocations. The network features are presented and unusual features (twist character and low APB energies), not previously reported, are discussed.
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Superlattice intrinsic stacking faults (SISF) are the main culprit for the low temperature creep deformation of modern nickel-based superalloys used in jet engines. While these faults ...were identified over fifty years ago, their nucleation mechanism remains unclear. This work provides the first ever experimental evidence, via transmission electron microscopy, of a SISF nucleating from a cross-slip event in a polycrystalline alloy. Such an instance was identified in a grain with a near-〈001〉 tensile loading orientation. In the nucleation mechanism proposed, cross-slip allows the two dissimilar a2〈110〉 dislocations required to form a SISF to meet on adjacent planes at a precipitate interface. The concept of a nascent fault is introduced: the initial stacking fault that forms on a crystallographic plane and the dislocations of which continue to form coplanar faults as they glide away. This nucleation mechanism and the subsequent dislocation evolution are detailed taking into consideration the shear stresses on the individual Shockley partials and the full dislocations involved, as well as the stress orientation dependence of the energy barrier for cross-slip. These findings will guide future characterisation efforts in the field and inform the modelling of more realistic predictive models of creep behaviour.
Literature results show that creep properties differ significantly with specimen thickness. This behaviour can be attributed to the influence of oxidation. In the present work the effect of oxidation ...on creep of single crystal nickel-based superalloy M247LC SX samples with varying thickness was studied. Results reveal that aluminium depletion during oxidation results in a modified matrix/γ′-microstructure in the near-surface regions. The differences from nominal alloy composition were measured as a function of distance from the oxidised surface. From these measurements five compositions were chosen representing different positions within the oxidised specimen. All of these alloys were cast as single crystals with different γ′ fractions.
Creep experiments were carried out at 980°C in vacuum to determine the influence of the varying γ′ fraction on secondary creep. The dependence of stress level on creep rate was approximated by a Norton creep law. The stress exponent n plotted against the γ′ fraction shows a S-shaped behaviour almost doubling at 50% volume fraction. This behaviour suggests a change in the dominant creep mechanism. Using precipitate hardening theories this progression can be attributed to the transition of dislocation movement from cutting ordered γ′ precipitates to Orowan bowing mechanism with decreasing γ′ fraction. These results enable a correlation of oxidation-affected γ′ fraction and secondary creep, which is essential for the design and the improved prediction of creep deformation of thin sections of single crystal turbine blades.
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.
The superalloy CMSX-4 was tested under non-isothermal creep conditions and in situ analysed using synchrotron X-ray diffraction. The influence of temperature jumps between 950 °C and 1160 °C on the ...γ′-volume fraction and the lattice misfit were recorded. From this dataset, a phase fraction model was derived using a diffusion driven Johnson-Mehl-Avrami approach.
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