Microstructural and compositional design of Ni-based single crystalline superalloys ― A review

Ni-based superalloys play an important role in aircraft engine propulsion. However, many experiments demonstrated that these alloys as well as Ni always show ductility loss at intermediate temperature, which constrains further development. A comparison of published papers by various authors reveals considerable differences in understanding the mechanism of intermediate temperature embrittlement. To clarify this situation, the present article first confirms the generality of intermediate temperature embrittlement of Ni and Ni-based alloys by the experimental results reported in the literature. The existing interpretations of the mechanism are then outlined. Based on the generality, these interpretations are discussed through the representative investigations on intermediate temperature embrittlement. It is shown that the mechanism of intermediate temperature embrittlement has not been satisfactorily explained yet and “nonequilibrium interface segregation” of impurities taking into account the effect of str...

Mechanism of Intermediate Temperature Embrittlement of Ni and Ni-based Superalloys

Mechanisms of creep deformation in nickel-base superalloy single crystals in the directional coarsening regime have been studied in alloys with large variations in γ-γ′ lattice misfit and phase composition, achieved by Ru additions and variable levels of Cr and Co. Interfacial dislocation spacings established by long-term annealing experiments under no externally applied stress indicate that the experimental alloys have high-temperature lattice misfits ranging from near-zero to as large as −0.65 pct. Variation in misfit influences the stress-induced directional coarsening (rafting) behavior during creep deformation at 950 °C and 290 MPa. In postcreep deformed material, the density of excess dislocations (defined as the dislocations beyond those necessary to relieve the lattice misfit) at the γ-γ′ interfaces varied with alloy composition, with the most creep-resistant alloy containing the highest excess interfacial dislocation density. In the directional coarsening creep regime, continued deformation requires shearing of the γ′ rafts and is strongly influenced by the resistance of the precipitates to shearing as well as the interfacial dislocation structure. A preliminary model for creep in the rafting regime is developed.

Interfacial Dislocation Networks and Creep in Directional Coarsened Ru-Containing Nickel-Base Single-Crystal Superalloys

The influence of Re and Ru on phase diagram, as-cast microstructure, and segregation behavior has been investigated using a series of 12 experimental nickel-base superalloys derived from the parent alloy CSMX-4. Up to 2 at. pct Re and 4 at. pct Ru were added at the expense of nickel, while all other alloying elements were kept constant. The Re is found to increase the liquidus temperature and to decrease the γ′-solvus. The Re additions also strongly increase the amount of eutectic and the tendency for microsegregation. The Ru, on the other hand, displays much weaker effects or even no effect at all. The effects of Re on the eutectic formation and on segregation are shown to be related. Calculations based on ThermoCalc software were carried out and compared with the experimental results. ThermoCalc is found to be able to predict the changes in the phase diagram rather well. However, the calculation of the eutectic fraction with the Scheil–Gulliver model is not satisfying, which is due to the one-dimensionality of the model.

Solidification Characteristics and Segregation Behavior of Nickel-Base Superalloys in Dependence on Different Rhenium and Ruthenium Contents

The partitioning behavior and sublattice site preference of Re or Ru in the Ni3Al (L12) γ′- precipitates of model Ni–Al–Cr alloys are investigated by atom-probe tomography (APT) and first-principles calculations. Rhenium and Ru are experimentally observed to partition to the γ(fcc)-phase, which is consistent with the smaller values of the γ-matrix Re and Ru substitutional formation energies determined by first-principles calculations. APT measurements of the γ′-precipitate composition indicate that Re and Ru occupy the Al sublattice sites of the Ni3Al (L12) phase. The preferential site substitution of Re and Ru at Al sublattice sites is confirmed by first-principles calculations.

The partitioning and site preference of rhenium or ruthenium in model nickel-based superalloys : An atom-probe tomographic and first-principles study

Single crystal nickel-base superalloys with spherical, intermediately shaped and cuboidal γ′ precipitates have been observed in Ru-containing superalloys with substantial amounts of Re and W. The γ′ precipitates experienced stress-induced directional coarsening (rafting) during creep deformation at 950 °C and 290 MPa indicative of a wide range of γ–γ′ lattice misfit, which vary from negative to near-zero to positive. The precipitate morphology, lattice misfit and compositions of the γ and the γ′ phases are interrelated, and therefore, the elemental partitioning between the phases were investigated to determine the origin of the unusual microstructural features. TEM-based EDS analysis of the γ and γ′ phases indicate that the combination of Ru and Cr influences the partitioning of Re, which displays a wide range of partitioning behavior within the investigated alloy compositions. In alloys containing 5.7–9.6 wt.% Ru and no Cr, Re partitioned to both the γ and γ′ phases. With high levels of Cr (6.7 wt.%) the Re strongly partitioned to the γ matrix, while in alloys with intermediate Cr and Ru levels, the Re displayed intermediate partitioning behavior. The Ru and Co partitioned nearly equally between the γ and the γ′ phase in all of the experimental Ru-containing alloys investigated. The degree of Cr partitioning was also similar among the alloys investigated, however, it partitioned more strongly to the γ phase. These investigations demonstrate that Ru and Cr control the lattice misfit and precipitate shape, through the associated changes in system thermodynamics that alter the γ and γ′ phase compositions.

Elemental partitioning in Ru-containing nickel-base single crystal superalloys

Ruthenium-containing multicomponent Ni-base superalloys with large variations in refractory alloying elements (Re, Ru, Ta, and W) have been investigated with respect to solidification, segregation characteristics, and the tendency to develop grain defects during directional solidification. Phase transformation temperatures and the effects of alloy composition on the liquidus temperature were determined by differential thermal analysis (DTA). The liquidus temperatures for most Ru-containing superalloys are generally higher than those of current commercial single-crystal superalloys. The partitioning behavior of individual constituents under the influence of alloy chemistry was characterized using a quantitative segregation mapping technique combined with a Scheil-type analysis. Whereas ruthenium partitioned preferentially to the dendrite cores during soldification, segregation of Ru is much less pronounced than Re and W. A higher degree of rhenium segregation was observed in Ru-containing superalloys. For the fixed processing conditions and moderate levels of Ru+Re, single-crystal solidification occurred without freckle formation or convection-induced breakdown of the solidification front. However, with high levels of Ru (9.6 ∼ 14.1 wt pct) and Re (7.2 wt pct), grain defects or the complete breakdown of single-crystal solidification was observed. Results from segregation and DTA analyses were used to estimate the corresponding Rayleigh numbers present during solidification of the experimental alloys. The Rayleigh criterion is effective for predicting the conditions under which the grain defect formation occurs during directional solidification of Ru-containing superalloys.

Soldification segregation in ruthenium-containing nickel-base superalloys

The cyclic oxidation behavior of Ru-containing single crystal superalloys at 1100 °C has been investigated in alloys with Ru contents ranging from 3.5 to 9 at.% (5.7–14.1 wt.%) and Cr additions ranging from 0 to 8 at.% (0–6.7 wt.%). High levels of Cr additions (8 at.%) significantly improved oxidation resistance, while increased levels of Ru adversely affected the oxidation performance. A multi-layered scale formed on the alloy surface, and it generally consisted of an external layer of NiO, an intermediate layer of spinel and an Al 2 O 3 inner layer. The presence of a continuous α-Al 2 O 3 inner layer clearly improved oxidation performance. Ru-rich precipitates were observed in the spinel layer of alloys which displayed poor oxidation resistance. High levels of Cr additions not only reduced the propensity for formation of Ru-rich phases in the oxide, but also promoted the formation of a continuous alumina layer adjacent to the superalloy, without stabilization of any detrimental phases in the superalloy.

Cyclic oxidation of Ru-containing single crystal superalloys at 1100 °C

The effects of ruthenium and chromium contents have been investigated on phase transformation temperatures and the morphology of γ’ precipitates as well as microstructural stability in high refractory Ni-base superalloys. The solidus and liquidus temperatures were determined by differential scanning calorimetry (DSC), suggesting that the addition of Cr resulted in a decrease in solidus/liquidus temperatures while the Ru addition (3.5 at.%) had the neutral effect. The morphology of γ’ precipitates in the heat-treated microstructure was changed under the influence of the Cr and Ru additions, suggesting that Ru and Cr contents affected the γ−γ’ lattice misfit through changes in the associated partitioning to the constituent phases. The microstructural instability has been investigated at 1000 oC. High levels of Cr addition (8 at.%) strongly promoted the formation of TCP phases, while Ru improved the microstructural stability to some extent.

L.J. Carroll

Papers

Elemental partitioning in Ru-containing nickel-base single crystal superalloys

Soldification segregation in ruthenium-containing nickel-base superalloys

Cyclic oxidation of Ru-containing single crystal superalloys at 1100 °C

Influence of Ru and Cr on the heat-treated microstructure of Ni-based superalloys