Phases In Ni-based Superalloys Research Articles

Experimental Investigations of Ni-Ti-Ru System: Liquidus Surface Projection and 1150 °C Isothermal Section.

Ruthenium addition inhibits the formation of the topologically close-packed phases in Ni-based superalloys and improves the solid solution strength of Ni-Ti shape memory alloys. Therefore, the Ni-Ti-Ru phase stability is a very valuable indicator of the effects of Ru in Ni-based superalloys and Ni-Ti shape memory alloys. In this study, the isothermal section at 1150 °C and liquidus surface projection of the Ni-Ti-Ru ternary system were determined experimentally using the equilibrated alloy method and diffusion couple method, respectively. Alloys were prepared through the arc-melting of Ni, Ti, and Ru (all 99.99% purity), and then vacuum encapsulation in quartz tubes, followed by annealing at 1150 °C for 36 to 1080 h depending on the alloy composition. Diffusion couples were fabricated by joining one single-phase block (τ1) with one two-phase block (Ni3Ti + γ(Ni)), and the couples were annealed under vacuum at 1150 °C for 168 h. Reaction temperatures of as-cast alloys were determined by differential scanning calorimetry performed with heating and cooling rates of 10 °C/min. Scanning electron microscopy and X-ray diffraction were used to analyze the microstructure. Seven three-phase regions were found at the 1150 °C isothermal section. Seven primary solidification regions and five ternary invariant reactions were deduced in the liquidus surface projection. A new ternary compound τ1 was discovered in both the isothermal section at 1150 °C and liquidus surface projection. The results aid in thermodynamic modeling of the system and provide guidance for designing Ni-based superalloys and Ni-Ti shape memory alloys.

First-principles study on effects of X (Ta, Ti, and W) on the nitriding behavior of γ′-Ni3Al in Ni-based superalloys: From the surface adsorption and diffusion of nitrogen

The role of X (Ta, Ti, and W) doping in affecting the adsorption and diffusion behavior of nitrogen at Ni3Al(110) and Ni3Al(111) surfaces was studied based on first-principles calculations to understand the nitriding mechanism of γ′-Ni3Al in Ni-based superalloys. The calculated results show that the substituting of X atom on the 1st-layer (top layer) Al site could enhance the adsorption ability of N on Ni3Al surface, especially on Ni3Al(111), owing to the stronger bonding strength between N and X atoms. For comparison, the effect of the X atom on the adsorption behavior of nitrogen is not significant when it is doped in the 2nd-layer or 3rd-layer. Nitrogen diffusion into the Ni3Al surfaces as the rate-determining step of nitriding is calculated based on the climbing image nudged elastic band (CI-NEB) method. The doping of Ti could significantly decrease the energy barriers of N penetration into γ′-Ni3Al surface, regardless of surface orientation, which contributes to promoting the nitriding process of γ′ phase. The strongest strengthening effect of Ti explains the experimental results that γ′-Ni3Al contains Ti element having a great nitriding behavior in Ni-based superalloys. Our results can provide an in-depth insight into the diffusion mechanism of nitriding γ′-Ni3Al phase in Ni-based superalloys.

Distinct nitriding behaviors of γ and γ′ phases in Ni-based superalloys: A first-principles study

The diffusion of nitrogen through γ(Ni)/γ'(L12-Ni3Al) interface in Ni-based superalloys was investigated based on first-principles calculations and the different nitriding behaviors of γ matrix and γ' precipitate were determined. At the interface, nitrogen atoms prefer to occupy the 6-Ni octahedral interstitial site with a large physical volume rather than the octahedral site constituted by Ni and Al with good chemical affinity. This indicates that γ matrix can provide more trapping sites for nitrogen atom than γ' precipitate. Moreover, the migration energy barrier of N crossing the (002)γ/γ' coherent plane is higher than that in γ phase. Nitrogen atoms can move into γ' phase unless the diffusion front has higher nitrogen concentration and O4 * (5Ni-Al octahedral site) is occupied. Inside the γ' matrix, nitrogen atoms are also easily trapped by the 6-Ni octahedral interstitial site, but their escape rates are much lower. It is the potential barrier role of the (002)γ/γ’ coherent plane and the low diffusion ability of N in γ' that are responsible for the unfavorable nitriding behavior of γ' phase in Ni-based superalloys.

Promotion of topologically close-packed phases in a Ru-containing Ni-based superalloy

We report on the effects of Ru addition on the formation of topologically close-packed (TCP) phases in a Ni-based superalloy during high-temperature exposure. The Ru addition reduces the lattice misfit between the γ and TCP phase, unexpectedly promoting nucleation of the TCP phase. Furthermore, the γ matrix is destabilized by Ru, while the stability of TCP phase is less affected according to the prediction of an electronic structure map. The results of this work indicate that γ destabilization is another important factor for the role of Ru in TCP phase formation.

Exploring the alloying effects on generalized stacking fault energy and ideal strength of Ni and Ni3Al phases in Ni-based superalloys

To clarify the different effects of alloying elements on the strength of Ni and Ni3Al, this study employs First-principles methods to investigate the effects of refractory elements Re, Ru, Ta, Mo and W on the generalized stacking fault energy and ideal strength of Ni and Ni3Al, respectively. The results reveal that the alloying elements Re, Ru, Ta, Mo, and W can help to improve the creep strength of Ni but reduce that of Ni3Al. Among these alloying elements, Re and Ru are more helpful for improving the strength of Ni, and for Ni3Al, Ru has the weakest enhancing effect on the strength of Ni3Al. Moreover, except for Ru, other alloying elements have more significant enhancing effects on the strength of Ni3Al. The electronic structure analysis shows that the d orbitals of alloying elements except Ru can form deep and wide pseudogap near the Fermi energy level, which causes the obvious enhancement of the strength of Ni3Al. Furthermore, the regular areas of charge distribution between atom Ru and its nearest neighbor atoms Ni are more vulnerable to be destroyed, resulting in Ni3Al–Ru owning a relatively lower ideal tensile strength and reaches its ideal tensile strength under a smaller strain.

Segregation behavior of alloying elements and its effects on stacking fault of γ′ phase in Ni-based superalloys: First-principles study

In this paper, the segregation tendency of elements M (Re, Co, Cr, Mo, Ti) near the complex and intrinsic stacking fault (CSF and SISF) on γ′- Ni3Al (111) plane as well as the influence of the segregation on the mechanical properties were studied by the first-principle method. The results show that Co and Cr atoms tend to segregate to CSF and Co also has segregation tendency for SISF. Comparison of metallic radius of alloying element can predict preliminary the segregation tendency which decreases along with the increase of metallic radius of M, except for Re. Segregation of M depends on their strong interactions with their surrounding Ni atoms near SFs, rather than the minimization of elastic strain. In addition, the Re effect is not induced by its interaction with the two SFs in γ′ phase. Meanwhile, the effects of segregated elements Co and Cr on the mechanical properties are further discussed. The results show that segregation of the two elements to CSF results in higher interfacial stability and the improvement of plastic deformation ability, but Co near SISF is beneficial to increase work-hardening. This work will provide insight for the composition design and improvement of processing technology of Ni-based superalloys and even other alloys.

Crystal plasticity modeling of non-Schmid yield behavior: from Ni3Al single crystals to Ni-based superalloys

A crystal plasticity finite element (CPFE) framework is proposed for modeling the non-Schmid yield behavior of L12 type Ni3Al crystals and Ni-based superalloys. This framework relies on the estimation of the non-Schmid model parameters directly from the orientation- and temperature-dependent experimental yield stress data. The inelastic deformation model for Ni3Al crystals is extended to the precipitate (γ′) phase of Ni-based superalloys in a homogenized dislocation density based crystal plasticity framework. The framework is used to simulate the orientation- and temperature-dependent yield of Ni3Al crystals and single crystal Ni-based superalloy, CMSX-4, in the temperature range 260–1304 K. Model predictions of the yield stress are in general agreement with experiments. Model predictions are also made regarding the tension–compression asymmetry and the dominant slip mechanism at yield over the standard stereographic triangle at various temperatures for both these materials. These predictions provide valuable insights regarding the underlying (orientation- and temperature-dependent) slip mechanisms at yield. In this regard, the non-Schmid model may also serve as a standalone analytical model for predicting the yield stress, the tension–compression asymmetry and the underlying slip mechanism at yield as a function of orientation and temperature.

Re-precipitation mechanisms of the γ′ phase with sphere, near-sphere, cubic, octets and finally-dendrite in as-cast Ni-based superalloys

The morphology and size distribution of γ′ phases have a significant impact on the thermal processing and manufacturing of components made of superalloys and their service performance under adverse conditions. The homogenization treatment of as-cast superalloys ingot is the first modulation of the morphology, size distribution and other characters of γ′ phase. Therefore, investigation on the re-precipitation mechanism of γ′ phase is of critical theoretical value and engineering significance. In this research, the morphology evolution and size distribution of re-precipitation γ′ phase in an as-cast Ni-based superalloy during homogenizing treatment were investigated extensively and systematically. The re-precipitation mechanism of γ′ phase with sphere, near-sphere, cubic, octets and finally-dendrites was identified under various cooling conditions. The γ′ phase size is increased while the nucleation density is decreased with the cooling conditions changing from air cooling (AC) to furnace cooling (FC). The γ′ phase stability during growth was discussed and the reprecipitation γ′ phase is generated from a refine spherical shape at fast cooling condition (AC); while irregularity growth even splitting happened at slow cooling condition (FC). The findings help understand the re-precipitation mechanism of the γ′ phase of Ni-based superalloys and provide a basis for controlling of the microstructure and tailoring of the properties of the alloys.

Atomic configurations of planar defects in μ phase in Ni-based superalloys

μ topologically close-packed phase commonly forms in superalloys and its intrinsic deformation is critical for their performance. Atomic configurations of planar defects in μ phase have been investigated by aberration-corrected scanning transmission electron microscopy and geometrical structure analysis. It is found that the basal slip in μ phase is accomplished by synchroshear inside Laves triple-layers, whereas shear deformations on non-basal planes are associated with long-range diffusions or local atomic rearrangements. Distinct from that on (11¯02) pyramidal planes, displacement vectors of planar faults on (1¯101) pyramidal planes deviate from the slip plane, leading to the contraction or dilation faulted regions.

Effect of alloying elements on stacking fault energies of γ and γʹ phases in Ni-based superalloy calculated by first principles

The first-principle calculations were employed to investigate the effect of alloying elements on stacking fault energies of γ-Ni and γʹ-Ni3Al phases in Ni-based single crystal superalloys. The results showed that Co, Cr, Mo, Ta, Ti, W, Re and Ru elements decreased the stacking fault energy of γ-Ni phase with increasing the concentration, but increased the stacking fault energy of γʹ-Ni3Al phase except Co and Cr elements. And, Co had a more significant effect than Cr in decreasing the unstable stacking fault energies of two phases. Furthermore, the interaction effect of Co and Cr elements was calculated that it significantly reduced the stable stacking fault energy of γ′ phase but had no obvious effect on the that of γ phase. Finally, two alloys with different Co contents were crept at 750 °C/750 MPa condition, and the creep life in 12Co alloy was about 118 h, which was basically half of that in 9Co alloy (242 h). The microstructure observation indicated that the width of stacking fault in γ′ phase in 12Co alloy was increased by 51.8 nm, and the stable stacking faults energy of γ′ phase was reduced by 9.1 mJ/m2 compared to those in 9Co alloy.

Partitioning behavior and lattice misfit of γ/γ′ phases in Ni-based superalloys with different Mo additions

A low-density single crystal (LDS) alloy with the composition of high Mo content was designed. The extra 1.5 wt% Mo was added in the Alloy A with the composition of Ni–6.5Al–8.0Mo–2.4Cr–6.2Ta–4.9Co–1.5Re–(0.01–0.05)Y (wt%) to study the influence of Mo on the lattice parameter and partitioning behavior. Scanning electron microscope (SEM) with energy-dispersive spectrometer (EDS), transmission electron microscopy (TEM) and high-temperature X-ray diffraction (HT-XRD) were used to observe the microstructure, analyze the elemental content and measure the lattice parameter of the alloys. The natural lattice misfit was calculated by lattice constants which were measured by HT-XRD at the temperature from 25 to 1150 °C, and the results showed that the lattice misfit would be more and more negative with temperature increasing. It was found that 1.5 wt% addition of Mo will increase the absolute value of the lattice misfit of γ/γ′ phases and the volume fraction of γ′, and at the same time, influence the elemental distribution in γ and γ′ phases, especially Re and Cr. Re has a higher partitioning ratio (k) after the addition of Mo.

Doping effects on the stacking fault energies of the γ′ phase in Ni-based superalloys**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0701502).

The doping effects on the stacking fault energies (SFEs), including the superlattice intrinsic stacking fault and superlattice extrinsic stacking fault, were studied by first principles calculation of the γ′ phase in the Ni-based superalloys. The formation energy results show that the main alloying elements in Ni-based superalloys, such as Re, Cr, Mo, Ta, and W, prefer to occupy the Al-site in Ni3Al, Co shows a weak tendency to occupy the Ni-site, and Ru shows a weak tendency to occupy the Al-site. The SFE results show that Co and Ru could decrease the SFEs when added to fault planes, while other main elements increase SFEs. The double-packed superlattice intrinsic stacking fault energies are lower than superlattice extrinsic stacking fault energies when elements (except Co) occupy an Al-site. Furthermore, the SFEs show a symmetrical distribution with the location of the elements in the ternary model. A detailed electronic structure analysis of the Ru effects shows that SFEs correlated with not only the symmetry reduction of the charge accumulation but also the changes in structural energy.

Effect of alloying elements on the γ’ antiphase boundary energy in Ni-base superalloys

Mechanical and fatigue performance of γ-γ′ Ni-base superalloys are strongly affected by the antiphase boundary energy (APBE) of the γ′ precipitates which, in turn, is dictated by the alloy's composition. Due to the multicomponent character of these alloys, establishing composition-APBE relationships are challenging, even though the qualitative effect of individual solutes on the APBE may be known. This work attempts to utilize density functional theory-based cluster expansion calculations to systematically assess the effect of composition on the APBE of the γ′ phases in Ni-base superalloys. We aim to elucidate the influence of not only one single element but also multiple coexisting alloying elements on the γ′ APBE. By explicit consideration of configurational disorder via Monte Carlo sampling, the effect of temperature on the APBE has also been analyzed. This work reveals that (1) effects of individual solute element M on the APBE energy obtained in an isolated, ternary condition (i.e. in Ni3-xAl1-yMx+y) does not directly translate to a multi-solute case and that (2) the mutual synergistic interactions among different solute elements are not negligible. Based on the present results, an empirical master equation that predicts the APBE based on the composition of the γ′ phases has been obtained.

Comparison of Methods for Quantification of Topologically Close-Packed Phases in Ni-Based Superalloys

The ability to quantify accurately the formation of topologically close-packed phases in nickel-based superalloys is key to assessing their thermal stability and ensuring that their performances will not deteriorate during long-term exposure at high temperatures. To investigate the effectiveness of synchrotron XRD for the detection of such minority phases in Ni-based superalloys, the commercial polycrystalline alloy RR1000 was analyzed following exposures of varying times at 800 °C. Data were collected from both solid samples and extracted residues, and additional laboratory X-ray diffraction was performed on the residues. The minor phases were successfully detected in solid samples using synchrotron radiation, and a comparison of the results from these quantification methods shows that the extraction method gives results of the right order of magnitude to reflect the phase quantities that are present in the alloy. However, the results indicate that the synchrotron route is not a suitable method for the quantification of phases present in quantities less than approximately 0.3 wt pct.

Abnormal increase of TCP phase during heat treatment in a Ni-based single crystal superalloy

Generally, elevated solution heat treatment temperature decreases the residual dendritic segregation of refractory elements (i.e. Re and W) and suppresses the precipitation of deleterious topologically close-packed (TCP) phases in Ni-based superalloys. However, in the present study, elevated solution temperature resulted in an abnormal increase of TCP phases in a Ni-based single crystal superalloy after long-term thermal exposure at 900 °C. Microstructural analysis suggested that higher supersaturation in γ phase which promoted the formation of TCP phases was induced by the increased γ′ precipitation after heat treated at elevated solution temperature.

Abnormal Increase of TCP Phase During Heat Treatment in a Ni-Based Single Crystal Superalloy

Generally, elevated solution heat treatment temperature decreases the residual segregation of refractory elements (i.e. Re and W) and suppresses the precipitation of deleterious topologically close-packed (TCP) phases in Ni-based superalloys. However, in the present study, elevated solution temperature resulted in an abnormal increase of TCP phases in a Ni-based single crystal superalloy after long-term thermal exposure at 900 oC. Microstructural analysis suggested that higher supersaturation in γ phase which promoted the formation of TCP phases was induced by the increased γ' precipitation after heat treated at elevated solution temperature.

Grain boundary precipitation behavior of δ-Ni3Nb (D0a) phase in a Ni-Nb-Fe ternary model alloy

Aiming at covering grain boundary (GB) with a geometrically close-packed (GCP) type intermetallic phase in Ni-based superalloys, GB precipitation behavior of the δ-Ni3Nb phase with the D0a structure in the A1 matrix is investigated using a model alloy Ni-12Nb-3Fe. The D0a phase precipitates on GB prior to grain interior (GI). The precipitation of D0a phase tends to take place in a continuous manner above its nose temperature (1373 K) and in a discontinuous manner below the temperature. The area fraction of a GB covered by the precipitate (ρ) significantly differs by GB in aging at 1423 K, i.e. between 0 and 80% in short time aging and between 30 and 100% in long time aging, while their average area fraction (ρ¯) increases up to about 75% after long time aging. The observed large difference in ρ is found to be enhanced in the growth stage of the precipitation. A crystallographic orientation analysis indicates that the difference may be caused by the geometry of the habit planes, {111} in the matrix phase, with respect to the GB plane; that is, in the case that one of the habit planes is nearly parallel to the GB plane, the D0a phase grows along the GB, resulting in high ρ, while the phase grows towards GI when any habit planes are inclined to the GB plane, resulting in low ρ. A multi-step heat treatment is proposed to suppress the growth of the precipitates towards GI.

On the influence of gamma prime upon machining of advanced nickel based superalloy

Whilst gamma prime (γ′) phase is the strengthening phase in Ni-based superalloys its influence on machining has been seldom investigated. This paper reports for the first time on the effect of γ′ upon machining of Ni-based superalloys when cutting with parameters yielding different cutting temperature intervals which lead to strengthening/softening effects on the workpiece (sub)surface. In-depth XRD, SEM/FIB, EBSD analysis and unique micro-pillar testing in the workpiece superficial layers indicated that with the increase of γ′ fraction the grain plastic deformation significantly decreased, while specific cutting energy can switch from low to high values influenced by the real cutting temperature.

Simulation of γ′ Precipitation Kinetics in a Commercial Ni-Base Superalloy

The ability to accurately simulate the precipitation kinetics of γ′ and other strengthening phases in Ni-base superalloys is of great interest to industry. Several commercial simulation tools such as TC-PRISMA (Thermo-Calc, Sweden) and PanPrecipitation (Computherm, USA) have been made available in recent years. This paper reports the outcome of a validation study on wrought Ni-base superalloy HAYNES® 282® alloy for two scenarios of commercial interest: (1) the precipitation of γ′ during continuous cooling, and (2) the precipitation of γ′ during two-step aging. The simulation results are validated against experimental data. Any discrepancies are discussed in the context of the uncertainty in key material properties (such as interfacial energies), model assumptions, and experimental errors.

Charge optimized many-body (COMB) potential for dynamical simulation of Ni–Al phases

An interatomic potential for the Ni–Al system is presented within the third-generation charge optimized many-body (COMB3) formalism. The potential has been optimized for Ni3Al, or the γ′ phase in Ni-based superalloys. The formation energies predicted for other Ni–Al phases are in reasonable agreement with first-principles results. The potential further predicts good mechanical properties for Ni3Al, which includes the values of the complex stacking fault (CSF) and the anti-phase boundary (APB) energies for the (1 1 1) and (1 0 0) planes. It is also used to investigate dislocation propagation across the Ni3Al (1 1 0)–Ni (1 1 0) interface, and the results are consistent with simulation results reported in the literature. The potential is further used in combination with a recent COMB3 potential for Al2O3 to investigate the Ni3Al (1 1 1)–Al2O3 (0 0 01) interface, which has not been modeled previously at the classical atomistic level due to the lack of a reactive potential to describe both Ni3Al and Al2O3 as well as interactions between them. The calculated work of adhesion for this interface is predicted to be 1.85 J m−2, which is in agreement with available experimental data. The predicted interlayer distance is further consistent with the available first-principles results for Ni (1 1 1)–Al2O3 (0 0 0 1).