Increase of misfit during creep of superalloys and its correlation with deformation

In superalloys the loss of coherency during creep results in the increase of misfit of the γ/γ′-interface. The kinetics of this process were measured locally by TEM (Moiré fringes) and X-ray diffraction. Two materials were creep tested (SRR99 and CMSX-4) in two temperature ranges (stable γ′-morphology and rafting), and the morphology changes were quantified. A microstructural model allows calculation of the equilibrium misfit and the increase of plastic strain on the basis of these data. At high temperatures and low stresses the model describes quantitatively creep kinetics up to 30 h. Here the processes controlling primary creep are propagation of dislocation loops along matrix channels and thickening of the matrix channels orientated perpendicular to the load direction.     

Improved High-Temperature Microstructural Stability and Creep Property of Novel Co-Base Single-Crystal Alloys Containing Ta and Ti

The influence of Ta and Ti additions on microstructural stability and creep behavior in novel Co-Al-W base single-crystal alloys has been investigated. Compared to the ternary alloy, the γ′ solvus temperature and γ′ volume fraction were raised by individual additions of Ta and Ti, and increased further in the quinary alloy containing both alloying additions. In contrast to ternary and quaternary alloys, an improved microstructural stability with the stable γ–γ′ two-phase microstructure and more than 60% γ′ volume fraction existed in the quinary alloy after prolonged aging treatment at 1050°C for 1000 h. The creep behavior at 900°C revealed lower creep rates and longer rupture lives in the quaternary alloys compared to the ternary alloy, whereas the quinary alloy exhibited even better creep resistance. When the creep temperature was elevated to about 1000°C, the creep resistance of the quinary alloy exceeded the previously reported Co-Al-W-base alloys and first-generation Ni-base single-crystal superalloys. The improved creep resistance at approximately 1000°C was considered to be associated with high γ′ volume fraction, γ′ directional coarsening, and dislocation substructure, which included γ–γ′ interfacial dislocation networks and the sheared γ′ precipitates containing stacking faults and anti-phase boundaries.     

Mechanical and electrical characterisation in age hardened Waspaloy microstructures

Abstract

Nickel-base superalloys such as Waspaloy possess excellent mechanical properties at elevated temperatures. This is possible through precipitation hardening, whereby submicron precipitate phases (γ′) are formed in the nickel-rich matrix phase (γ) under controlled aging heat treatments. The current work is focused on studying microstructural evolution in systematically aged Waspaloy specimens through mechanical and electrical characterisation conducted via Vickers microhardness and four probe resistivity measurements respectively. Four bars of industrial grade Waspaloy were solution treated at 1145°C and aged at temperatures of 600, 725, 800 and 875°C for times ranging from 2 min to 763·5 h. For the specimens aged at 600 and 725°C, the maximum in hardness was not reached. However, maxima in hardness and conductivity occur at similar aging times for the samples aged at 800 and 875°C. The present studies indicate that conductivity measurements show potential for ascertaining the hardness of the alloy relative to the over-aged state.     

Effects of rhenium addition on the temporal evolution of the nanostructure and chemistry of a model Ni–Cr–Al superalloy. I: Experimental observations

The temporal evolution of the nanostructure and chemistry of a model Ni–8.5 at.% Cr–10 at.% Al alloy, with the addition of 2 at.% Re, aged at 1073 K from 0.25 to 264 h, was studied. Transmission electron microscopy and atom-probe tomography were used to measure the number density and mean radius of the γ′ (L1₂ structure)-precipitates and the chemistry of the γ′-precipitates and the γ (face-centered cubic)-matrix, including the partitioning behavior of all alloying elements between the γ- and γ′-phases and the segregation behavior at γ/γ′ interfaces. The precipitates remained spheroidal for an aging time of up to 264 h and, unlike commercial nickel-based superalloys containing Re, there was not confined (nonmonotonic) Re segregation at the γ/γ′ interfaces.     

Silver segregation to θ′ (Al2Cu)–Al interfaces in Al–Cu–Ag alloys

θ′ (Al₂Cu) precipitates in Al–Cu–Ag alloys were examined using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The precipitates nucleated on dislocation loops on which assemblies of γ′ (AlAg₂) precipitates were present. These dislocation loops were enriched in silver prior to θ′ precipitation. Coherent, planar interfaces between the aluminium matrix and θ′ precipitates were decorated by a layer of silver two atomic layers in thickness. It is proposed that this layer lowers the chemical component of the Al–θ′ interfacial energy. The lateral growth of the θ′ precipitates was accompanied by the extension of this silver bilayer, resulting in the loss of silver from neighbouring γ′ precipitates and contributing to the deterioration of the γ′ precipitate assemblies.     

Lattice-parameter misfits between the γ and γ′ phases in single crystals of nickel superalloys

X-ray diffractometry has been used to study the transformation of the profiles of X-ray diffraction lines (222), (004), (003) and to determine the values of the lattice misfit between the γ and γ′ phases (γ/γ′ misfit) for single crystals of nickel superalloys ZhS6U, ZhS30, ZhS32, VZhM1, and VZhM4 in the as-cast state and after heat treatment. It has been established that with an increase in the γ/γ′ misfit there increase tetragonal distortions of the fcc lattice of the γ solid solution. For the single crystals of VZhM4 alloyed with rhenium and ruthenium, splitting of the superlattice reflection of the strengthening γ′ phase has been revealed. The structural regularities revealed testify to qualitative changes in the factors that govern the redistribution of refractory alloying elements between the lattices of the γ and γ′ phases, for example, of Mo in the presence of Re and Ru.     

Microstructures of Rene 142 nickel-based superalloy fabricated by electron beam melting

Rene 142, a commercial, columnar grained, gas turbine airfoil Ni-based superalloy, has been fabricated from a pre-alloyed, atomized powder by additive manufacturing using electron beam melting. Monolithic components having [0 0 1] oriented, columnar grain structures exhibited a creep-optimized 59% volume fraction of cuboidal, coherent, γ′-phase precipitates averaging 275 nm on the side, and with γ/γ′ channel widths ranging from 25 to 75 nm. Transmission electron microscopy, utilizing bright and dark field imaging of optimally oriented γ/γ′ interfaces showed prominent misfit coherency strains as δ-fringe patterns. Corresponding hardness measurements also indicated the possibility of creep strength comparable with the commercial alloy. The notable feature of this study was the monolithic development of desirable superalloy properties without conventional, multi-step heat treatments.     

The Microstructure and Gamma Prime Distributions in Inertia Friction Welded Joint of P/M Superalloy FGH96

A gamma prime (γ′) precipitation (~35% in volume)-hardened powder metallurgy (P/M) superalloy FGH96 was welded using inertia friction welding (IFW). The microstructure and γ′ distributions in the joints in two conditions, hot isostatic pressed state and solution-treated and aged state, were characterized. The recrystallization of grains, the dissolution and re-precipitation of γ′ in the joints were discussed in terms of the temperature evolutions which were calculated by finite element model analysis. Regardless of the initial states, fully recrystallized fine grain structure formed at welded zone. Meanwhile, very fine γ′ precipitations were re-precipitated at the welded zone. These recrystallized grain structure and fine re-precipitated γ′ resulted in increasing hardness of IFW joint while making the hardness dependent on the microstructure and γ′ precipitation.     

Effect of cooling rate on microstructure and tensile properties of powder metallurgy Ni-based superalloy

The effects of size distribution, morphology and volume fraction of γ′ phase and grain size on tensile properties of powder processed Ni-based superalloy were investigated by using two different quenching methods. Oil quenching and air cooling were adopted with cooling rate of 183 °C/s and 4–15 °C/s, respectively. The experimental results show that the average size of the secondary γ′ after oil quenching is 24.5 nm compared with 49.8 nm under air cooling, and corresponding volume fractions of γ′ are 29% and 34%, respectively. Meanwhile, the average grain size remains nearly equivalent from both oil-quenching and air-cooling specimens. The tensile strength at room temperature is higher for the oil-quenched specimen than the equivalent from the air-cooled specimen, but the difference approaches each other as the temperature increases to 650 °C. The fractography clearly demonstrates that transgranular fracture governs the failure process at ambient temperature, in contrast to the intergranular fracture at 650 °C or even higher temperature. These two mechanical responses indicate the strengthening effects of γ′ precipitates and grain boundary for polycrystalline Ni-based superalloys at different temperatures.     

Elemental partitioning of platinum group metal containing Ni-base superalloys using electron microprobe analysis and atom probe tomography

The partitioning of platinum group metal (PGM) additions in polycrystalline Ni-base superalloys has been investigated. Alloys with a baseline composition of 15Al–5Cr–1Re–2Ta–0.1Hf (at.%) with systematic variations in Pt, Ir, Ru and W were cast and heat-treated to produce bimodal two-phase γ–γ′ microstructures. Electron microprobe analysis was used to measure the composition of coarse γ′ particles. Selected alloys were studied using atom probe tomography. Pt and Ru weakly partition to the γ′ and γ phases, respectively, whereas Ir partitions equally between both phases. Pt and Ru do not change partitioning behavior appreciably with additional W and Re. Interestingly, Ir lowers γ′ volume fraction and reduces partitioning of W, Re, Ru, Al and Pt. Tungsten partitioning coefficients exhibit an unusually strong temperature dependence, with increased partitioning of W to the matrix at lower temperatures. The thermodynamics of these PGM-containing systems are analyzed and the implications for partitioning are discussed.     

Microstructure and stress-rupture property of an experimental single crystal Ni-base superalloy with different heat treatments

The influence of heat treatment on the microstructure and stress-rupture property at 1,100 °C/140 MPa was investigated in a 5.0 wt% Re containing experimental single crystal Ni-base superalloy. The results indicate that the γmorphology is nearly cuboidal in the dendrite core after conventional heat treatment. The lattice misfit of alloy becomes more negative after modified heat treatment and results in more cuboidal γ precipitates than that after conventional heat treatment. The increased stress-rupture life after modified heat treatment is attributed to higher γ volume fraction, more negative lattice misfit, well-rafted structure, and narrower c channel width.     

Effect of Interactions Among Elements on Diffusion Process Associated with γ′ Coarsening in a Ni-Based Single-Crystal Superalloy

Coarsening of cuboidal γ' precipitates and relevant diffusion process in Ni-based single-crystal superalloy CMSX-4 were investigated at 1000,1020 and 1040℃ for specific times.The y' coarsening kinetics followed a cubic rate law with time and was presumably controlled by bulk diffusion of elements in y matrix.The associated diffusion activation energy was experimentally determined to be about 300 kJ/mol when it is considered the temperature-dependent thermo-physical parameters in modified Lifshitz-Slyozov-Wagner theory.The influence of temperature on γ/γ' microstructure is briefly discussed based on pseudo-binary [Ni]-[Al] phase diagram.Interactions among elements can effectively raise the local vacancy formation and vacancy-atom exchange barriers close to γ-and γ'-partitioning elements,respectively.Thus,it can significantly reduce the inter-coupling migrations of atoms during the macroscopic cross-diffusion process associated with γ' coarsening of Ni-based superalloys.     

Phase-field modeling of the γ′-coarsening behavior in Ni-based superalloys

Microstructural changes during heat treatment of the Ni-based CMSX-4 and CMSX-6 superalloys have been investigated experimentally and simulated using a phase-field multi-component model incorporating elastic driving forces in the presence of a lattice misfit. Furthermore, a theoretical model of the coarsening of anisotropic particles is proposed for the prediction of the main kinetic parameters of the coarsening process. A comparison of the main characteristics of the microstructural evolution during non-directional γ′-coarsening, provided from both experiments and phase-field simulations, gives a good agreement of the coarsening kinetics of the CMSX-4 superalloy. However, for the CMSX-6 superalloy, phase-field simulation results and theoretical predictions are not entirely consistent with experimental results, and show that additional effects, for example, those caused by plastic deformation, might be a reason for the slow coarsening rate.     

Modelling the orientation and direction dependence of the critical resolved shear stress of nickel-base superalloy single crystals

The orientation and direction dependence of the critical resolved shear stress was determined experimentally for the chromium-rich superalloy SC 16 at 650, 750 and 850°C and a constant strain rate of 10⁻³ s⁻¹. The results are used to establish an extended Schmid law for octahedral slip in the temperature and orientation range in which cross-slip pinning of dislocation pairs in the γ′ phase takes place. Normal Schmid behaviour was assumed for orientations near [111], for which cube slip was activated on a macroscopic level. Differences between some commercial superalloys are worked out and can be attributed to morphology and volume fraction of the γ′ phase. The orientation dependence and asymmetry effects increase in the order NIMONIC 105, SC 16, René N4. The orientation range where macroscopic cube slip can be expected increases in the same order. A close inspection of the parameters which are responsible for non-Schmid behaviour suggests that, in addition to cross-slip pinning, a matrix effect must be operating as well, partly counteracting the behaviour expected for mono-phase γ′ crystals.     

Thermodynamic stability of Co–Al–W L12 γ′

Co-based superalloys in the Co–Al–W system exhibit coherent L1₂ Co₃(Al,W) γ′ precipitates in an face-centered cubic Co γ matrix, analogous to Ni₃Al in Ni-based systems. Unlike Ni₃Al, however, experimental observations of Co₃(Al,W) suggest that it is not a stable phase at 1173 K. Here, we perform an extensive series of density functional theory (DFT) calculations of the γ′ Co₃(Al,W) phase stability, including point defect energetics and finite-temperature contributions. We first confirm and extend previous DFT calculations of the metastability of L1₂ Co₃(Al_0.5W_0.5) γ′ at 0 K with respect to hexagonal close-packed Co, B2 CoAl and D0₁₉ Co₃W using the special quasi-random structure (SQS) approach to describe the Al/W solid solution, employing several exchange/correlation functionals, structures with varying degrees of disorder, and newly developed larger SQSs. We expand the validity of this conclusion by considering the formation of antisite and vacancy point defects, predicting defect formation energies similar in magnitude to Ni₃Al. However, in contrast to the Ni₃Al system, we find that substituting Co on Al sites is thermodynamically favorable at 0 K, consistent with experimental observation of Co excess and Al deficiency in γ′ with respect to the Co₃(Al_0.5W_0.5) composition. Lastly, we consider vibrational, electronic and magnetic contributions to the free energy, finding that they promote the stability of γ′, making the phase thermodynamically competitive with the convex hull at elevated temperature. Surprisingly, this is due to the relatively small finite-temperature contributions of one of the γ′ decomposition products, B2 CoAl, effectively destabilizing the Co, CoAl and Co₃W three-phase mixture, thus stabilizing the γ′ phase.     

Coarsening of a multimodal nickel-base superalloy

The coarsening of γ′-Ni₃Al precipitates in the nickel superalloy Ni115 has been examined and compared to the results of a numerical model based on LSW coarsening theory. Ni115 has a γ′ fraction of around 60%, and at the coarsening temperatures of interest the γ′ distribution is bimodal, with two populations ～5 nm and ～90 nm in radius. It is found that during the initial transient (around 2000 h at 800 °C), the fine γ′ dissolve, leading to a rapid increase in the mean radius followed by a plateau. At long times, the expected steady-state unimodal t^1/3 coarsening is observed. The model reproduces these features in form and approximately in magnitude, a first for LSW model-experiment comparisons in nickel superalloys.     

Coarsening kinetics of γ′ precipitates in cobalt-base alloys

The expeditious development of novel cobalt-base γ–γ′ alloys as possible next generation superalloys critically depends on achieving a comprehensive understanding of the coarsening kinetics of ordered γ′ precipitates. This paper discusses the coarsening of L1₂ ordered Co₃(W, Al) precipitates in a model ternary Co–10Al–10W (at.%) alloy during isothermal annealing at 800 and 900 °C. The experimentally determined temporal evolution of average size of the γ′ precipitates suggests classical matrix diffusion limited Lifshitz–Slyozov–Wagner coarsening at both temperatures. The γ′ coarsening rate constants have been determined using a modified coarsening rate equation for non-dilute solutions. Furthermore, using the Cahn–Hilliard formulation for interfacial energy, the γ/γ′ interfacial energies at the respective annealing temperatures have been correlated to the concentration profile across the interface that has been experimentally determined using atom probe tomography. The calculated interfacial energies are in comparable range with those observed in nickel-base superalloys. Additionally, this analysis has permitted, for the first time, the determination of the gradient energy coefficient for γ/γ′ interfaces in Co-base alloys, a critical input for phase-field and other simulation models for microstructural evolution.     

Coarsening of ordered intermetallic precipitates with coherency stress

The morphological evolution and coarsening kinetics of ordered intermetallic precipitates with coherency stress were studied using a diffuse-interface phase-field model in two dimensions (2D). The emphasis is on the effects of precipitate volume fraction. The average aspect ratio of the precipitates in the microstructure is found to increase with time and decrease with volume fraction. Contrary to all the existing coarsening theories but consistent with a number of experimental measurements on the coarsening kinetics of ordered γ′ precipitates in Ni-base superalloys, we found that the coarsening rate constant from the cubic growth law decreases as a function of volume fraction for small volume fractions (≲20%) and is constant for intermediate volume fractions (20–50%). From the simulation results, we infer that the two length scales in a stress-dominated coherent two-phase microstructure, the average precipitate size and average spacing between arrays of aligned precipitates, follow different growth exponents. It is demonstrated that as the volume fraction increases, the precipitate size distributions become broader and their skewness become increasingly positive.