Numerical simulation of precipitation in multicomponent Ni-base alloys

A comprehensive particle size distribution model has been developed for the simulation of γ′ precipitation in multicomponent Ni alloys. Nucleation, growth and coarsening of the precipitates are described by a particle size distribution. The growth rate of each precipitate class is calculated with a multi-component diffusion model formulated for non-diagonal matrices of diffusion coefficients. The model is fully coupled with CALPHAD calculations of the thermodynamic equilibrium at the interface, including a direct treatment of the effect of curvature through modification of the Gibbs free energy. An optimization strategy was developed to minimize the computational cost. The model was used to simulate ageing heat treatment at 600 °C of Ni–7.56 at.% Al–8.56 at.% Cr, which was studied experimentally by Booth-Morrison and others (Booth-Morrison C, Weninger J, Sudbrack CK, Mao Z, Noebe RD, Seidman DN. Acta Mater 2008;56:3422; Mao Z, Booth-Morrison C, Sudbrack CK, Martin G, Seidman DN. Acta Mater 2012;60:1871). The comparisons showed that the precipitation stages of γ′ precipitates are correctly captured by the numerical model. It was shown that non-diagonal diffusion coefficients substantially influence the selection of the operating tie-line and the overall transformation kinetics. With non-diagonal diffusion matrices, complex phenomena such as uphill diffusion of Cr due to the Al gradients were evidenced and explained.     

Mullite shell mould for casting of advanced CG and SX components in nickel based superalloys

Abstract

Mullite base shell mould system is used for casting of superalloys in columnar and single crystal grains in temperature range of 1480–1550°C. The colloidal silica gel+mullite filler with and without fine alumina slurries was prepared followed by two shell moulds: one without alumina (shell system I) and the other with alumina (shell system II). Shell made with slurry system II resulted in increase in green strength by 10% and fired (950°C) strength by 125% respectively. Sag resistance capability was observed more for shell system II for tested temperatures from 1500 to 1650°C. Dimensional stability of low pressure turbine blade cast at 1550°C was also studied. No shell bulging effect was observed for both shell moulds. The importance of mullite filler material for shell mould to be used for investment casting and its capability to withstand high temperature without metal mould reaction have been highlighted.     

Microstructural response of an Al-modified Ni–Cr–Fe ternary alloy during thermal processing

A thermodynamic package was used to predict the phase transformations that occurred during thermal processing of a superalloy based on the composition of a ternary Ni–Cr–Fe alloy. The effect of the addition of 6 w/o Al on phase transformation in the material sintered were estimated and compared with results obtained experimentally by X-ray diffraction and metallography, while the transformation temperature of the modified alloy was corroborated by differential scanning calorimetry (DSC). Mechanical property of the alloy was estimated in terms of Vickers hardness. These results suggest that despite potential problems encountered in high-temperature powder processing of superalloys that often tend to influence the feasibility of using thermodynamic predictions to model such alloy systems, the software and predictions used in this study offer a way to simulate both design and characterisation of the experimental alloy.     

Oxidation of a chromia-forming nickel base alloy at high temperature in mixed diluted CO/H2O atmospheres

Corrosion of a chromia-forming nickel base alloy, Haynes 230^®, has been investigated under impure helium containing a few Pa of CO and H₂O at 900 °C. It has been found that this alloy reacts simultaneously with CO and H₂O. Oxidation by CO has been revealed to occur mainly in the first hours. CO diffuses through the scale via short-circuit pathways and oxidizes Al, Cr and Si at the oxide/metal interface. Kinetics of CO oxidation has been investigated and several rate limiting steps are proposed. In the long term, H₂O is the major oxidant of chromia-forming nickel base alloys in impure helium.     

Chemical Composition Design of Superalloys for Maximum Stress, Temperature, and Time-to-Rupture Using Self-Adapting Response Surface Optimization

We have adapted an advanced semistochastic evolutionary algorithm for constrained multiobjective optimization and combined it with experimental testing and verification to determine optimum concentrations of alloying elements in heat-resistant austenitic stainless steel alloys and superalloys that will simultaneously maximize a number of the alloy's mechanical properties. The optimization algorithm allows for a finite number of ingredients in the alloy to be optimized so that a finite number of physical properties of the alloy are either minimized or maximized, while satisfying a finite number of equality and inequality constraints. Alternatively, an inverse design method was developed, which uses the same optimization algorithm to determine chemical compositions of alloys that will be able to sustain a specified level of stress at a given temperature for a specified length of time. The main benefits of the self-adapting response surface optimization algorithm are its outstanding reliability in avoiding local minimums, its computational speed, ability to work with realistic nonsmooth variations of experimentally obtained data and for accurate interpolation of such data, and a significantly reduced number of required experimentally evaluated alloy samples compared with more traditional gradient-based and genetic optimization algorithms. Experimentally preparing samples of the optimized alloys and testing them have verified the superior performance of alloy compositions determined by this multiobjective optimization.     

Development of an anisotropic constitutive model for single-crystal superalloy for combined fatigue and creep loading

A unified anisotropic viscoplastic constitutive model for single-crystal superalloys is developed based on a modification of a phenomenological isotropic model. Orientation-dependent viscoplastic behaviour was observed in experiments. The model is used to simulate the orientation and cyclic mechanical response of single-crystal SRR99 under combined fatigue and creep conditions at 950°C. The results from the simulations are then used to estimate the fatigue-creep lives of the single-crystal nickel-base superalloy SRR99 using a new life prediction methodology. The predicted fatigue-creep lives are in good agreement with the experimental results.     

Progress in application of rare metals in superalloys

Rare metals play an important role in development of superalloys. Over the last two decades, the application of the rare metals in superalloys has achieved progress significantly. They present multi-beneficial effects for strengthening the matrix and the c0phase, increasing the lattice misfit, cleaning the grain boundary, improving the carbides and eutectics, refining the grain, stabilizing the oxidation film, etc., so that the elevated temperature rupture life and elevated temperature oxidation resistance are improved significantly, leading to a broad application in the superalloys. In order to meet the higher demand for better superalloys in the future, more intensive research is necessary on the effects of the rare metals on the superalloy, and especially on the combination effect of various rare metals and mutual influence among them. Utilization of the computational materials science and combinatorial high throughput experiment will be of importance in application of rare metals in superalloys.     

Improved Spallation Resistance of the Oxide Scale by Hf/Y Co-doping in Ni-Based Superalloy at High Temperature

Ni-based superalloys added with comparably higher concentrations of single-doped Hf and co-doped Hf/Y are prepared by vacuum induction melting (VIM). The oxidation properties up to 300 h at 900 °C, 1000 °C, and 1100 °C are investigated. The undoped alloy exhibited a minimum oxidation rate at 900 °C and 1000 °C. The co-doped alloy showed a higher oxidation rate; however, it possesses better scale adhesion, and no spallation occurs. Hf-doped alloy showed a lower oxidation rate and better scale adhesion at 900 °C and 1000 °C, but exhibited a shorter lifetime at 1100 °C. The spallation of the Hf-doped alloy is attributed to the precipitation of the HfO₂ in and beneath the oxide scale. The spallation in the undoped alloy is accredited to the thermal expansion mismatch between the growing oxide scale and superalloy substrate. Incorporating two reactive elements (REs) in alloy minimized the precipitation of RE oxide in the oxide scale, diminished internal oxidation in the alloy, and decreased oxide scale spallation.     

Effect of High Density Current Pulses on Microstructure and Mechanical Properties of Dual-Phase Wrought Superalloy

In this study,high density electric current pulse (ECP) treatment was introduced instead of the conventional solution treat-ment,and the γ'phase was completely dissolved under the ECP treatment within only several milliseconds at 1148 ℃.Due to the extremely short treatment time and high cooling rate,the growth of γ-phase matrix grain and γ'phase precipitate was effectively retarded.By comparing with the conventional heat process,the grain size of ECP treated sample was controlled to about 15 μm,the size of the re-precipitated γ'phase reduced from 65 to 35 nm,and the number density of γ'precipitate increased from 1.46 × 108 to 3.03 × 108/mm2.The Vickers hardness,ultimate tensile strength and yield strength of the ECP treated sample were significantly improved.According to the theoretical derivation of kinetics,the ECP treatment introduces an extra electrical free energy which promoted the dissolution of γ'phase.The ECP treatment may provide a new method for solution treatment of the Ni-based superalloy.