Phases In Ni-based Superalloys Research Articles

Effect of ruthenium on the precipitation of topologically close packed phases in Ni-based superalloys of 3rd and 4th generation

The precipitation of topologically close packed (TCP) phases is detrimental for the high temperature strength of high refractory Ni-based superalloys. The beneficial influence of Ru with respect to this so called instability is nowadays well accepted. In the present paper the precipitation of topologically close packed (TCP) phases is studied quantitatively in two experimental alloys (one Ru-free and one with addition of Ru) to clarify the mechanism of the Ru effect. It is confirmed that the TCP phase precipitates undergo sequential phase transformation with the tetragonal σ-phase precipitating first. Ru retards the phase transformation and leads to decreased equilibrium volume fraction of TCP phases. The results clearly indicate that Ru decreases the driving force for TCP phase precipitation. Investigations of crystallography and chemistry of the TCP/matrix interface point to an additional effect by increase of misfit strain energy.

Quantitative Phase Field Study on the Precipitation Dynamics of γ′ Phase in Ni-Based Superalloys with Different Concentration

A quantitative phase field simulation was performed on the dynamics evolution of γ′ (L12-Ni3X) phase in Ni-based superalloys, the microstructure, the volume fraction and the particle size distribution (PSD) of γ′ phase for Ni-Al alloys aged at 1173K with the Al concentration c=0.178, 0.180 and 0.182 were investigated, and the results were compared with Lifshitz-Slyozov-Wagner (LSW) theory and Brailsford-Wynblatt (BW) theory. As the Al concentration increases the γ′ phase morphology changed from the separated cuboidal shape to the connected rectangle shape, the nucleation and growth of γ′ phase became faster and the volume fraction of the γ′ phase increased. The average particle radius <r> of γ′ phase and the aging time t has a exponent relationship <r> ~ tn at the coarsening stage with the exponents n=0.313, 0.235 and 0.204 for c=0.178, 0.180 and 0.182, respectively. The peaks of the fitted PSDs are less than the predicted value from the LSW theory and the fitted PSDs are wider than that of LSW predicted, while they are similar to that of the BW theory as the Al concentration increases. The peaks appear at a smaller r/<r> than the predictions of the LSW and BW theories.

Effects of Ru on elemental partitioning and precipitation of topologically close-packed phases in Ni-based superalloys

Two Ni-based superalloys (one Ru-free and one containing 1 at.% Ru) were comparatively studied by Atom Probe Tomography and complimentary techniques. Ru addition impedes precipitation of the σ phase at 950 °C. Ru partitions nearly equally to the γ and σ phase. Neither reverse elemental partitioning nor destabilization of the γ′ phase is detected when adding Ru. We propose an increase in the γ/σ lattice misfit caused by Ru as a major reason for impeded nucleation of the σ phase.

Topologically Close-Packed Phase Precipitation in Ni-Based Superalloys

The addition of refractory elements is effective in improving the comprehensive performance of Ni-based superalloys, and meanwhile, the precipitation trend of TCP phases increases with the elements content due to the segregation of refractory elements during high temperature service. The precipitation of TCP phases obviously decreased the creep properties and creep rupture life of superalloys. The toughness and plastic of superalloys also dramatically deteriorated. The addition of Ruthenium (Ru) is shown to suppress the formation of TCP phases in Ni-based superalloys, resulting in much improved the creep resistance and the microstructural stability. The studies on TCP phases in Ni-based superalloys including crystallography and precipitation of TCP phases and the effect of alloy composition on TCP phases are reviewed.

TCP phase predictions in Ni-based superalloys: Structure maps revisited

The traditional methods for predicting the occurrence of deleterious topologically close-packed (TCP) phases in Ni-based superalloys have been based on the PHACOMP and newPHACOMP methodologies. These schemes use the average number of holes N ¯ h or the centre of gravity of the elemental d-bands M ¯ d to predict whether or not a given multicomponent alloy will be prone to TCP formation. However, as both these one-dimensional methodologies are well-known to fail with respect to new generations of alloys, a novel two-dimensional structure map ( N ¯ , Δ V / V ¯ ) is introduced where N ¯ is the average electron concentration and Δ V / V ¯ is a composition-dependent size-factor difference. This map is found to separate the experimental data on the TCP phases of binary A–B transition metal alloys into well-defined but sometimes overlapping regions corresponding to different structure types such as A15, σ, χ, R, P, δ, μ, M and Laves. Detailed investigations of ternary phase diagrams and multicomponent systems show that TCP phases, regardless of the number of constituents, are located in the same regions of the structure map that are favoured by the binary compounds of the same structure type. The structure map is then used in conjunction with CALPHAD computations of σ phase stability to show that the predictive power of newPHACOMP for the seven component Ni–Co–Cr–Ta–W–Re–Al system studied recently by Reed et al. [24] is indeed poor. This supports a growing consensus that robust methods of TCP phase prediction in multicomponent alloys will require the inclusion of reliable first-principles thermodynamic databases within the semi-empirical CALPHAD scheme.

Etching and High-Resolution Backscatter Electron Imaging for Semi-Automated Segmentation and Stereology of the Gamma Prime Phase in Ni-based Superalloys

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Phase-Field Simulation on Coarsening of the γ'Phase Particles in Ni-Based Superalloys Considering Elastic Inhomogeneity

A phase-field simulation is performed to examine the effect of elastic inhomogeneity between the  and ’ phases on coarsening of the ’ phase in Ni-based superalloys. In the calculation of elastic strain energy, the mechanical equilibrium equation in elastically inhomogeneous system is solved by an iterative-perturbation scheme. On the basis of the elastic constants of a practical Ni-based superalloy, a series of simulations is performed in which both elastic anisotropy and shear modulus are varied independently. The variation of elastic anisotropy gives significant effect on both morphology and size distribution function of the ’ particles, whereas the variation of shear modulus gives little effect on them. Furthermore, it is found that the coarsening rate constant of the cubic growth raw changes and increases with increasing the standard deviation of the ’ size distribution.

Semi-automated characterization of the [formula omitted] phase in Ni-based superalloys via high-resolution backscatter imaging

The size distribution and volume fraction of the γ ′ phase in Ni-based superalloys plays a critical role in microstructural evolution and mechanical properties. Automated analysis of images is often desired for rapid quantitative characterization of these microstructures. Backscatter electron imaging of specimens in which the γ ′ phase has been selectively etched yields images that can be more readily segmented with image processing algorithms than other imaging techniques. Utilization of this combination of sample preparation and imaging technique allows for the rapid collection of microstructural data.

Phase-Field Simulation of the Effect of Elastic Inhomogeneity on Microstructure Evolution in Ni-Based Superalloys

Phase-field simulation is performed to examine the effect of elastic inhomogeneity between theand � 0 phases on microstructure evolution of the � 0 phase in Ni-based superalloys. For solving the elastic equilibrium equation numerically in the elastically inhomogeneous system, an iterative approach is used. In this simulation, both elastic anisotropy and shear modulus are varied independently based on the elastic constants of a practical Ni-based superalloy. Four different types of anti-phase domains in the � 0 phase are also considered in this simulation. The variation of elastic anisotropy affected significantly the manner of both morphology of the � 0 phase and distribution function of the � 0 particle size, whereas the variation of shear modulus did not affect them. (doi:10.2320/matertrans.MBW200826)

Temperature dependence of ordering in the ? -phase of Ni-based superalloys

Short-range order (SRO) in model γ-phases of several Ni-based superalloys is investigated by elastic diffuse neutron scattering as a function of temperature. The composition of the single-crystalline samples has been modified by small amounts of additional elements like Mo, Re, Ru and W in order to study their influence on SRO, which is clearly dominated by peaks of the {1 1/2 0} type. The intensity of SRO scattering remains nearly unchanged up to 600 °C, above this temperature SRO decreases gradually and finally vanishes around 1000 °C. Correlation lengths do not change significantly with temperature.

Numerical Resolution Enhancement of X-ray Diffraction Patterns

AbstractA numerical resolution-enhancement method was developed for x-ray diffraction data measured with a conventional x-ray diffractometer. This method removes the instrumental broadening due to x-ray optics, including the spectral distribution of the x-ray source such as the CuKα doublet. The advantages of this method are to separate the cluster of peaks in x-ray powder patterns into individual peaks without previous knowledge of the number of peaks, and to remove CuKα2 reflection peaks automatically.The instrumental window function, which was approximated by a modified pseudo-Voigt function, was calculated from the measured diffraction pattern of NBS Standard Reference Materials (640B) by a non-linear least squares method. The simple diffraction patterns, including no CuKα2 peaks, were obtained from the diffraction patterns measured with the conventional x-ray diffractometer by using the window function mentioned above.The application of the method of determination of the lattice misfit between γ and γ phases in Ni-base superalloys was also described.

X-ray Diffractometric Determination of Lattice Misfit Between γ and γ' Phases in Ni-Base Superalloys

AbstractThe lattice misfits between γ and γ' phases in Ni-base superalloys (single crystal) were accurately determined for filings of specimens by using both a conventional X-ray tube focusing diffractometer(CXRFD) and a synchrotron-radiation parallel beam X-ray diffractometer (SRPXRD). All reflection peaks measured with the CXRFD were in a cluster of overlapping peaks because of the very small differences in the lattice parameters of both phases and the instrumental broadening due to X-ray optics including the spectral distribution of Xray source such as CuKα doublet. The deconvolution method was applied to remove the instrumental broadening from the peaks measured with the CXRFD. The window functions for the deconvolution method were calculated from CuKα doublet reflection of Si standard by a nonlinear least-square method.The instrumental broadening of SRPXRD was much smaller than that of CXRFD since the monochromatic X-rays produced single peak profiles and constant profile shape over a wide 2θ range. A profile fitting with a pseudo-Voigt function was used to determine 2θ angles to 0.0005 deg. for the synchrotron powder data. The peak angle and shape reflected from γ' phases in γ-matrix and those fron electrochemically extracted γ'-phase were significantly different.