Precipitation In Superalloys Research Articles

A sharp-interface model for diffusional evolution of precipitates in visco-plastic materials

This paper describes a 3D implementation of the sharp-interface theory for material heterogeneities and is, hence, able to identify equilibrium shapes of precipitates in superalloys. The theory is adopted from Morton E. Gurtin and extended by crystal plasticity in the bulk. Crystal plasticity relaxes stresses at the phase interface, which leads to subsequent coalescence of the precipitates. The fully implicit model employs the extended finite element method (XFEM) in conjunction with level sets. The level set is advected in a velocity field computed by the stress-modified Gibbs-Thomson interface condition. Mechanical equilibrium and level set update are solved in a staggered procedure. Jump quantities are treated by means of a suitable enriched least square smoothing. Multiple schemes for the computation of curvature of surfaces in the context of the XFEM are presented and compared. Equilibrium shapes at different levels of misfit are computed. A cuboidal equilibrium shape is retrieved in a rotated mesh in order to quantify mesh-independence, a linear volume-time relationship during Ostwald ripening is reproduced and merging of particles under tension is reported.

Design of corrosion-resistant high-entropy alloys through valence electron concentration and new PHACOMP

Various methods can be used to predict the phase composition of conventional alloys. However, it is relatively difficult to predict high-entropy alloys (HEAs) owing to their complexity. There remain many challenges in designing HEAs with desirable properties. In this study, two strategies were applied to design corrosion-resistant non-equal-molar CoCrFeMoNi HEAs with a single phase. The two strategies were prediction for valence electron concentration dependent structural behavior to evaluate the matrix of HEAs, and average d-orbital energy level (M̅d) that was initially used to predict precipitation in superalloys under different temperatures. The validity of these two approaches was determined by thermal stability. The corrosion behaviors of the designed HEAs were investigated by polarization tests. In conclusion, the phase compositions of CoCrFeMoNi HEAs were predicted using a combination of the two approaches. The corrosion resistance was also competitive compared to that of Hastelloy C276. This research can provide a reference for the future development of corrosion-resistant HEAs with a single phase.

A crystal plasticity model incorporating the effects of precipitates in superalloys: Application to tensile, compressive, and cyclic deformation of Inconel 718

An elasto-plastic polycrystal plasticity model is developed and applied to an Inconel 718 (IN718) superalloy that was produced by additive manufacturing (AM). The model takes into account the contributions of solid solution, precipitates shearing, and grain size and shape effects into the initial slip resistance. Non-Schmid effects and backstress are also included in the crystal plasticity model for activating slip. The hardening law for the critical resolved shear stress is based on the evolution of dislocation density. Using the same set of material and physical parameters, the model is compared against a suite of compression, tension, and large-strain cyclic mechanical test data applied in different AM build directions. It is demonstrated that the model is capable of predicting the particularities of both monotonic and cyclic deformation to large strains of the alloy, including decreasing hardening rate during monotonic loading, the non-linear unloading upon the load reversal, the Bauschinger effect, the hardening rate change during loading in the reverse direction as well as plastic anisotropy and the concomitant microstructure evolution. It is anticipated that the general model developed here can be applied to other multiphase alloys containing precipitates.

The effect of magnetic field on precipitation phases of single-crystal nickel-base superalloy during directional solidification

The effect of magnetic field on precipitation phases in directional solidified single-crystal nickel-base superalloy was investigated. The magnetic field significantly decreases the size of γ′ and the content of carbides and γ/γ′ eutectic. The decreased magnitude is firstly sharp and then gentle with the increasing magnetic field intensity. The γ′ phase refinement could be attributed to a decrease in nucleation activation energy during solid phase transformation. The segregation reduction of alloying elements is responsible for the content decrease of carbides and γ/γ′ eutectic. The magnetic- field-assisted directional solidification provides an effective way to control the size and content of precipitation in superalloys.

A ternary phase-field model incorporating commercial CALPHAD software and its application to precipitation in superalloys

Abstract A ternary phase-field model was developed that is linked directly to commercial CALPHAD software to provide quantitative thermodynamic driving forces. A recently available diffusion mobility database for ordered phases is also implemented to give a better description of the diffusion behavior in alloys. Because the targeted application of this model is the study of precipitation in Ni-based superalloys, a Ni–Al–Cr model alloy was constructed. A detailed description of this model is given in the paper. We have considered the misfit effects of the partitioning of the two solute elements. Transformation rules of the dual representation of the γ + γ ′ microstructure by CALPHAD and by the phase field are established and the link with commercial CALPHAD software is described. Proof-of-concept tests were performed to evaluate the model and the results demonstrate that the model can qualitatively reproduce observed γ ′ precipitation behavior. Uphill diffusion of Al is observed in a few diffusion couples, showing the significant influence of Cr on the chemical potential of Al. Possible applications of this model are discussed.

Stability of nanoscale co-precipitates in a superalloy: A combined first-principles and atom probe tomography study

Inconel 718 is a nickel-iron based superalloy widely used in the aerospace industry. Its high temperature strength is attributed to the thermal stability of dense nanoscale precipitates ${\ensuremath{\gamma}}^{\ensuremath{'}}$ $[{\mathrm{Ni}}_{3}\mathrm{Al}]$ and ${\ensuremath{\gamma}}^{\ensuremath{''}}$ $[{\mathrm{Ni}}_{3}\mathrm{Nb}]$. There is experimental evidence that ${\ensuremath{\gamma}}^{\ensuremath{'}}$ and ${\ensuremath{\gamma}}^{\ensuremath{''}}$ often form co-precipitates ${\ensuremath{\gamma}}^{\ensuremath{'}}∕{\ensuremath{\gamma}}^{\ensuremath{''}}$ or sandwichlike structure ${\ensuremath{\gamma}}^{\ensuremath{'}}∕{\ensuremath{\gamma}}^{\ensuremath{''}}∕{\ensuremath{\gamma}}^{\ensuremath{'}}$ or ${\ensuremath{\gamma}}^{\ensuremath{''}}∕{\ensuremath{\gamma}}^{\ensuremath{'}}∕{\ensuremath{\gamma}}^{\ensuremath{''}}$. But how they stabilize under heat treatment or in service is still not well-understood. We have investigated the interfacial structure and chemistry of fine co-precipitates ${\mathrm{Ni}}_{3}(\mathrm{Al},\mathrm{Ti},\mathrm{Nb})∕{\mathrm{Ni}}_{3}\mathrm{Nb}({\ensuremath{\gamma}}^{\ensuremath{'}}∕{\ensuremath{\gamma}}^{\ensuremath{''}})$ in Inconel 718, using both first-principles density functional theory calculation and the three-dimensional atom probe technique. Our calculations confirm that Al atoms in the ${\ensuremath{\gamma}}^{\ensuremath{'}}$ phase segregate to the ${\ensuremath{\gamma}}^{\ensuremath{'}}∕{\ensuremath{\gamma}}^{\ensuremath{''}}$ interface. The enrichment of Al helps to impede the assimilation of Nb from ${\ensuremath{\gamma}}^{\ensuremath{'}}$ to ${\ensuremath{\gamma}}^{\ensuremath{''}}$ and reject Al from ${\ensuremath{\gamma}}^{\ensuremath{''}}$ to ${\ensuremath{\gamma}}^{\ensuremath{'}}$, and therefore keeps such secondary precipitates at fine size. In the absence of Ti in the ${\ensuremath{\gamma}}^{\ensuremath{'}}$ phase, our calculations predict an enhanced driving force for Al to accumulate at the interface. We have also characterized the microstructure of the ${\ensuremath{\gamma}}^{\ensuremath{'}}∕{\ensuremath{\gamma}}^{\ensuremath{''}}$ interface for an Inconel 718 sample taken from a commercial compressor blade serviced in an airplane engine for over $10\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}\mathrm{h}$ at a temperature up to $600\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ using three-dimensional atom probe analysis. Interestingly, we find that Al enrichment sustains long-term service, suggesting that the coarsening of secondary precipitates is interface-controlled. The success of first-principles density functional theory computation in reproducing the experimental observation encourages extensive application of this powerful tool in the study of precipitates in superalloys.

Localized Surface Instabilities of Stressed Solids

AbstractLocalized instabilities formation on the free surface of solids has been studied when sources of non-homogeneous stress such as dislocations or precipitates are present in the bulk. This formalism of localized perturbations has been used to describe the butterfly transformation of cubic precipitates in superalloys and the contraction of rectangular specimens under stress.

A High-Temperature Study of Phase Precipitation in Superalloys

AbstractMany of the iron- and nickel-base superalloys exhibit brittle properties on heat treatment, welding, or other fabrication processes at temperatures of about 2000°F or higher. Studies have been carried out by means of electron microscopy, electron diffraction, and X-ray diffraction and fluorescence analysis of the precipitation in the metal and in an isolated form.Results of the electron microscope study of the surface of the metal show a grain boundary constituent to be present which increases in amount as the temperature is increased. Studies on the isolated residue of such samples show a very thin “featherlike” film to be located at the grain boundaries and enclosing the grains. Electron diffraction, X-ray diffraction, and X-ray fluorescence analysis studies of the thin films indicate that they are a TiC phase with very little alloying elements in solution.At temperatures above 2000°F the thin film becomes quite thick and tends to force the grains apart. It is believed that this form of the TiC phase promotes the severe embrittling nature of these alloys at high temperatures. Suitable heat treatment at lower temperatures causes the TiC film to agglomerate and the grain boundaries become “tight,” and a more ductile condition results.